201 c Document

201C Script

Corrosion Prevention and

Mitigation

IInnttrroodduuccttiioonn

Welcome to the Corrosion 201-C course, which provides details on preventing and

mitigating corrosion of systems, equipment, and infrastructure.

Since the impact of corrosion is particularly devastating to the Department of

Defense, this course focuses on the prevention and treatment of corrosion of

weapon systems, support equipment, and the associated infrastructure. Enjoy the

course. Remember, you can refer to the glossary at any time.

CCoorrrroossiioonn RReevviieeww

Remember, corrosion is defined as the deterioration of a material or its

properties as it reacts with the environment. Remember also, that the rate at

which corrosion attacks a material is directly proportional to the corrosivity of

the environment, to which the material is exposed, and is inversely proportional

to the corrosion resistance of the material being exposed.

Of course, the ideal solution is to prevent corrosion but corrosion can also be

managed. You can manage corrosion if you understand causes and effects and

implement effective decision-making.

The content of Course 201-C is directed at both of these conditions and addresses

corrosion mitigation.

CCoorrrroossiioonn CCeellll RReevviieeww

A corrosion cell, which consists of: an

anode, a cathode, an ionic current path in an

electrolyte, and an electronic path in a

conductor between the anode and cathode must

be present for corrosion to occur. As shown

in the corrosion cell animation, removing any

one of these 4 elements stops corrosion.

201C Ch1 Introduction to Corrosion Control

Section 1: Introduction

201C Ch.1: Introduction to Corrosion Control

Section 2: Managers Roles

CCoorrrroossiioonn MMaannaaggeerrss

Corrosion managers are decision makers at every management level who have a stake

in and responsibility for preventing and controlling the onset of corrosion,

mitigating the destructive effects of corrosion, assessing the impact of

corrosion technology alternatives and associated trade-offs, and implementing

effective management tools and controls to achieve corrosion prevention and

control objectives.

CCoorrrroossiioonn MMaannaaggeerrss:: CCoonnssiiddeerraattiioonnss

Corrosion managers need to consider:

1. The importance of corrosion prevention and control;

2. Those systems, products, or facilities that need to be protected and how to

provide that protection;

3. The types of vulnerable materials and to what environments they are exposed;

4. The risk associated with potential corrosion and the methods for managing that

risk; and

5. The development and application of strategic and operational plans for

corrosion prevention and control.

CCoorrrroossiioonn DDeecciissiioonn PPooiinnttss

Corrosion managers need to: Understand the nature and impact of corrosion and

insist on and perform adequate planning for corrosion prevention and control.

Oversee or perform legitimate corrosion affordability acquisition and support trade-offs

between cost, performance and availability before committing to initial designs or

design changes, materials utilized, manufacturing methods, or maintenance

processes.

If corrosion managers dont make effective decisions:

Critical research and development in corrosion technology, product design,

materials selection, manufacturing processes, and maintenance methods will be

overlooked.

Up-front investment in corrosion prevention will not be implemented, thus denying

leverage of significant downstream life-cycle savings in corrosion maintenance

costs.

Systems, equipment, and facilities will continue to suffer from reduced

performance capability, lower availability rates, and increased vulnerability to

unsafe conditions


Section 2: Managers Roles

CCoorrrroossiioonn DDeessiiggnn:: RRiisskkss

It is critical to consider corrosion risks early in the design of systems, equipment or

facilities, to balance the performance aspects of the design with the corrosion

prevention aspects of the design, also, to balance the downstream corrosion

maintenance approach with the availability requirements of the end product, and

to minimize product total life cycle costs by trading off up-front corrosion

prevention investment with projected long-term corrosion operations and

maintenance costs.

CCoorrrroossiioonn DDeessiiggnn:: PPoooorr PPllaannnniinngg

This montage shows the result of poor design for both up-front corrosion

prevention and downstream corrosion maintenance on:

military ground vehicles, munitions in storage and inventory, and equipment in

harsh marine environments, etc.

CCoorrrroossiioonn DDeessiiggnn:: SSyysstteemmss EEnnggiinneeeerriinngg

The most cost-effective design approach is to design corrosion resistance into a

system, a piece of equipment, or a facility.

This should be part of the systems engineering process. Design approach is

similar to other risk-related activities such as survivability, manufacturing,

and reliability.

The corrosion engineer needs to identify risks during the earliest conceptual

stages and develops designs, strategies and mitigation practices to address

corrosion and prolong the life of the product.

The corrosion engineer must have access and be involved with the early stages.


Section 3: Corrosion Design

CCoorrrroossiioonn DDeessiiggnn:: PPeerrffoorrmmaannccee RReeqquuiirreemmeennttss

Make corrosion control a performance requirement based on, mission objectives and

system performance requirements such as required materials properties which

include:

Physical properties such as shape and size; mechanical properties such as

strength and durability; electrical properties based on function and thermal

properties based on environment.

An additional performance requirement is the corrosion risk affecting the

mission. Finally, approaches to preventing and/or mitigating corrosion risk must

be considered when establishing system performance requirements.

CCoorrrroossiioonn DDeessiiggnn:: EEccoonnoommiiccss

Use economic analysis early in the design phase to support risk prevention and

mitigation approach as shown in these two diagrams.



A typical bathtub curve shows product-operating costs over time. It shows the

Infant mortality period in which early failure costs decrease as performance

stabilizes as well as Normal life period with somewhat constant failure costs

and finally the End-of-life period in which failure costs increase as product

wears out.

Maintenance costs can be reduced during the normal life period by effective

corrosion mitigation and control planning.

Corrosion mitigation can have a significant effect on the product life. Some

small investment late in normal life may even extend the useful life.

CCoorrrroossiioonn DDeessiiggnn:: EEaarrllyy CCoonnssiiddeerraattiioonn

Another reason for considering corrosion control at the early design stages is

shown here.

Acquisition program managers tend to consider performance risks almost

exclusively early in the design phase, at the expense of support risks. The cost

to resolve an acquisition design problem goes up by an order of magnitude each

time you proceed to the next acquisition phase.

Increasing up-front investment to address corrosion risks early in the

acquisition cycle will reduce high downstream costs to resolve the issues and

reduce mitigation costs during system, equipment or facility operation.



The bottom line is up-front investment in corrosion prevention and mitigation

leverages large downstream savings in design and operational costs.

CCoorrrroossiioonn DDeessiiggnn:: EEnnttiirree SSyysstteemm

Taking a systems approach to design for corrosion prevention and mitigation means

analyzing the corrosion risks within the entire system and examining the

interfaces between system components.

It is important to define how each component affects the potential for corrosion

in other components and to determine how electrochemical corrosion cells can form

and assess the potential impact of corrosion on the reliability, availability and

overall performance of the entire system.

CCoorrrroossiioonn DDeessiiggnn:: DDeessiiggnn SSyysstteemmss

Some corrosion design shortfalls and pitfalls that might be experienced during

the design phase are explained here by two DoD subject matter experts.

Theres a lot of geometric features as well that you would like to design against

from a corrosion standpoint. You dont want things that are going to, you dont

want geometry features that are going to trap water or allow water to not drain

properly, because then if you have some sort of coatings defects then the

corrosion will start taking place and most of the time it's gonna be at a place

that you dont want it to occur: places that are hard to see, difficult to

inspect. From a design point, we want to make sure that we take care with the

small detailed geometric features. Those are the features that can keep water

from draining properly or moisture from draining and collecting in places that

are very difficult to see from visual inspection, which is the bulk of the

inspection procedures, and in the aircraft industry, for sure. So if we can guard

against that, essentially, not letting corrosion take a foothold like keeping the

environment out of the critical areas, thats a big step right there reducing it

completely. If you pick up from that, that says really when youre gonna go ahead

and start the design youre designing for life. So, part of the life cycle

considerations are not only: can you select materials?

can you bolt them together? Its how you gonna manage them for the lifetime so

there are some things where you're gonna have hidden and inaccessible areas and

youre gonna have to have them in that condition so part of that design is to

understand that youre not only picking the materials and youre picking their

performances in the environment but that youre also gonna consider how you're

gonna inspect those things that are hard to get to. So its not just whats the

geometry? how do they fit together? how do they react to the loads?



Its once thats assembled, are you ever gonna be able to get in there again or

do you wanna interrupt that structure? So youre designing to: do you wanna have

a corrosion sensor in there?

do you want the capability where you have to have non-destructive test equipment

that you can see through structure and understand when you have damage thats

occurred and understand how to manage it?

So youre designing and selecting materials in processes for the reliability and

the life sustainability of whatever it is that youre gonna put into service. And

theres a separate point when you talk about keeping the environment out or

designing for the environment. One of the things that folks dont realize often:

they go to the handbooks for the structural information, some of the data thats

available, and dont understand that that data has been produced in a very clean

environment so its fatigue crack growth data that was generated in the

laboratory, very different from the exterior environment and you need to consider

where it's going to be, for how long its gonna be, and design around those

features.

Probably a good summary on all of that is we tend to be overly optimistic so you

anticipate a good response for selections that you believe youve chosen wisely

based on what you know.

We tend to put really perfect systems on the test and dont account for the

variability that you actually see when you have something that has been

manufactured out there on a manufacturing floor with standard handling. And so

the failure to put on test components that have seen at least some exemplar of

damage so scrubbing and scratching them, flexing them, creating some of the

mechanical damage that you know is going to have to happen or will occur in

service, and testing pristine structures instead of those that have at least have

a form of realistic damage, is a pitfall, and what happens is you get a test

result that gives you really an unwarranted warm feeling about how your design is

going to behave when its in service and thats just its disingenuous; that

leads us to poor results in the field.


If youre gonna design a system for corrosion control, one of probably the most

important things for a design engineer to do is kind of step back and recognize

that every decision that hes gonna make involves a number of trades and so it

requires the expertise of a number of folks with different backgrounds so you

probably need someone that understands materials, materials properties. Probably

one of the most helpful things Ive ever heard that would probably kick off a

design engineer, moving forward when youre talking about corrosion prevention

and control is to understand that a material its a dance with what you come

with so it brings all of its properties and all of its susceptibilities to the

design and you have to treat it holistically.



In consistent with the systems engineering approach, the materials that the

corrosion engineer wants to have in their structure and those that the structures

folks have sometimes theyre at competing ends so if on the materials science

we're on the MNP side, the engineers are picking or selecting a certain material.

They really cant wait and finish the design to give it to you to the structures

people that need to use that material and those properties for static strength

and fatigue resistance as well so it has to be in concert every step of the way.


Say one of the shortfalls or a pitfall that is very easy to fall into is to take

commercial experience and assume that its sufficient to cover military

experience so aircraft systems that are designed to work on a commercial

environment which is nowhere near the length of time that we're operating

military aircraft and assume that the data and some of the material selection

that would support that kind of application sufficient would be short of what we

have to have for military performance.

As we go and do maintenance again with these systems engineering approach, the

coatings that we put on the airplane are really the first line of defense. We try

to pick corrosion resistant materials as much as we can but when it comes to

coatings thats going to keep it, keep corrosion from occurring over these very

long life cycles that we had. Whoever would have thought that wed fly a KC-135

for 80 years. No one ever thought that and thats whats gonna happen. So the

coatings that were putting on there both initially and during maintenance, we

have to make sure that we take care of those so that we dont have degradation of

the coatings again not to let corrosion start. So, there are just new challenges

for the designer, keeping in mind that we have this very long life cycle now and

we have to think of the solution over the entire life cycle but if you take in an

integrated systems engineering approach where you have the materials people, the

designers, the structures, the maintainers - all cooperating and seeing how the

design evolves - this is definitely a solvable problem. Its within our means to

do it.

CCoorrrroossiioonn DDeessiiggnn:: CCoorrrroossiioonn MMaannaaggeemmeenntt

As has been discussed, effective

corrosion management consists of defining

and understanding the corrosion risks and

making informed decisions regarding how

to prevent or mitigate those risks.



MMaatteerriiaallss SSeelleeccttiioonn AApppprrooaacchh

This first segment covers the process of material selection that incorporates

corrosion analysis in the traditional materials selection approach. Materials

selection involves three major activities:

1. Define the requirement,

2. Select the material, including processing and production methods, and

3. Defining the corrosion management process to ensure material fitness for

operational use.

MMaatteerriiaallss SSeelleeccttiioonn:: PPrroocceessss CCoonnssiiddeerraattiioonnss

Materials selection is important, particularly for metals. The material selection

process should consider the following:

1. Expected corrosivity of the materials environment,

2. Resistance of the material to corrosion in all potential operating

environments,

3. Relative position of interfacing metals on the galvanic scale,

4. Cost versus savings regarding unique or expensive materials.

5. Strength and durability of material versus ability to resist corrosion and

6. Coatings and corrosion prevention compounds that can increase corrosion

resistance of specific materials.

MMaatteerriiaallss SSeelleeccttiioonn:: TTrraaddee OOffffss

Performing trade-offs between material alternatives is inevitable. Make the

corrosion tradeoffs knowingly, judiciously and prudently, and avoid a default

approach.

Consider the effect of a material on system performance. Consider all potential

corrosion risks associated with the materials selected in all potential operating

environments.

Consider the impact on material corrosion resistance of production methods,

including heat treatment, cold working, addition of coatings or other corrosion

prevention treatments.

Consider cost and effectiveness of corrosion maintenance alternatives during

operation: coatings, sealants, and CPCs; sensing, diagnostics and prognostics;

repair of corroded systems and replacement of corroded components.

201C Ch.2: Materials Selection and Corrosion Maintenance

Section 1: Materials Selection

MMaatteerriiaallss SSeelleeccttiioonn:: DDeeffiinniinngg tthhee RReeqquuiirreemmeennttss

You define the material requirements by: reviewing the system design and

performance specifications, determining mission requirements in terms of how and

where the mission will be accomplished, and developing corrosion design

performance specs that define all system operating environments and specific

system operation, including functions to be performed, and required and expected

system lifetime.

MMaatteerriiaallss SSeelleeccttiioonn:: SSeelleeccttiinngg tthhee MMaatteerriiaall

You select materials, processes and methods by: identifying candidate materials

from the various classes of materials including metals, alloys, polymers,

ceramics, composites, or nanomaterials by its structure or composition. We must

identify materials processes to achieve desired mechanical and corrosion-

resistant properties by using alloying processes, forming operations, and heat

treating. Selecting fabrication and assembly processes that reduce residual

stresses and retain corrosion resistant properties, including welding and

joining, other fastening methods, and assembly involving dissimilar metals is

important.



MMaatteerriiaallss SSeelleeccttiioonn:: DDeeffiinniinngg CCoorrrroossiioonn RReeqquuiirreemmeennttss

Selecting materials to prevent or control corrosion is a complex decision-making

process for the manager, designer or engineer involved.

For example, the perfect material for a corrosion resistance application might

not be acceptable because it is not compatible with operational requirements, or

the cost may be too high, and/or tradeoff analysis indicates that desired

corrosion resistance capability could reduce desired operational performance

capability. Analyses of material performance variables are complicated in regards

to mechanical, electrical and thermal properties, as well as impact of multiple

operational environments, and optimal balance of requirements, costs, and

benefits.

Corrosion requirements often receive low priority, thus materials and fabrication

and assembly methods are considered later in the design process, and subsequent

changes to system design to accommodate corrosion risk reduction are more

difficult and expensive, and often less effective.

MMaatteerriiaallss SSeelleeccttiioonn:: SSiixx--SStteepp PPrroocceessss

AMMTIAC has identified a 6-step approach to incorporate corrosion analysis into

the traditional materials selection process:



1. Screen materials based on experience cited in technical documents and

literature.

2. Conduct an environmental assessment that considers the impact of the

environment on the material to be exposed.

3. Evaluate the corrosion risks associated with a material based on the

materials potential corrosion failure modes determine what type of corrosion

to which the material is vulnerable in a particular environment.

4. Select appropriate methods to reduce and manage corrosion risks in other

words to prevent and control corrosion in these environments.

5. Assess other factors that affect the selection of the material such as cost,

availability, manufacturability, supportability, and maintainability.

And finally,

6. Select the best material and methods for corrosion prevention and control.

MMaatteerriiaallss SSeelleeccttiioonn:: SSyysstteemm aanndd PPrroocceessss DDeessiiggnnss

System design, materials selection and manufacturing process design are closely

interrelated. None can be done in isolation.

Design and performance specifications and environmental conditions dictate the

range of affordable materials and manufacturing processes that can be considered.

Materials properties, availability and cost constrain the design both physically

and functionally, and constrain the manufacturing processes that can be used.

Manufacturing process capability, including assembly, and cost constrain the

degree to which desired material properties can be attained and to which

specifications can be met.



MMaatteerriiaallss SSeelleeccttiioonn:: KKnnoowwlleeddggee BBaassee

When selecting materials for an application, you can draw upon a significant

Knowledge Base generated from past experience. It is very likely that designers,

engineers, and other decision-makers have already addressed the requirements and

conditions that you must consider. For example, particular classes of alloys

classes are prone to specific forms of corrosion:

Carbon steel is prone to general or uniform corrosion. High strength steels are

quite susceptible to Hydrogen Embrittlement cracking.

Stainless steels are vulnerable to localized forms of corrosion such as pitting

or crevice corrosion.

Aluminum alloys can also succumb to localized forms of corrosion such as pitting,

exfoliation, or intergranular corrosion.

Nevertheless, there are some natural combinations of materials and the

environments that are compatible, like:

Aluminum alloys in non-chloride atmospheric conditions, and copper alloys in non-

oxidizing environments.

On the other hand, its imperative that you avoid incompatible combinations of

materials and the environment. For example, stainless steel exposed to hot

chloride solutions is highly susceptible to corrosion.

High strength aluminum alloys exposed to marine environments are likely to

generate significant intergranular corrosion.

Steel in industrial and marine environments is another unnatural combination

subject to corrosion.

MMaatteerriiaallss SSeelleeccttiioonn:: EEnnvviirroonnmmeenntt

Since it is unlikely that you can either choose or change the operating

environment in which materials will perform, you will first need to conduct an

environmental assessment.

This means analysis of macro environments and their characteristics such as

industrial, rural and marine atmospheric environments for humidity and rainfall,

wind, temperature, and contaminants.

Also, marine, fresh, brackish, and polluted water environments for composition,

pH level, temperature, water velocity, agitation, and biological organisms.



We need to analyze various soil environments for acidity and alkalinity (pH),

permeability, ease of penetration, conductivity, resistivity, and chemical

content.

Another important up-front assessment is the effect of local conditions on the

microenvironment. Wet/dry cycles can be significantly more aggressive than

complete moisture immersion.

Start-up and shut-down cycles can also affect corrosion resistance of systems and

their materials. When systems are shut down, the non-operational or storage

microenvironment can be different than the operational environment, and might

initiate and accelerate corrosion. Biological microbial activity in a local

environment can change composition of the chemical solution and influence

corrosion.

MMaatteerriiaallss SSeelleeccttiioonn:: EEnnvviirroonnmmeenntt AAsssseessssmmeenntt

This slide illustrates a logical progression of environmental assessment steps in

order to identify the corrosivity of the macro- and micro- environments.



It identifies the main corrosivity constituents including concentration levels.

It also identifies minor constituents, such as impurities including

concentration levels characterizes the ambient and work-cycle conditions

temperature, humidity, pressure, and acidity or alkalinity. It determines the

aeration level, which is the degree of oxygen concentration, identifies fluid

velocities and agitation levels, and characterizes cyclic and residual material

stresses. Extend this assessment through the expected life cycle of the system or

structure being assessed.

MMaatteerriiaallss SSeelleeccttiioonn:: RRiisskk EExxppoossuurree IInnddeexx

You have now established a foundation for assessing corrosion risks associated

with the use of specific materials. Corrosion risk can be defined in terms of a

risk exposure index, where exposure is a combination of probability of corrosion

occurrence, and consequence of corrosion. Risk probability and corrosion

consequence are based on evaluation of potential failure modes associated with

the materials selected, and the expected operating environment. The risk exposure

index is usually quantified by:

assigning an ordinal scale value to probability of corrosion (for example, 1

being low; 2 being moderate; 3 being high)

assigning an ordinal scale value to consequence of corrosion and multiplying the

ordinal scale values and/or depicting on a matrix as shown here.

Managers can use the risk exposure index to narrow down the choices of materials,

and to determine if risk mitigation methods are needed.

MMaatteerriiaallss SSeelleeccttiioonn:: EEvvaalluuaattiioonn MMeetthhoodd



After you identified risks and level of risk exposure, evaluate methods of

protecting the selected materials from corrosion risks by:

maintenance and monitoring, use of inhibitors, changing the potential, cathodic

protection, coatings and sealants, sheltering and dehumidification.

Re-evaluate the risk exposure associated with each potential corrosion prevention

and control method.

MMaatteerriiaallss SSeelleeccttiioonn:: SSuummmmaarryy

In summary, material selection decision-making needs to be performed during the

design phase; is an iterative process based on the interrelationship between

design, materials selection and manufacturing processes; must consider materials

behavior based on experience and assessment in a given environment; and must

ensure cost effective operational systems where corrosion management has been

made an integral part of the design process.



MMaaiinntteennaannccee PPllaann

Ideally, we have been able to select materials and manufacturing methods that

prevent corrosion from initiating during system or facility operation. However,

this cannot always be achieved.

Thus, effective maintenance and condition monitoring capability must be planned

for and designed into the system before it is placed into operation.

SSyysstteemm aanndd PPrroocceessss DDeessiiggnn

During the design and material selection stages, it is crucial that decision-

makers at all levels: first, consider and plan for corrosion maintenance

requirements and methods during early system development; Revisit those

requirements and methods throughout systems engineering and production; Next,

include requirements for preventative and other routine maintenance during system

operation; Then specify processes, equipment and facilities for major maintenance

activities such as repair, replacement and rehabilitation and finally plan for

decommissioning, recycling and disposal.


Section 2: Corrosion Control Maintenance

MMaaiinntteennaannccee CCoonnttrrooll:: AAiirrccrraafftt

Aircraft systems are some of the systems most vulnerable to the effects of

corrosion. Decision-makers must consider the following when planning for aircraft

corrosion control:

Selection of effective protective materials and processes during design and

manufacturing - for example, coatings need to be properly applied and maintained.

Finishing treatments during material processing must follow prescribed published

standards. Specifying operational-phase maintenance methods for retaining

corrosion protection - for example - washing and cleaning materials and

processes, recoating materials and processes, application of corrosion

inhibitors, such as corrosion preventative compounds and lubricants.

Reviewing aircraft design configuration for accessibility to corrosion prone

areas, adequate drainage, avoidance of designs that promote formation of

corrosion cells.

Corrosion inspection requirements, such as specific inspection methods and

processes, required inspection equipment, specified inspection intervals, and

finally, corrective actions for specific deficiencies.

MMaaiinntteennaannccee CCoonnttrrooll:: SShhiippss aanndd SSuubbmmaarriinneess

Ships and submarines are exposed to harsh marine environments, in which they may

be immersed periodically or for the life of the craft.

Decision-makers must consider the following when planning for ship and submarine

corrosion control: Extended periods of immersion in a highly conductive chloride

solution require selection of materials that endure such an environment and

specific methods to reduce corrosion effects, such as, impressed current cathodic

protection and sacrificial anodes.

Extended exposure of topside surfaces, internal areas, voids, and equipment to

the corrosive atmosphere require:

1. Coatings that are properly applied and maintained

2. Designs that include provisions for drainage

3. Plans for frequent inspections of areas vulnerable to corrosion

4. Periodic washing and cleaning of vulnerable areas and

5. Application of corrosion inhibitors in hard to access areas

Tanks, which contain solutions that are inherently corrosive or can stimulate

microbial activity require:

1. Specific, long-life coatings selected for the environmental conditions



2. Periodic inspections and recoating when dictated and

3. Accessibility for inspection and recoating.

MMaaiinntteennaannccee CCoonnttrrooll:: GGrroouunndd VVeehhiicclleess

The vast number and types of ground vehicles in use are subject to almost every

type of corrosive environment on earth.

Decision-makers must consider the following when planning for ground vehicle

corrosion control:

Selection of effective, protective materials and processes during design and

manufacturing which can include:

Coating systems that need to be properly selected and applied; Designs that must

prevent moisture and dust accumulation, particularly in inaccessible areas.

Designs also must avoid configurations that can initiate formation of corrosion

cells, particularly those that generate Poultice Corrosion a special form of

crevice corrosion resulting from a buildup of mud, salt and other road debris and

Manufacturing processes needed to be strictly controlled to reduce design

variances that can induce corrosion; Different environments will require a

tailored maintenance program; Periodic and as-required washing of vehicles should

be standard Periodic inspection must be planned and tailored to the environment

and type of vehicle; Protective coatings should be repaired or replaced as

dictated by inspection Corrosion preventive compounds or other corrosion

inhibitors should be applied in hard-to-access areas when appropriate.

MMaaiinntteennaannccee CCoonnttrrooll:: SSttrruuccttuurreess aanndd FFaacciilliittiieess

Structures and facilities such as bridges, buildings, large equipment, storage

tanks, pipelines, and processing equipment are prone to corrosion because they

are typically constructed of low to medium carbon steels and steel reinforced

concrete. Decision-makers must consider the following when planning for

structures and facilities corrosion control:

For effective performance and economy, structures and facilities require:

1. Proper application of effective coatings is often required to prevent or

resist corrosion;

2. Cathodic protection for steel and steel rebar;

3. Protective liners to protect pipelines and equipment that contain or process

corrosive fluids; and

4. Materials that are environmentally compatible;

Planned monitoring and maintenance during operational use is very important.

Inspect, repair and refurbish coatings early in the degradation process.



Coatings require real-time monitoring of chemical and water spillages. They

require immediate clean up and rinsing of spillages. We must monitor, assess

effectiveness, and if necessary, adjust cathodic protection. Steel, reinforced

rebar structures require a formal monitoring and assessment program provide for

cathodic protection or chloride removal processes, if needed.

MMaaiinntteennaannccee MMaannaaggeemmeenntt

When decision-makers consider corrosion maintenance strategies and requirements,

the ability to determine material condition, predict corrosion growth rates, and

determine required maintenance or repair actions depends on capabilities and

methods to monitor and inspect materials and then to interpret the results.

CCoorrrroossiioonn MMaannaaggeemmeenntt SSttrraatteeggyy

Corrosion monitoring and inspection is part of a comprehensive corrosion

management strategy that identifies and monitors corrosion, and/or takes some

corrective action or preventative action. Corrosion monitoring depends on

technologies that assess the corrosivity of a system either continuously or

periodically to detect and locate defect formation, and follow defect growth.

Corrosion inspection consists of periodic checking of the system for corrosion-

related defects.



CCoorrrroossiioonn MMoonniittoorriinngg

Corrosion monitoring uses probes, sensors and coupons to assess the nature and

growth of a potential corrosion defect and to predict the component wear-rate and

life.

Corrosion probes or sensors can monitor the chemical or electrochemical nature of

the environment, track corrosion damage, and indicate material corrosion rates.

Corrosion coupons are samples of the material being monitored. They are placed in

a similar environment, and used to measure corrosion rates of the material.

The reliability of these monitoring methods can be adversely affected by

environmental conditions.

CCoorrrroossiioonn IInnssppeeccttiioonn

There are a number of corrosion inspection methods. One of the most useful

inspection techniques is visual inspection.

Other methods include enhanced visual inspection using fiber optics and

magnification for observation in restricted areas like:

Liquid penetrant, Magnetic particles, Eddy Current Imaging Systems, Mobile

Automatic Ultrasonic Scanners, Radiography, and Thermography.



EEnnvviirroonnmmeennttaall EEffffeeccttss MMiittiiggaattiioonn

This segment addresses methods to control the corrosive environment. It

describes: strategies to control the corrosion environment, the use of inhibitors

and water treatment, the chemical composition of inhibitors, various types of

inhibitors and methods of treating boiler water.

EEnnvviirroonnmmeennttaall CCoonnttrrooll SSttrraatteeggiieess

The methods described in this segment for controlling the environment are based

on applying one or more of the following strategies alone or in combination:

1. Add chemical compounds that help reduce corrosion behavior such as: chromates,

phosphates, and molybdates, which act as corrosion inhibitors and organic

compounds that feature self-forming and self-healing surface films.

2. Remove or control elements that initiate and promote corrosion such as:

aggressive compounds like chlorides in solution, compounds that promote and

sustain cathodic reactions, such as oxygen dissolved in water, heavy metals, such

as soluble iron and copper ions that cause aluminum pitting

and finally, 3. remove the moisture, which effectively eliminates the corrosion

cell environment.

CCoorrrroossiioonn IInnhhiibbiittoorrss

One of the primary strategies is the use of corrosion inhibitors. Inhibitors are

chemicals that are applied to the surface of a material to form a protective

film, increasing the resistance to corrosion.

For example additives in coatings, primers, sealants, surface treatments and

corrosion preventive compounds (CPCs).

Inhibitors need to be dispersed though out the operating environment to interact

with its elements and reduce its corrosivity.

For example, additives in recirculating systems such as automotive radiators,

water used to wash vehicles and equipment and boiler water to adjust pH and

remove oxygen.

CCoorrrroossiioonn IInnhhiibbiittoorrss TTyyppeess

There are several types of inhibitors that use different mechanisms to reduce the

incidence and effects of corrosion. Some include:

201C Ch.3: Environmental Effects and Mitigation Section 1: Mitigating The Effects of Environment

Passivating inhibitors, cathodic inhibitors, organic inhibitors, precipitation

inhibitors, vapor-phase inhibitors and boiler and steam generator inhibitors.

IInnhhiibbiittoorr CCoommppoouunnddss

This table lists some inhibitor compounds, designed to neutralize components of

the corrosion cell.

In general, the mechanisms of inhibition accomplish one or more of the following:

Remove moisture and, therefore, the electrolyte; isolate the material from the

potentially corrosive environment; and stem reactions at the cathode and/or

anode.


PPaassssiivvaattiinngg IInnhhiibbiittoorrss

Passivating inhibitors are referred to as anodic inhibitors because they promote

passivity of the anodes in the corrosion cell. There are two classes of

passivating inhibitors.

They are: 1. oxidizing compounds such as chromates, nitrates, and nitrites; and

2. non-oxidizing compounds such as phosphates, tungstates, and molybdates.

In both cases, a critical concentration of the inhibitor must be maintained.

There must be sufficient inhibitors in solution to treat all anodes.

If some anodes are exposed, the system is vulnerable to localized corrosion.

CCaatthhooddiicc IInnhhiibbiittoorrss

Cathodic inhibitors protect corrosion cell cathodes by blocking or slowing the

reduction reaction. This also reduces or eliminates anodic corrosion reactions.

One of the three strategies are employed:

1. Block the cathodic sites where chemicals added to the solution cause an

insoluble organic precipitate to cover the cathodes; this results in the

precipitate blocking corrosion at the cathode. Or

2. Slow the cathodic reduction reaction itself by adding cathodic poisons like

solutions of arsenic, bismuth, and antimony, which slow the reduction process of

acid ions at the cathode.

Or 3. Use oxygen scavengers to remove reducible chemicals because oxygen in the

electrolyte contributes to the current flow, and removing oxygen slows reduction

reactions at the cathode.

OOrrggaanniicc IInnhhiibbiittoorrss

Organic inhibitors form films that cover the metal surface that is vulnerable to

corrosion. As shown in the illustrations, corrosion will occur at specific sites

on the metal surface.

Organic inhibitor compounds bond themselves to the surface creating a self-

forming, self-healing film that can be regenerated if damaged; and displacing

water and shielding the surface from aggressive ions.


PPrreecciippiittaattiioonn IInnhhiibbiittoorrss

Precipitation inhibitors are chemicals in the electrolyte that form a thick,

inorganic layer at the cathode. The layer is formed by precipitation of inorganic

compounds, such as phosphates and silicates, on the metal. These precipitates

tend to cover the entire surface of the metal.

They act as a barrier coating within the corrosive environment, and they react

specifically at the cathodes, blocking their reduction capability.

Precipitation inhibitor chemicals are also used as surface treatments with

primers and topcoats to promote coating adhesion and inhibit corrosion.

VVaappoorr--PPhhaassee IInnhhiibbiittoorrss

Vapor-phase inhibitors (VPIs), also known as volatile corrosion inhibitors (VCIs) are

molecules of combined organic and inorganic compounds that are vaporized for delivery

in the atmosphere and/or deposited as a film on vulnerable metal surfaces.

They are useful in enclosed spaces such as electronics cabinets, limited access

spaces that are subject to corrosion, packaging for electronic, photographic, or

computer equipment, and shrink-wrapped storage containers.

They are often encapsulated to release the inhibitors within packaging and/or

combined with a desiccant to absorb moisture. Theyre composed of chemical

compounds that can be metal or alloy specific.

SStteeaamm GGeenneerraattoorrss IInnhhiibbiittoorrss

Boilers or steam generators generate steam or heated water that circulates

through heating systems. They usually have a water pH that is neutral or slightly

acidic.

although, the best operation is with mildly alkaline boiling water. However, too

much alkalinity can initiate caustic corrosion. There are three approaches to

combating boiler corrosion:

1. Add water treatment chemicals, to change the pH into a region of passivity.

2. Reduce the electrical potential to the level where cathodic protection

provides immunity to corrosion. Finally,

3. Add oxygen scavengers such as sodium sulfate or hydrazine to the boiler waters

to remove oxygen from the solution.

However, use hydrazine with extreme caution if copper condensers are part of the

system.


CCaatthhooddiicc PPrrootteeccttiioonn

This segment describes methods for adjusting the electrical potential at the

corrosion cell electrodes to assist in corrosion control.

Cathodic Protection is a widely used electrochemical method for protecting a

structure or materials buried or immersed in soil, water, or concrete.

The objective is to reduce corrosion of the buried or submerged system often in

fresh water or seawater. The engineering principle is to induce or force a

cathodic current to flow onto the metal surface to be protected, using either

impressed current from electrical power supplies, or galvanic action associated

with sacrificial anodes.

Cathodic protection is not effective for systems in air or other environments

that resist current flow between the anode and the cathode.

CCaatthhooddiicc PPrrootteeccttiioonn SSyysstteemm

A cathodic protection system must be an electrochemical cell with the four

components shown: a cathode, an anode, an electrical path between them, and an

electrolyte connecting the anode and cathode.

This shows a steel tank buried in the soil, with general corrosion on the

unprotected steel surface. The steel surface of the tank provides the electric

path between the anodes and cathodes.

The corrosion environment is the moist soil containing the electrolyte that

conducts the ionic current. The potential difference between the anodes and

cathodes drives corrosion reactions.

Cathodic protection is provided by the anode buried near the steel tank. An

above-ground power supply is connected to the steel tank and the buried anode

with electric cables.

When power is applied, current flows from the anode, through the electrolyte in

the soil, onto the steel structure, and back through the electric cable from the

tank to the power supply.

The current forces the entire structure to be a cathode, thus eliminating the

anodes on the metal surface and removing that component of the corrosion cells on

the steel structure.

201C Ch.4: Cathodic and Anodic Protection Section 1: Cathodic Protection

IImmpprreesssseedd CCuurrrreenntt CCaatthhooddiicc PPrrootteeccttiioonn

This sketch depicts a similar impressed current cathodic protection system. In

this case, AC power is provided from commercial electric power lines.

A rectifier converts the AC power to direct current (DC), and can also control DC

current flow. The buried steel structure is a tank or pipeline vulnerable to

corrosion.

A group of five or six impressed current anodes are composed of high silicon cast

iron, graphite, iron oxides or other similar materials, and the current passes

through the electrolyte in the soil to the structure.

The impressed current forces the entire tank or pipeline to be a cathode, and

therefore completes the circuit back to the rectifier via an electric cable

welded to the buried structure. The absence of anodes in the buried structure

eliminates the corrosion cell.

GGaallvvaanniicc CCaatthhooddiicc PPrrootteeccttiioonn

Cathodic protection can also be provided using sacrificial anodes, which are

passive corrosion control devices. External power supplies are not used.


Current flows produced by galvanic action between materials. Remember that

current flows from the active metals in the galvanic series to the noble metals.

The noble metals are more positive, so they become cathodes. The active metals

are more negative and become anodes. We protect a more noble metal say steel

by connecting it in a corrosive environment to an active metal say zinc, which

makes the steel a cathode. The more positive the cathode, the more negative the

anode.

The anode and cathode must have electrical contact to complete an electrochemical

cell circuit that provides the desired protection. An electrolyte, such as

seawater, is also needed in the corrosion cell. The galvanic action between the

metals will cause a current to flow from the active metal anode, through the

electrolyte in the corrosive environment, to the nobler metal cathode, and back

to the anode. The reaction at the cathode prevents it from corroding. Anodes are

consumed by the galvanic action, hence the term sacrificial anodes.


They must be replaced periodically to maintain the cathodic protection, and they

cannot be coated because they will no longer be a component of the corrosion cell

formed to provide cathodic protection.

IImmpprreesssseedd CCuurrrreenntt VVSS.. GGaallvvaanniicc CCPP

In comparing impressed current cathodic protection systems to sacrificial anode

systems, sacrificial cathodic protection is simpler to install and operate but

must be replaced at periodic intervals. It incurs low costs for small system

installation, but requires a conductive environment.

Conversely, impressed current cathodic protection systems are more complex. They

require more inspection and maintenance, but low capital investment for large

systems. They have longer useful system life, but can cause unintended corrosion.

RReeiinnffoorrcceedd CCoonnccrreettee SStteeeell CCoorrrroossiioonn

Corrosion of steel in reinforced concrete can cause severe degradation of bridge

decks, parking garages and support columns, as shown in these photographs.

Steel rods are imbedded in concrete to strengthen the concrete. The steel rods

are normally passive under the mild alkaline condition few corrosion problems

occur in that environment.

If chloride ions from salt or marine environments permeate the concrete and reach

the steel surface, the passive film breaks down and the steel corrodes. The steel

corrosion products expand, taking up more volume than the corroded steel and


causing concrete cracking and spalling. Cathodic protection is one of the few

ways to mitigate further damage.

CCoonnccrreettee CCaatthhooddiicc PPrrootteeccttiioonn

The method for providing cathodic protection of concrete rebar is shown here. A

distributed anode consisting of expanded metal mesh is attached to the structure

frequently the mesh is covered with a concrete grout layer. A power supply is

connected to the rebar and to the metal mesh anode.

Current flows from the power supply through the anodes and through the concrete

onto the steel rebar surface, and then back to the power supply.

While this mitigates further corrosion, the process does not reverse or repair

damage that has already occurred.

IICCCCPP SSttrraayy CCuurrrreenntt

Impressed current cathodic protection depends on induced currents flowing through

the soil in direct paths from the anodes to the metal surface to be protected.

Since current will take the path of least resistance, another metal structure

near the structure to be protected can attract the impressed current. That stray

current can enter the foreign structure at holidays or exposed areas, flow along

that structure on the path of least resistance, and return through the soil to

the structure to be protected.

Where the stray current leaves the foreign structure, it goes into the

electrolyte at an anode. Severe damage in a short time can result the effect

appears as electrochemical machining.

The stray current also has a detrimental effect on the intended cathodic

protection system. Stray currents can be mitigated by bonding structures together

electrically to prevent stray current, and/or using grounding cells to drain off

the stray current.


AAnnooddiicc PPrrootteeccttiioonn TTeecchhnniiqquuee

Anodic protection techniques are only useful in limited conditions - the metal

being protected must have an accessible passive zone in which to apply anodic

currents. The objective is to shift the electrical potential of the metal surface

to the passive zone as shown on the pH diagram.

The schematic shows an example of the configuration

for anodic protection. The structure to be protected

is a steel tank filled with sulfuric acid. The power

supply is a potentiostat, which provides an electric

current to the auxiliary electrode. A reference

electrode is also attached to the power supply to

enable the potentiostat to regulate the current

level.

Current flows from the metal structure

to the auxiliary electrode, which

promotes enhanced metal structure

passivity.

AAnnooddiicc PPrrootteeccttiioonn MMeettaall--EEnnvviirroonnmmeenntt

The combinations of metals and

environments amenable to anodic protection

are quite limited. The table shown here

lists the metals and solutions that can be

anodically protected.

A major caution is also noted if the

passive film breaks down, anodic

protection currents cause severe,

accelerated corrosion attack.

201C Ch.4: Cathodic and Anodic Protection Section 2: Anodic Protection

MMeetthhooddss ooff CCoorrrroossiioonn CCoonnttrrooll:: CCooaattiinnggss

This segment addresses methods to control the corrosive environment using

protective coatings. It describes:

Engineering principles regarding the use of coatings for corrosion protection;

The types of coatings, sealants and adhesives available for long-term

protection of materials;

The protective coating planning and selection process;

The modes by which coatings provide protection from corrosion; and finally,

Protective coatings degradation and failure modes.

TTyyppeess ooff CCooaattiinnggss

Barrier coatings shield the reactive metal from the surrounding environment,

moisture and corrosive agents.

Sacrificial coatings function as sacrificial anodes, where a more active metal

corrodes to protect the more noble metal substrate. Sealants and adhesives

provide corrosion protection by preventing moisture from penetrating enclosed

areas, and acting as a barrier between the electrolyte and the active metal

surface, as well as incorporating added inhibitors for additional protection.

Metallic coatings provide enhanced corrosion resistance by acting as a barrier or

functioning as a sacrificial anode.

Organic coatings protect the metal substrate by acting as an impermeable barrier

promoting inhibition, or providing a form of cathodic protection.

Ceramic coatings are inorganic, non-metallic coatings that act as barriers and

are added to seal the protected substrate.

SSuurrffaaccee TTrreeaattmmeenntt CCooaattiinnggss

Surface treatment or surface engineering, provides metallic coatings by modifying

a materials surface to make it more corrosion resistant.

Types of surface engineering include conversion coatings and anodization, which

use chemical reaction on the metal surface to create a corrosion resistant oxide

layer or an inorganic chemical layer.

Shot peening is where round steel balls are shot against the metal surface.

Residual compressive stresses are induced to mitigate stress corrosion cracking

or corrosion fatigue.

201C Ch.5: Coatings and Other Methods of Protection

Section 1: Coatings

Laser peening, which heats the metal surface to modify its structure, aids in

chemical modification of the surface, and induces compressive residual stresses

to increase resistance to stress corrosion cracking and corrosion fatigue.

MMeettaalllliicc CCooaattiinnggss

Metallic coatings enhance corrosion resistance by providing a barrier between a

metal and its environment, or a sacrificial coating for more active metals.

Metallic coatings can be applied by: forming and bonding layers of cladding,

electroplating, spraying, hot dipping, diffusion, chemical vapor deposition, or

ion vapor deposition.

AAlluummiinnuumm

Aluminum, which can be applied by hot dipping to protect metal from atmospheric corrosion

and oxidation at high temperatures. Spraying to provide uniform, impermeable

protection.

CCaaddmmiiuumm

Cadmium, which can be applied by several methods to protect steel in moist and

marine environments; provide a smooth, conductive coating that resists fretting

and fatigue.

CChhrroommiiuumm

Chromium provides a hard chromium oxide coating on the surface.

Note that certain types of chromium are environmentally incompatible and may be

restricted in their usage.

TTiinn

Tin is a coating that provides metal substrates with corrosion resistance either

barrier or sacrificial protection of steel and copper.


Section 1: Coatings

ZZiinncc

Zinc, primarily to provide sacrificial protection through galvanizing, can be

applied by hot dipping, electro deposition, spraying.

OOrrggaanniicc CCooaattiinnggss

Organic coatings protect metal surfaces by providing an impermeable barrier

between the metal substrate being protected and the corrosive environment, and

provide sacrificial cathodic protection.

Types of organic coatings and their characteristics are shown in the table:

Paint a pigmented liquid, which converts to a solid, tough, adherent film after

application;

Oil paint contains oil or varnish to cause film formation as well as alkyd,

epoxy or vinyl resins for drying and durability;

Water paint uses a water to disperse the liquid emulsion; Enamel paint with a

very smooth surface film;

Varnish a clear combination of drying oil and resin that dries by oil oxidation

to form a solid translucent film;

Lacquer a combination of polymeric esters or ethers and plasticizers in a

solvent that dries by solvent evaporation to form a solid film;

Baking finish paint or varnish baked above 150 degrees Fahrenheit to obtain

desired properties.

MMuullttii--LLaayyeerr CCooaattiinngg SSyysstteemm

Many applications consist of multiple layers to form an organic coating system.

There are multiple coating layers, each to protect each other. For example, in an

automobile theres cleaning, a pretreatment which is mostly inorganic and the

pretreatment is meant to protect the metal substrate against corrosion.

A primer goes on top of that, after the pretreatment is dried and cured and the

primer is meant to protect the pretreatment.

A top coat goes on top of that and the top coat is meant to protect the primer.

So in other words, its a three separate coating steps all working as a system


Section 1: Coatings

SSeeaallaannttss aanndd AAddhheessiivveess

Sealants and adhesives are widely used to control corrosion by serving as a

barrier between the electrolyte and the active metal, providing adhesive control,

and sealing moisture from crevices to prevent corrosion. Corrosion preventative

compounds (CPCs) are temporary protective organic coatings used to provide short

to mid-term protection against corrosion.

They also repair areas where the primary coating has been damaged, and separate

galvanic couples when dissimilar metals are present. Common CPCs are shown here.

LLiinniinnggss aanndd CCllaaddddiinnggss

Linings and claddings, which are usually thicker than the coatings already

described, are also used for corrosion protection.

They include rubber linings, glass linings, porcelain enamel, clad metals, and

concrete and cementatious coatings and linings.

EEffffeeccttiivvee CCooaattiinngg PPeerrffoorrmmaannccee

Proper surface preparation and surface cleanliness is crucial to effective

coating performance. It is important for adhesion, which can be enhanced by

preparing a textured surface; also, coating integrity, which requires a

completely clean surface to avoid trapped dirt, debris, or organic material

between the coating and protected surface that can form voids in the coating.

Proper surface preparation also protects against the formation of corrosion cells

in voids, cracks and crevices, delaminated materials where moisture and

aggressive ions can collect, and accumulation of soluble salt particles that

attract water through osmosis can form concentrated corrosive solutions.


Section 1: Coatings

CCooaattiinngg AApppplliiccaattiioonn SSeelleeccttiioonn

Selection of a coating application process is important because application

quality is critical to avoid defects or porosity that can result in localized

corrosion.

Each application method has advantages and disadvantages that must be considered

and traded-off such as: type of coating, area to be covered, type of substrate to

be coated, and environmental restrictions.

Organic coatings can be brushed, rolled, and sprayed, while Ceramic coatings can

be applied using high temperature diffusion processes, spraying or chemical

conversion.

CCooaattiinngg AApppplliiccaattiioonn FFaaccttoorrss

Factors to be considered regarding effective coating applications are: proper

surface preparation, proper selection and application of primer, and proper

topcoat selection.

An effective protective coating plan, developed as part of the initial system

engineering effort, should address the following:

First, design and selection of the protective coating;

Second, surface preparation methods;

Third, the coating application process;

Fourth, follow-on inspection and quality assurance processes; and

Fifth, coating maintenance and refurbishment during system operation.

CCooaattiinngg DDeessiiggnn aanndd SSeelleeccttiioonn PPrroocceessss

The coating design and selection process is almost the same as the material

selection process. Determine the material coating requirement in terms of:

The vulnerability of the material to be protected, the use to which that material

will be applied and the environment in which that material will be used.

Determine the performance required by the coating in terms of:

Overcoming the materials corrosive vulnerability, adaptation to the material

shape and configuration, desired appearance of the coating, and expected coating

service life.


Section 1: Coatings

Determine how the coating will be applied to the material regarding:

method of application, labor force skills required and available, equipment

required and available, and safety requirements during and after application.

Lastly, assess life-cycle consequences of using selected coatings:

Modes and likelihood of coating failure, investment and operating costs, and

potential maintenance and repair requirements and associated methods.

Perform affordability trade-offs considering cost, coating, and labor skill

availability, and required coating performance in the expected corrosive

environments.

CCooaattiinnggss aanndd EEnnvviirroonnmmeenntt ZZoonneess

Matching coating performance to the aggressiveness of the environment is

particularly important. The table shown here describes types of exposure for

coatings in terms of zone conditions.


Section 1: Coatings

Zone zero is the least severe an interior area that is dry, not exposed to

sunlight, and has no extreme temperatures.

Zone one is normally dry interiors or exteriors.

Zone two is frequently wet by fresh water including frequent immersion, salt

water including frequent immersion, full fresh water immersion and/or full salt

water immersion.

Zone three is affected by chemical exposure that might be acidic, neutral,

alkaline; associated with mild solvents and/or associated with severe oxidizing

chemicals and strong solvents.

The coating selected should have sufficient resistance to the corrosion

environment in the zone to which the coated material will be exposed.

CCooaattiinngg PPrrootteeccttiioonn:: MMeettaalllliicc SSaaccrriiffiicciiaall CCooaattiinngg

Coatings can provide four modes of corrosion protection as a metallic

sacrificial coating, the coating is more active, and thus more negative than the

metal being protected; becomes an anode and the metal being protected becomes a

cathode; requires an electrolyte and electric path; corrodes due to galvanic

action.


Section 1: Coatings

CCooaattiinngg PPrrootteeccttiioonn:: PPaassssiivvaattiinngg CCooaattiinngg

Coatings can provide four modes of corrosion protection as a passivating

coating, the coating forms an insoluble film with blocking particles; promotes

passivation of the metal through the use of oxides and phosphates. In every case,

if there is a defect in the coating, there will be no protection adjacent to that

defect the coating only protects locations where it is intact and continuous on

the metal surface.

CCooaattiinngg PPrrootteeccttiioonn:: PPeerrmmeeaabbllee BBaarrrriieerr

Coatings can provide four modes of corrosion protection as a permeable barrier,

coating acts only as a partial barrier, allowing moisture and oxygen to penetrate

the coating; inhibits corrosion as the coating or primers below the coating

modify the corrosive environment.


Section 1: Coatings

CCooaattiinngg PPrrootteeccttiioonn:: IImmppeerrmmeeaabbllee BBaarrrriieerr

Coatings can provide four modes of

corrosion protection as an

impermeable barrier, a coating stops

the penetration of moisture,

eliminating the electrolyte in the

corrosion cell; prevents chlorides

and other aggressive chemicals from

getting to the metal surface;

increases electrical resistance

which reduces the mobility of

chemical ions.

CCooaattiinngg FFaaiilluurree MMooddeess

Coating failure modes are shown here. Loss of adhesion in coating systems can

occur at the interface between the coating and the metal substrate it is

protecting, within a layer of coating, or as a delamination of a polymer close to

the metal substrate. The loss of adhesion at the interface between the coating

and the metal substrate surface can be due to contamination from salts and oils,

loss of metal oxide, or a flaw in the coating. Degradation of the coating over

time can be due to polymer sensitivity to ultra-violet rays, wide variations in

temperature, and use of organic solvents and other chemicals.


Section 1: Coatings

MMeetthhooddss ffoorr EEnnvviirroonnmmeennttaall MMooddiiffiiccaattiioonn

Now, lets explore methods for eliminating or modifying the environment that

causes corrosion.


Section 1: Coatings

SShheelltteerriinngg

Methods for modifying the environment to reduce corrosion.

Sheltering provides a barrier between a military aircraft atmospheric

contaminants and rain. This helps to minimize the extent of corrosion on the

critical mainframe component of a sheltered aircraft or other equipment. By

sheltering a military asset, in this case a Fighter-Aircraft, the environment, a

key factor of an electrochemical corrosion cell is reduced or eliminated

completely.

SShhrriinnkk--WWrraappppiinngg

Methods for modifying the environment to reduce corrosion. Shrink-wrapping is

used to protect this USMC CH-46E "Sea Knight helicopter from atmospheric

contaminants, moisture, and therefore corrosion.

Shrink-wrapping is used during storage and transport of systems and equipment to

provide temporary protection from moisture and other weather-related factors.

This protection depends on clean, dry initial conditions achieved by pretreatment

of vulnerable material, and cleaning and drying prior to packaging.

Covers are also used to protect a variety of assets from harsh environments.

A lot of the corrosion challenges that we face here in Wheeler and on deployment

are the majority are the moisture accumulation that happens in the recessed areas

of the aircraft.

Mostly back in the F pores of the aircraft where your tail boom fitting

attachments are. A lot of the major issues come with the dust accumulation when

were on deployments along with weather - ambient moisture is there - which

creates more of a problem. The severity of the types of corrosion that you find

here is more severe due to the high salt content as for other places.

Theres more dry places out in Kansas; Texas; Fort Bragg, North Carolina. The

intent to apply the sheltering of helicopters is tried to be used as far as army-

wide as we possibly can but given the facilities and the nature of the areas that

we operate in, sometimes, it becomes difficult to do. The nose covers and covers

in general have kind of been mixed and those covers work quite well in keeping

the aircraft dry and a lot of water accumulation out as far as the nose

compartment area it goes.


Section 1: Miscellaneous Protection Methods

The use of cover as far as aircraft go in the army is a widespread practice. It

has been used by several units in different operating environments.

DDeehhuummiiddiiffiiccaattiioonn

Dehumidification is performed through air conditioning. It removes moisture from

operating components, eliminates thin films of moisture condensed on the parts,

and provides continued corrosion resistance if properly maintained.

CCoorrrroossiioonn PPrreevveennttiioonn aanndd MMiittiiggaattiioonn SSuummmmaarryy

In summary, we control or eliminate the environmental component of the

electrochemical corrosion cell by:

controlling or modifying the internal environment by using coatings, sealants,

adhesives, corrosion preventative compounds, linings and claddings or eliminating

the external environment by isolating the vulnerable material from that external

environment.


Section 1: Miscellaneous Protection Methods

Documents

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