201 b Document

201B

Characteristics and

Impact of Materials

IInnttrroodduuccttiioonn

Corrosion 201B discusses engineering fundamentals for controlling corrosion and

provides basic concepts and tools you will need to understand how corrosion

initiates and propagates, how it affects materials, and what you can do to

prevent or mitigate corrosion.

CCoorrrroossiioonn DDeecciissiioonn MMaakkeerrss

System managers, designers and engineers must make choices between design,

alternative materials, structural designs and manufacturing processes that have

an influence on corrosion. The environment in which the material is used will

affect how corrosion initiates and grows, and the effectiveness of corrosion

prevention and control methods.

CCoorrrroossiioonn CCoonnttrrooll FFuunnddaammeennttaallss

This section of Corrosion 201-B addresses engineering fundamentals for corrosion

control.

BBaassiicc CCoonncceeppttss ooff CCoorrrroossiioonn

Corrosion is the deterioration of materials metals, polymers, ceramics or

composites or their properties due to a reaction with the surrounding

environment.

201B Ch1 Fundamentals of Corrosion Control

Section 1: Introduction


Section 1: Introduction

Material strength or durability might be lost or degraded, or the damage might be

just aesthetic. The deterioration can be due to an electrochemical reaction, a

dissolution reaction, or an interaction between the material and the environment.

The environmental effects could result from the chemical composition of an

aqueous environment such as chlorides or from factors such as moisture,

Temperature, flow rate, wet/dry cycles, stresses or abrasion. These examples

indicate that corrosion occurs many places for many reasons.

It is important to understand corrosion causes and behavior, to anticipate how

and where corrosion can occur, and to take actions to avoid corrosion induced

damage and failures. Avoiding such damage and failures depend on better corrosion

management and mitigation.

CCoosstt ooff CCoorrrroossiioonn

Why is the impact of corrosion of such great concern? The costs of corrosion are

staggering estimates generated in 2000 by the Department of Transportation

placed the cost of corrosion in the United States at more than 276 billion

dollars a year, and that figure continues to rise. By 2010 studies by the DoD,

determines Military cost of corrosion alone at over 20 billion dollars.

Corrosion damage affects weapon systems and infrastructure performance,

availability and safety, all of which adversely impact the ability to accomplish

the military mission.

CCoorrrroossiioonn RRiisskk AAwwaarreenneessss

Better corrosion management and mitigation means there is an increased awareness

of corrosion risks. A basic technical understanding and effective implementation

program, with sound mitigation strategies and plans, can lead to sound corrosion

performance during a systems life cycle.

During a systems design phase, the objective is to identify and avoid risks, and

plan risk mitigation. During production and manufacturing, the objective is to

specify processes and practices that mitigate risk.

During the operational phase, the objective is to plan for and perform effective

corrosion inspection and maintenance to include root cause analysis, repair or

replacement.


Section 2: Risk-Based Corrosion Management

RRiisskk--BBaasseedd AApppprrooaacchh MMaannaaggeemmeenntt PPrroocceessss

A risk-based approach to corrosion management incorporates an analysis of these

items:

Identify risks

Identify barriers to those risks

Determine allowable corrosion

Develop corrosion management plan for acceptable performance

Implement, validate and monitor the plan

RRiisskk--BBaasseedd AApppprrooaacchh CCoorrrroossiioonn CCoonnttrrooll

Some aspects of a risk-based approach are described here by a subject matter

expert:

To develop a risk-based approach to corrosion prevention and control,

first, you need to define what facilities and/or equipment youre trying to

protect. Secondly, once you have that defined. What are these things made of?

What materials? Is it ceramics? Is it polymers? Are there metals? What kind of

metals? There are numerous materials that can be used. Once you know the

materials that are being used and the materials you have to protect. Then, youve

got to define the environment to which they are exposed: that can be submerged;

it can be in the atmosphere. It can be severe. It can be mild. If its a

submerged situation - it could be turbulent conditions or it could be very calm

conditions. Each of those impacts the corrosion rates.

Finally, and a very important question, once you understand the operating

environments, you also need to understand the other environments impacting

corrosion. For instance, storage environments - a lot of times we have equipment

or systems and subsystems that are stored for a large portion of their life

before theyre used so when that happens, you need to understand the impact of

those storage conditions on that equipment.



Once you develop that first set of questions, then you need to ask a second set

of questions. First, what are the fundamental principles that we can employ to

gain corrosion protection? Once we understand the fundamentals of the science-

related or engineering -related fundamentals, we then need to look at the

concepts that we would use to protect those assets. We would also need to look at

the type of corrosion that were concerned with. For instance, if you had pitting

corrosion and if you have a situation where a system was very prone to crack

initiation - the pits would be very critical. On the other hand, if you had

uniform corrosion and it just took away the thickness of a material and we were

worried about crack growth rate, it would be uniform corrosion that we would

worried about crack growth rate, it would be uniform corrosion that we would

worry about.

So once we define the types of corrosion then we go to the corrosion

mitigation methods and practices we might use to protect those and those could be

anything from sheltering to corrosion inhibitors to coatings. Theres an array of

various types of mitigation, prevention and mitigation procedures that can be

used and they might be deployed at different points in the life-cycle of that

system. Thus, we have to decide how were gonna actually deploy or implement

those particular methods.

RRiisskk--BBaasseedd AApppprrooaacchh EEffffeeccttiivvee UUssaaggee

To effectively use the risk-based approach and answer the decisive corrosion

questions, you need to understand the corrosion behavior of metals, and the

characteristics and conditions of the corrosion environments.



MMeettaall EEnnvviirroonnmmeenntt BBeehhaavviioorr

A specific material in a specific environment determines corrosion behavior. For

example, steel is active in a hydrochloric acid solution and corrodes rapidly,

while it is passive in a mildly alkaline environment and the corrosion rate is

very low. Similarly, aluminum is passive in neutral environments and active in

acidic or alkaline solutions. Stainless steels are passive in many environments,

but in certain aggressive environments, passivity breaks down and localized

corrosion can cause serious damage. Likewise many nonmetallic materials will

degrade in their operating environment. An example of this would be fiberglass

composites, which deteriorate when exposed to severe UV environments, while they

remain stable if protected.

TThhrreeee BBeehhaavviioorrss ooff MMeettaallss

Environmental and Metallurgical Effects on Materials. One of three things will

happen when you put metal into an environment in this case a liquid.

If the metal is noble and is immune to the environment, there is no reaction and

no corrosion as shown on the left. In the center the metal reacts with the

environment so there is corrosion. The degradation process results in the

formation of soluble products, such as iron atoms on the metal surface going into

solution as ferrous ions. The behavior on the right is passivity, where the metal

reacts and corrosion products form on the exposed surface, but these corrosion

products provide an insoluble oxide film that can protect the metal from further

corrosion. So depending on the metal and the environment, behavior can be noble,

active, or passive.

201B Ch1 Fundamentals of Corrosion Control Section 3: Environmental and Metallurgical Effects

on Materials

NNoobbllee aanndd PPaassssiivvee BBeehhaavviioorrss

Both noble and passive behaviors can be advantageous when developing solutions to

corrosion problems, but both have practical liabilities. Noble behavior is

functionally ideal, but noble metals are expensive and have limited strength and

ductility. Passive behavior is also advantageous as long as the metal stays

passive. If an unanticipated breakdown of the passive film occurs, pitting,

stress corrosion or crevice corrosion may result with catastrophic failures.

AAccttiivvee BBeehhaavviioorr

Active behavior results in corrosion, but a well conceived strategy provides the

foundation for effective management of active corrosion behavior. Corrosion can

be managed with improved designs and manufacturing processes, and with built-in

corrosion allowances. But corrosion management must be applied early in the

design stage. Specific corrosion management strategies include coating

susceptible materials to isolate them from the corrosive environment, application

of cathodic protection, or inducing immune or passive behavior.

IImmppoorrttaanntt CChhaarraacctteerriissttiiccss ooff MMeettaallss

Metallurgical characteristics of metals are important to describe their physical

composition and structure. These metallurgical characteristics are shown here.


on Materials

Inherent reactivity is a dynamic characteristic for which there is a wide range

of behavior highly noble metals such as gold, platinum, and iridium feature

very low inherent reactivity while very active metals such as magnesium feature

high reactivity with the environment.

Another important dynamic characteristic is the ability to become passive by

self-forming an insoluble protective surface layer.

MMeettaall IInnhheerreenntt RReeaaccttiivviittyy

Inherent reactivity of each metal can be ranked according to the electromotive

force series shown in the table. The metals at the top of the electromotive force

series are the least reactive while those with the lowest electromotive force are

the most reactive. Titanium and aluminum are extremely reactive and easily go

into solution. But they also can behave passively in some environments, often

protecting them from corrosion even though they are highly reactive metals.


on Materials

CChhaarraacctteerriissttiiccss ooff SSoolluuttiioonnss

Solutions are characterized by their pH, oxidizing potential, conductivity,

ionization potential and solubility. These properties can be measured and the

effect on metals determined quantitatively, enabling engineers to predict the

likelihood and type of corrosion for combinations of metals and solutions in

specific environments.

ppHH SSccaallee

Here we show how pH units define the acidity or alkalinity, where a pH from 1 to

7 reflects decreasing acidity, a pH of 7 indicates a neutral condition, and a pH

from 7 to 14 reflects increasing alkalinity.

OOxxiiddaattiioonn PPootteennttiiaall vvss.. ppHH

This graph combines oxidation potential with pH, where each quadrant of the graph

characterizes the particular environment in terms of oxidation or reduction

potential and relative acidity or alkalinity.


on Materials

OOxxiiddaattiioonn PPootteennttiiaall aanndd ppHH ooff EEnnvviirroonnmmeennttaall SSoolluuttiioonnss

The previous graph is shown here reflecting the combined oxidation potential and

pH zones of selected environmental solutions.


on Materials

This enables designers and engineers to relate the observed behavior of a test

solution to a common environment, and to predict its behavior in terms of that

common environment.

MMeettaall''ss CCoorrrroossiioonn TTeennddeennccyy vvss.. EEnnvviirroonnmmeennttaall OOxxiiddaattiioonn PPoowweerr

The relationship between a metals tendency to corrode and the oxidizing power of

the environment is important in designing systems to resist corrosion.

On the left the graph shows that the

tendency to corrode is highest for

the active metals at the bottom of

the scale and decreases as the metal

becomes more noble. On the right the

graph show tendency to oxidize is

highest at the top of the scale and

decreases as the oxidizing power

decreases.

For corrosion to occur, the oxidizing power of the environment has to be greater

than the metals tendency to corrode. So desecrated acid could corrode iron, and

would definitely corrode magnesium, but would not corrode copper or gold.

However, aerated acid would corrode copper, iron, and magnesium. And to corrode

gold, highly concentrated nitric acid or the equivalent would be required.


on Materials

MMeettaall BBeehhaavviioorrss aanndd tthhee PPoouurrbbaaiixx DDiiaaggrraamm

When the three metallic behaviors are mapped to the two important environmental

characteristics pH and oxidizing potential the result is shown here.

Iron is immune to corrosion if there

is substantial reduction potential

for any pH. And its passivity

increases with a higher oxidation

potential in alkaline and some mild

acid solutions. It is active only in

acidic solutions where the oxidation

potential varies from high to quite

low. These maps are of considerable

value to designers and engineers

attempting to reduce the impact of

corrosion on structures and

equipment.

EEnnvviirroonnmmeenntt aanndd PPoouurrbbaaiixx DDiiaaggrraamm OOvveerrllaayy

This overlay permits you to predict a metals behavior if it was introduced to

the environment in each of the boxes in the left graph. So iron would corrode in

a nitric acid solution, could be passive in a bicarbonate hydroxide environment,

and might be immune in sodium hydroxide if there was sufficient reduction

potential.


on Materials

PPoouurrbbaaiixx DDiiaaggrraamm:: IIrroonn vvss.. GGoolldd

This graph compares the behavior maps of iron and gold. Gold is immune over

almost the entire diagram, but if exposed to highly oxidizing environments, it

could corrode.

PPoouurrbbaaiixx DDiiaaggrraamm:: IIrroonn vvss.. AAlluummiinnuumm

This graph compares the behavior maps of iron and aluminum. This reveals that

aluminum is almost never immune, and is very active in strong acid and alkaline

environments, but can be passive in near neutral pH solutions.


on Materials

PPoouurrbbaaiixx DDiiaaggrraamm:: IIrroonn vvss.. TTiittaanniiuumm

This diagram compares the behavior maps of iron and titanium. Like aluminum,

titanium is almost never immune, but has a much larger passive zone, which covers

all but the high oxidation potential, high acid zone and the high reduction

potential high alkaline zone.

IIrroonn CCoorrrroossiioonn CCoonnttrrooll

This graph provides information that facilitates development of design strategies

to prevent or reduce corrosion.

If iron will be used in the

environment found at the plus

sign in the graph, the iron

would likely corrode. If the

environment could be changed to

be quite alkaline, such as at

point A in the diagram, the

iron would be in a passive zone

and could be self-protected

from corrosion. Or, if the iron

is coupled to a more active

metal like magnesium or zinc,

the reduction potential would


on Materials

be increased such as shown at point B and the iron would be immune a process

called cathodic protection. Alternatively, the iron could be forced into the high

oxidation potential zone where the iron displays passive behavior, such as at

point C, by coupling it to a less active metal a process called anodic

protection.

Other alternatives are to add a passivating inhibitor to the environment, which

would move the iron to a point such as D; or to substitute an alloy for the iron,

which would shift the behavior to a different zone such as at point E. These

strategies can be widely applied to other corrosion control methods and

techniques.


on Materials

201B Ch2 Electrochemistry of Corrosion Section 1: Electrochemical Reactions

CCoorrrroossiioonn CCeellll RReevviieeww

All corrosion results from the operation of a corrosion cell so it is essential

that you understand a corrosion cells components, behavior, and operation. This

segment will discuss and distinguish between chemical and electrochemical

reactions; identify corrosion cell components and explain the rules by which they

operate; use a water pumping system as an analogy to describe electrochemical

system types and rates of flow; and explore some useful concepts and tools to

analyze corrosion cells.

EElleeccttrroocchheemmiiccaall RReeaaccttiioonn PPrroocceessss

Electrochemical reactions are distinguished from other chemical reactions by the

generation or consumption of electrons during the reaction process. The process

of generating electrons is called oxidation, and the process of consuming

electrons is called reduction. Electrochemical cells, where oxidation and

reduction take place, are the primary mechanisms found in batteries,

electroplating, fuel cells and corrosion cells. Each of these electrochemical

cells features an anode, where an oxidation reaction occurs and a cathode, where

a reduction reaction occurs. The first three examples are productive

electrochemical cells the fourth is destructive.

CChheemmiiccaall RReeaaccttiioonn TTyyppeess

You can relate chemical reactions to events with which you might be familiar.

Dissolving sugar in your coffee is a chemical reaction. Likewise, salt added to

your soup dissolves the sodium chloride and then disassociates, which is another

chemical reaction that forms separate sodium and chloride ions. And, if you

should mix two solid chemicals, silver nitrate and sodium chloride in sequence

with water.

First, adding silver nitrate, dissolves and

disassociates into silver and nitrate ions; then

adding sodium chloride, which dissolves and

disassociates into sodium and chloride ions; the

result is four ions in solution.

The silver and chloride ions then react and

precipitate into solid silver chloride. Likewise,

the sodium and nitrate ions react and form sodium

nitrate.

EElleeccttrroocchheemmiiccaall RReeaaccttiioonn TTyyppeess

You might also relate electrochemical reactions to familiar events. If you leave

unpainted iron items outdoors, the iron oxidizes and forms rust in this classic

corrosion reaction. If you place an iron sample in hydrochloric acid, it

dissolves into ferrous ions another oxidation reaction resulting in corrosion.

Aluminum exposed to a non-marine atmosphere reacts with the atmosphere, but forms

a protective oxide on the surface an example of passivity.

However, if you place that aluminum in a marine atmosphere, or in a strong acid

or alkaline environment, the resulting corrosion causes the aluminum to dissolve

into a non-protective film which continues to spall.

CChheemmiiccaall RReeaaccttiioonn MMeecchhaanniissmm

During reaction between chemical elements, valence refers to the number of

electrons in an atom that can be gained or lost. In the chemical reaction

example, where silver nitrate and sodium chloride react in water, silver and

sodium each have a valence of (+1) before and after the reaction, and chlorine

and nitrogen each have a valence of (-1) before and after the reaction. Here, the

chemical reaction features no oxidation or reduction, and no electron generation

or consumption occurs.


EElleeccttrroocchheemmiiccaall RReeaaccttiioonn MMeecchhaanniissmm

On the other hand, in the electrochemical reaction example of iron and

hydrochloric acid, oxidation and reduction take place at anodes and cathodes that

form on and move about the surface of the iron.

The hydrochloric acid dissociates into hydrogen and chloride ions, and the iron

oxidizes (corrodes) at the anodes and goes into solution as a ferrous ion by

losing two ferrous electrons an electrochemical dissolution oxidation reaction.

Meanwhile, at the cathodes, hydrogen ions in solution migrate to the metal

surface and react with the free electrons to become hydrogen atoms a process

called hydrogen adsorption. Pairs of these absorbed hydrogen atoms join to form

hydrogen gas molecules a hydrogen evolution reaction.

Throughout the electrochemical reaction process, electrons flow through the metal

surface from anodes to cathodes where the electrons are consumed by the hydrogen

ions. The electrochemical cell can be described as consisting of an oxidation

half-cell and a reduction half-cell, which when combined, form the full

electrochemical corrosion cell.

CCoorrrroossiioonn HHaallff--CCeellll BBeehhaavviioorrss

A corrosion cell is composed of two half-cells, an anode and a cathode. The

behavior of each half-cell can be examined independently for effects of

temperature, pressure, chemical composition, and oxidizing potential.


When the half-cells are coupled together to make an electrochemical cell, the

more positive half-cell will be the cathode and the more negative half-cell will

be the anode. The potential difference between the anode and the cathode drives

the amount of direct current that flows through the metal circuit from cathode to

anode, and by ionic transport through the solution back to the cathode.

The magnitude of that current is controlled by the sum of resistances throughout

the cell. The current density at the anode and cathode can differ and depends on

the size of the exposed surface area of each.

CCoorrrroossiioonn aanndd CCuurrrreenntt

Corrosion occurs at the anode, which is where the material weight loss occurs due

to the oxidation reaction generating free electrons. There is no corrosion and no

weight loss at the cathode, but the reduction reaction must consume the electrons

generated at and flowing from the anode.

The cathodic current emanating from the cathode and flowing through the solution

must equal the current flowing from the anode. Since the electrochemical reaction

rate is inversely proportional to the current magnitude, you can reduce corrosion

by increasing the circuit resistance to reduce current flow.


EElleeccttrrooddee PPootteennttiiaall aanndd CCoorrrroossiioonn CCeellllss

The more positive corrosion half-cell forms a cathode and the more negative half-

cell forms an anode.

As mentioned above, the magnitude of the potential difference between the anode

and cathode is the driving force behind the oxidation and reduction reactions, so

the more positive the cathode in relation to the anode, the greater the driving

force.

On a scale of relative potential, shown on the left, noble metals such as gold

are the more positive and the least likely to corrode, and active metals such as

magnesium are the least positive and most likely to corrode.

The potential at the cathode determines the oxidizing power of the chemical

environment in other words how readily that environment can cause oxidation and

corrode a metal.

This scale indicates that the higher chemical concentration increases the

oxidation power, and adding air or oxygen to the chemical solution also increases

oxidation.


EElleeccttrriicc CCuurrrreenntt,, VVoollttaaggee,, aanndd RReessiissttaannccee

An electric circuit can be compared to a water flow circuit to help understand

the relationship between electrical voltage, current and resistance.

The water pump takes in water at low pressure and ejects it at high pressure. The

battery takes in a charge at low voltage and ejects it at high voltage. Both the

water pump and the battery do work on the input to produce energy. The available

water energy per unit of volume is pressure, where energy is expressed in joules

and volume is expressed in cubic meters. The available electrical energy per unit

of charge is expressed in volts, where energy is expressed in joules and charge

is expressed in coulombs. Water flow rate is volume of water per second, and

electric flow rate, called current, is coulombs per second, called amperes. The

severe constriction in the water circuit causes a significant pressure drop. The

resistor in the electric circuit causes a significant voltage drop.

The water flow rate that results from resistance to flow is equal to the change

in pressure over the length of the resistance divided by the resistance.

The electric flow rate (current) that results from the resistor equals the drop

in voltage across the resistor divided by the resistance.


OOhhmm''ss LLaaww

The important rule illustrated in the comparison of water and electric circuits

is Ohms law, which states that the change in energy (E) in an electrical

circuit, in volts, is equal to the current (I) in the circuit, in amperes,

multiplied by the resistance (R) of the circuit, in ohms. The formula E = I x R

can be used to determine any one variable if the other two are known.

Since current is equivalent to the corrosion rate, the corrosion reaction can be

slowed by lowering the change in E and/or increasing R.

MMaaggnniittuuddee ooff RReessiissttaannccee

Resistance must be calculated for a specific material based on its resistivity

multiplied by its length and divided by its cross-sectional area.


Resistivity is measured in ohms centimeter. Conductivity of a material is the

reciprocal of resistivity, so a high resistivity material has low conductivity

and vice versa.

VVaalluueess ooff RReessiissttaannccee

This table compares the resistivity of metals, fluids, soils and other materials.

Some interesting comparisons show that silver has significantly less resistivity

than steel; seawater is very conductive, while distilled water has high

resistivity because of few conductive ions; and dry soil has higher resistivity

than moist soil.


FFaarraaddaayy''ss LLaaww

Oxidation and reduction reactions in the corrosion cell result from electrolysis,

the passage of an electric current through a dissolved ionic substance causing

chemical reactions at the electrodes and separation of materials. Faraday

developed two laws concerning the material loss at the electrodes during

electrolysis.

The first law states that the mass of the material lost at the anode is directly

proportional to the electrical charge at the anode. The second law states that

the mass of the material altered at the anode is directly proportional to the

materials weight. Thus, you can calculate the quantity of electricity required

to corrode a specific amount of metal, or the amount of metal that could be

corroded using a specified current for a given time period.

CCuurrrreenntt DDeennssiittyy

Current density is the amount of current per unit area at any specific location

in a corrosion cell. Total current is the current density over a specific area

multiplied by that area.

The total anodic current in the corrosion cell must equal the total cathodic

current. That means the current density at the anode multiplied by the area of

the anode must equal the current density at the cathode multiplied by the area of

the cathode. So, if the corrosion cell contains a very large cathode and a very


small anode, the current density on the cathode is much smaller than the current

density on the anode because the total current has to be equal. This is a key

concept in corrosion design.

CCoonnsseerrvvaattiioonn ooff CChhaarrggee

The condition where total anodic current must equal total cathodic current

illustrates the principle of conservation of charge. The total electrons

generated at the anode must equal the total electrons consumed at the cathode.

The anode and cathode potential will self-adjust until a steady-state balance is

achieved. If you need to move away from the steady state to reduce corrosion, an

external current must be applied a technique used in a number of corrosive

environments.


CCoorrrroossiioonn CCeellll:: GGaallvvaanniicc CCoorrrroossiioonn

The galvanic corrosion cell will be used to provide a more in-depth explanation

of the corrosion cell, by examining a practical application.

In this example of a bronze fitting

connecting two buried sections of steel

pipe, the components of a galvanic

corrosion cell will be identified and the

rules of corrosion cell operation will be

applied.

Note that corrosion damage is occurring

where the current is leaving the steel

pipes surface and flowing onto the

bronze fitting.

CCoorrrroossiioonn CCeellll:: GGaallvvaanniicc CCoorrrroossiioonn CCoommppoonneennttss

When the bronze fitting and steel pipes are connected, they are in electrical

contact, and an electric circuit is completed through the ionic path in the moist

soil. So the four requirements of a corrosion cell are present one metal is the

cathode, the other is the anode, the connection between the two metals provides

the metallic current path, and the moist soil contains an electrolyte solution

that provides an ionic path.

201B Ch2 Electrochemistry of Corrosion Section 2: Galvanic Series

Remember that the more positive half-cell will be the cathode, and that current

flows from anode into the moist soil solution and onto the cathode surface.

CCoorrrroossiioonn CCeellll:: GGaallvvaanniicc CCoorrrroossiioonn MMeecchhaanniissmm

The bronze fitting becomes the cathode because its potential is one-half volt

more positive than the steel pipe (-0.2 volts compared to -0.7 volts). The more

active steel pipe corrodes, since the oxidation reaction of iron in the steel

alloy forms soluble ferrous ions and loses two electrons. At the same time,

oxygen reduction occurs in the neutral soil, where oxygen consumes free electrons

and combines with water in the soil to form hydroxyl ions. Later, some ferrous

ions will combine with hydroxyl ions to form iron oxide (rust).

Throughout these reactions, the ferrous ions in the soil flow in an ionic current

between the steel anode and the bronze cathode positive ions from anode to

cathode, negative ions from cathode to anode. The free electrons flow through the

metal surface from the steel anode to the bronze cathode.

GGaallvvaanniicc CCoorrrroossiioonn RRaattee

The galvanic corrosion rate is controlled by potential difference and the

resistance throughout the corrosion cell electric circuit. In this case, the

driving force is one-half volt.


The resistance of the soil is a key factor since soil resistivity varies widely

as shown in the table. The lower the resistivity; the higher the corrosivity.

Soil with a high salt content is very corrosive as is wet sand. Dry sand is only

slightly corrosive. The outside surface area of the steel pipe anode (outside

circumference multiplied by pipe length) can be important, as can the surface

area of the bronze fitting cathode. In this case the cathode area is much smaller

than the anode, so current density at the anode will not affect corrosion.

Polarization at the steel surface, which will be covered later, is neglected

here.

RReessiissttiivviittyy ooff EEnnvviirroonnmmeenntt

The effect on the galvanic cell of soil, water or moisture in the atmosphere is

its resistance to current flow.

The so-called ohmic drop (IR drop),

which is equivalent to the potential

energy drop (E) as shown in Ohms

Law, is caused by the current flow

through the resistive soil. The

higher the soils resistivity, the

greater the ohmic drop for a given

current flow. The effect is to

reduce corrosivity in the galvanic


cell since corrosivity is directly proportional to the current flow rate; and

as resistance increases, current flow decreases.

On the other hand, seawater is highly

conductive, and its very low resistivity

increases the current flow rate and the

galvanic cell corrosivity. To counter

this effect, off-shore pipeline

corrosion is reduced by placing

sacrificial zinc anodes in the galvanic

circuit, where the more active zinc

corrodes rather than the less active

pipeline material. These sacrificial

anodes can be spaced more than 200

meters apart because of seawaters high

conductivity.

A thin layer of moisture (electrolyte) on a

metal surface in the atmosphere provides

high resistance, even if that electrolyte

includes chlorides. As a result, corrosion

tends to be localized where the anode meets

the cathode. For example, a stainless steel

fastener inserted in steel plate in moist

atmospheric conditions would corrode only in

the contact area because the thin layer of

electrolyte resists current flow away from

the contact point.

CCaatthhooddee AAnnooddee RReellaattiivvee RRaattiioo aanndd CCuurrrreenntt DDeennssiittyy

The relative area of a cathode compared to an anode can have a significant effect

on galvanic corrosion. Remember, total corrosion current at the anode must equal

total corrosion current at the cathode. However, the current densities in amperes

per square centimeter will vary with the difference in electrode areas. If the

cathode is large compared to the anode, the current density at the anode will be

high, and that concentrated current will significantly increase corrosion.


Returning to the bronze fitting connecting the buried steel pipes, the pipes form

the anode, but the surface area is much larger than the cathode so the current

density is very small.

However, if you decide to coat the steel pipe to provide added protection from

the corrosive environment, and coating defects or later damages occur, you have

created a very small exposed anode area compared to the larger bronze cathode,

and the current densities at these defects are much higher. It would be better to

coat just the bronze fitting to remove the galvanic action.

BBeenneeffiicciiaall GGaallvvaanniicc CCoorrrroossiioonn

Another alternative to protect the

steel anode is to use beneficial

galvanic action. Since zinc is even

more active than steel, you can bury

a zinc bar in the soil and connect it

with a wire cable to the steel pipe.

The 0.3-volt potential drop from the

zinc to the steel causes current to

flow from the zinc through the steel

pipe to the bronze fitting, causing

the zinc anode to corrode, which

cathodically protects the steel.

GGaallvvaanniizziinngg CCaatthhooddiicc PPrrootteeccttiioonn

Galvanizing automotive body panels provides cathodic protection of these panels

from deicing salts, marine environments and corrosive materials embedded in soil

and debris deposits. This zinc-coated metal not only corrodes slowly, but

provides galvanic protection to the underlying steel. The application of organic

and inorganic coatings over the zinc coating provides more protection.

There are also cathodic protection schemes on the market that claim anodes

installed around automobile wheel wells, and connected to the electrical system,

provide impressed current cathodic protection like that used on naval vessels.

However, this scheme lacks one important element of the electrochemical corrosion

cell the electrolyte and its ionic path in each wheel well to complete the

electric circuit. Any thin layers of moisture or other electrolyte would be


insufficient to carry an ionic current from the anodes to the body panel

cathodes.

SSeeccoonndd LLaaww ooff TThheerrmmooddyynnaammiiccss

Rules of how systems behave are based on the second law of thermodynamics - these

rules explain causes of system reactions, systems behavior, equilibrium and

spontaneous reaction, and the effect of this behavior on corrosion.

FFllooww ooff EEnneerrggyy

Heat always flows from a hot object to a cold object until both objects reach the

same temperature. Higher pressure outside a balloon causes the balloon to shrink

until the pressure inside the balloon equals the outside pressure. A higher

electric energy charge will flow to a lower energy point in a circuit until the

electrical energy is the same throughout the circuit.

DDrriivviinngg FFoorrccee ooff SSyysstteemmss

The driving force behind these reactions is potential difference, and that

driving force continues until there is equilibrium throughout the system. The

flow always goes in one direction from a higher energy state to the lowest energy

state until equilibrium is reached. The systems stay at equilibrium until some

external force induces a potential difference, after which they again seek

equilibrium. Thermodynamic laws that govern their behavior and determine what

constitutes the point of equilibrium; how far the system is from the equilibrium

point; and how physical phenomena such as temperature, pressure, chemical

composition, and electrochemical potential change the point of equilibrium. These

laws provide tools for determining and calculating how to adjust systems for our

benefit or how to slow or stop undesirable reactions.

SSyysstteemm RRaattee BBeehhaavviioorrss

Systems react to these laws at different rates. Linear or constant rates feature

the same amount of change every time period like depositing the same amount of

money in your savings account every month.

201B Ch3Corrosion Rates and Passivity Section 1: Understanding Corrosion Rates

Increasing or exponential rates reflect an increasing increment in value every

time period like the sum of your savings account deposit and the interest it

accrues each month.

Decreasing or negative exponential rates show a decreasing increment in value

every time period like the remaining balance after withdrawing half of your

savings account every month.


Each of these behaviors can be seen in corrosion cell reactions depending on the

combination of material and environmental composition and conditions.

CCoorrrroossiioonn EExxaammppllee:: ZZiinncc iinn HHyyddrroocchhlloorriicc AAcciidd

If a zinc rod is immersed in hydrochloric acid, oxidation occurs at the anode

where two electrons are freed, zinc is consumed, and positive zinc ions are

generated. At the cathode surface, reduction occurs where free electrons are

consumed along with hydrogen ions, and hydrogen gas is generated from the

hydrogen ions and free electrons.


The rule is that the number of electrons consumed at the cathode must equal the

number of electrons freed at the anode.

MMiixxeedd PPootteennttiiaall TThheeoorryy

The reaction rate of zinc going to zinc ions is

a function of anode surface potential.

The diagram shows a plot of surface potential

on the Y-axis and the reaction rate (current)

on the X-axis for the anodic reaction. The

oxidation reaction of zinc producing two

positive zinc ions and two free electrons is

linear. If you know the zinc surface potential

on the Y-axis and the reaction rate (or

current) on the X-axis for the anodic reaction,

you can determine the current flow rate from

this graph.

The middle diagram shows a plot of surface

potential on the Y-axis and the reaction rate

(current) on the X axis for the cathodic

reaction. The reduction reaction of hydrogen

consuming two positive zinc ions and two free

electrons is also linear.

Since anodic current must equal cathodic

current according to the principle of

conservation of charge, superimposing the

zinc oxidation curve on the hydrogen

reduction curve shows that the anodic current

is equal to the cathodic current at the point

where two curves intersect. That point

reveals the corrosion current and the

corrosion potential values for a zinc rod in

hydrochloric acid under naturally corroding

conditions.


PPoollaarriizzaattiioonn

Polarization in a corrosion cell is the result of an electric current inducing a

potential difference that drives the circuit out of equilibrium. There are three

types: activation controlled polarization; concentration polarization, and ohmic

polarization.

Activation controlled polarization occurs very near an electrode surface due to

reaction resistance, which must be overcome by electrical activation energy.

Concentration polarization can occur at the anode and cathode surfaces, and

results from the exchange of reactants in the electrolyte and the electrode

material to form diffusion layers that provide high circuit resistance. Ohmic

polarization is caused by the much higher resistance in the ionic path than the

electric path in the corrosion circuit, which causes a large potential drop in

the electric path. It is important to be able to quantify the resistance caused

by polarization in order to determine the current available for the driving force

potential.

AAccttiivvaattiioonn CCoonnttrroolllleedd PPoollaarriizzaattiioonn

During the process of hydrogen adsorption, discussed in the first segment,

hydrogen ions disassociate from the electrolyte and combine with free electrons


at the cathode surface to form hydrogen atoms. Pairs of hydrogen atoms then

combine to form hydrogen gas molecules. However, these reduction reactions

require activation energy, which must be sufficient to overcome the energy

barrier imposed by equilibrium conditions.

The graph depicts the barriers that must be

overcome during the reduction reactions. The

energy level of an adsorbed hydrogen atom is

X, and the energy level of a positive

hydrogen ion in solution is Y. The activation

barrier is the total energy required for a

reaction at the peak of the curve less the

energy level of the atom or ion. So, the

activation energy (Ea) required to combine a

hydrogen ion with an electron is the

difference between Ea and Y, and the

activation energy required to remove an

electron from a hydrogen atom is the

difference between Ea and X. As the curve

shows, the reduction reaction requires less

activation energy (H). For hydrogen reactions

where electrons are consumed, the more

negative the cathode potential, the faster

the reaction. The rate of change of potential

versus current is exponential as shown by the

Nernst equation.

However, when current is plotted using a logarithm scale, the relation between

current and potential difference appears linear rather than exponential. The

higher the activation barrier; the slower the reaction rate. And the height of

the activation barrier differs between metals. The curve shows that zinc has a

higher activation barrier than platinum, so platinum, though more expensive, is

used in fuel cells and other electrochemical systems where fast reactions are

essential.

CCoorrrroossiioonn RRaattee:: AAnnooddiicc aanndd CCaatthhooddiicc RRaatteess

Changing the reduction reaction affects the corrosion rate. If you shift the

reduction curve to the right, the intersection point of the oxidation and

reduction curves shifts, and the new current and electrical potential is

indicated on the horizontal and vertical axes.


AAccttiivvaattiioonn aanndd OOhhmmiicc PPoollaarriizzaattiioonn

Ohmic polarization and activation polarization can combine to increase the

potential drop across a galvanic corrosion cell.

The copper electrode in this diagram is the cathode because it is more positive,

and the zinc electrode is the anode. The ohmic polarization caused by the

resistances throughout the circuit and the activation polarization caused by the

reaction resistance at the electrodes cause potential drops that are additive.


This combined potential difference determines the current flow through the

corrosion cell circuit.

OOhhmmiicc PPoollaarriizzaattiioonn

Ohmic polarization affects the corrosion rate due to the resistance in both the

electronic and ionic paths in the electric circuit.

The graph shows current on the horizontal axis and energy levels on the vertical

axis at three levels of resistance (R3, R2 and R1) for the lower anodic curve

(starting at Ea) and the higher cathodic curve (starting at Ec). R1 represents

the resistance of seawater, R2 the resistance of tap water and R3 the resistance

of distilled water. The two curves intersect where the resistance is lowest, the

conductivity highest, and the current the greatest. At points of greater

resistance, the curves diverge due to the ohmic resistance effects, and the

current is reduced. Since current flow is directly proportional to corrosion

rate, this curve explains why corrosion occurs more readily in seawater.

AAccttiivvaattiioonn PPoollaarriizzaattiioonn

As activation polarization progresses and more hydrogen ions combine with free

electrons, the cathodic surface becomes more negative and current flows more

rapidly. Activation polarization ceases once all the hydrogen ions are able to


react with free electrons, and a limiting current flow rate is reached. The

limiting current determines the diffusion rate of hydrogen ions, and as the

limiting current (iL) increases, the diffusion rate increases and the

concentration of hydrogen ions increases.

While this progression of concentration polarization is useful for certain

electrochemical processes, such as electroplating, it also promotes corrosion. As

the limiting current increases, the increasing diffusion rate also increases

corrosion.

CCoorrrroossiioonn RRaattee:: CCaatthhooddiicc RReeaaccttiioonn RRaattee

The cathodic reaction rate of a galvanic corrosion cell can influence the rate of

corrosion. For example, when a zinc bar is placed in hydrochloric acid, the

faster the positive hydrogen ions combine with free electrons to form hydrogen

atoms, the faster the corrosion rate.


The curves here show that as current density increases at any given surface

potential (E), expressed as the electrode potential variance from the standard

hydrogen electrode (V vs. SHE), the corrosion rate increases. Reducing the

cathodic reaction rate reduces the surface potential, the anodic reaction rate,

the current density and the corrosion rate.


PPaassssiivviittyy

Passivity is the phenomenon by which the surface of certain reactive metals

oxidize, and form a thin, stable, dense reaction layer that isolates the metal

substrate from the environment, and protects it from corrosion. That corrosion

resistance depends upon the durability and stability of the passive film and its

ability to reform if damaged. If the film can reform rapidly with little or no

metal loss, then corrosion resistance continues. But, if it fails to quickly

repassivate where the damage occurred, the metal substrate in those areas can

suffer from localized corrosion such as pitting, crevice corrosion, or stress

corrosion cracking. The remainder of the substrate remains protected, but the

local corrosion can propagate.

EEnnvviirroonnmmeenntt aanndd 33 MMeettaall BBeehhaavviioorrss

The surrounding environment can affect a metal in one of three ways. If the metal

is immune to the environment, it has no effect. If the metal is active, the

environment can cause corrosion. If the metal is passive, the environment can aid

in forming a protective film. Stainless steel can be used in hot water systems,

boilers and other steam generating systems by chemically treating the water to

facilitate the formation and durability of passive films. Aluminum alloys are

widely used in non-marine atmospheric environments because the aluminum develops

a passive film. Other corrosion resistant alloys (CRES) are composed of stainless

steel, nickel, chrome, iron, and other metals.

PPaassssiivvee MMeettaall CCoorrrroossiioonn RRaattee

The corrosion rates of passive metals can be extremely low, less than one micron

(a millionth of an inch) per year. It would take 16,000 years to penetrate metal

201B Ch3Corrosion Rates and Passivity Section 2: Understanding Passivity

the thickness of a quarter, hundreds of thousands of years to penetrate a stack

of ten quarters.

PPaassssiivvee FFiillmm DDuurraabbiilliittyy

Passive film durability is crucial. If the film breaks down and does not

repassivate after chemical, electrochemical or mechanical damage, significant

corrosion damage can result. In the laboratory experiment, a diamond-studded

scribe scrapes the passive film from a rotating electrode. The effect of the

damage is measured by the change in current flow rate. The current rate is high

during the scribing process, but almost immediately returns to a low, passive

current rate when the scribing ceases. This cycle repeats each time the metal is

scribed, showing that the metal alloy electrode remains corrosion resistant since

the passive film reforms after each instance of mechanical damage. This ability

to repassivate depends on the metal's behavior and the corrosivity of the

environment.

PPaassssiivvee FFiillmm BBrreeaakkddoowwnn SSuusscceeppttiibbiilliittyy

The susceptibility of different alloys to the breakdown of a passive film due to

crevice corrosion can be illustrated by plotting laboratory tests using the

electrochemical polarization curves. The oxidizing power (potential) of the

environment is plotted on the vertical axis, and the current density is plotted

on the horizontal scale. The upper curve depicts the behavior of a corrosion

resistant alloy composed of chromium, nickel and molybdenum.

The initial behavior is activation

polarization, and after reaching an

anodic peak, (Emax) the passive

corrosion area forms between 0 and 0.8

volt potential. Beyond that, the

behavior becomes transpassive, and even

if the potential is reversed, the

passive film readily forms. This means

that a damaged passive film will reform

in this potential range.


The curve for the less corrosion

resistant alloy follows the same

pattern up to the transpassive range,

but when the potential is reversed, a

large hysteresis loop forms. This

indicates that within the whole range

of passivity, there are regions where

the alloy could corrode very rapidly

if the passive film breaks down or is

damaged.

AAccttiivvee aanndd PPaassssiivvee CCoorrrroossiioonn BBeehhaavviioorrss

Active corrosion behavior can be compared to a passive corrosion behavior using

the electrochemical polarization curves featured in the previous segment.

For both active and passive metals, the variance of oxidizing power (potential)

from a reference electrode is plotted from positive to negative on the vertical

axis, and reaction rate (current) is plotted on the horizontal axis. As the

oxidizing reaction increases, the active metal reacts more quickly and corrosion

rate increases exponentially.

On the other hand, after initial active behavior, the passive metal reaches a

critical range of potential, and the corrosion rate decreases dramatically as the

metal enters the passive region. If higher oxidation conditions occur, a

transpassive range is reached where passive corrosion behavior is encountered.

The curve shows orders of magnitude lower corrosion rates in that potential

range.


OOxxiiddiizziinngg aanndd RReedduucciinngg RReeggiioonnss

In these diagrams, the oxidizing region is the positive region; the reducing

region is the negative region; the acidic region is pH less than 7 and the

alkaline region is pH greater than 7. The passive region for iron is the broad

hatched region in the upper right which is an oxidizing, mildly alkaline

environment. This is the environment you want to maintain if you want iron to be

passive. For aluminum to be passive, you want a neutral environment very little

to no acid or alkali. Though titanium is a very reactive material, there is a

huge passive region. But you must avoid highly acidic, highly reducing

conditions.

PPaassssiivviittyy:: WWaatteerr TTrreeaattmmeenntt

If steel is used in mildly reducing, mildly acidic water, general corrosion could

be a problem. To avoid this problem, adding phosphates or other chemicals to make

the water more alkaline changes the environment to the passive region. But if too

much alkali is added, the steel is subject to stress corrosion cracking a

condition that led to catastrophic boiler explosions in the past.


PPaassssiivviittyy:: LLooccaalliizzeedd CCoorrrroossiioonn

Localized corrosion poses the greatest danger to passive materials. The pictures

here show samples of pitting corrosion, crevice corrosion and stress corrosion

cracking. Each of these forms of corrosion is localized to the areas where

crevices, cracks or pits can occur. The result is a mostly pristine surface,

which is penetrated in small areas by corrosion that can eventually lead to

catastrophic failure.

PPaassssiivviittyy:: DDeessiiggnn CCaauuttiioonn

Use caution when designing a system that depends on passivity to resist

corrosion. If the passive film fails, localized corrosion can cause pitting,

stress corrosion or crevice corrosion that can result in unanticipated failures.

These can impact safety, health and the environment as well as economic factors

such as downtime and replacement costs. Passive film failure can result from

chlorides or other aggressive chemicals, electrochemical processes such as

oxidation and diffusion, or mechanical damage that causes cracks, crevices or

stagnant conditions.

PPaassssiivviittyy aanndd EEnnvviirroonnmmeennttaall CCoonnddiittiioonnss

It is important to recognize environmental conditions when using passivity for

corrosion resistance.

Acid, alkaline or oxidizing environments can all pose risks with the use of

specific metals. Strong acids or alkalis do not work with aluminum. Nitric acid

can be used with titanium, but be wary of titanium with hydrochloric acid. The

important point is to recognize that analytic tools exist, and while you may not

have access to all the analytic methods or know how to use them, you can find

competent engineers and analysts who can provide the answers needed to mitigate

these risks.


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201 b Document