Classification of Corrosion Protection Methods

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lassification of corrosion protectionmethods

[German version]

Active corrosion protection

Passive corrosion protection

Permanent corrosion protection

Temporary corrosion protection

Active corrosion protection

The aim of active corrosion protection is to influence the reactions which proceed during corrosion, it beingpossible to control not only the package contents and the corrosive agent but also the reaction itself in such

a manner that corrosion is avoided. Examples of such an approach are the development of corrosion

resistant alloys and the addition of inhibitors to the aggressive medium.

!ack to beginning

Passive corrosion protection

"n passive corrosion protection, damage is prevented by mechanically isolating the package contents from

the aggressive corrosive agents, for example by using protective layers, films or other coatings. #owever,

this type of corrosion protection changes neither the general ability of the package contents to corrode, nor

the aggressiveness of the corrosive agent and this is why this approach is known as passive corrosion

protection. "f the protective layer, film etc. is destroyed at any point, corrosion may occur within a very short

time.

!ack to beginning

Permanent corrosion protection

The purpose of permanent corrosion protection methods is mainly to provide protection at the place of use.

The stresses presented by climatic, biotic and chemical factors are relatively slight in this situation. $achines

are located, for example, in factory sheds and are thus protected from extreme variations in temperature,

which are fre%uently the cause of condensation. Examples of passive corrosion protection methods are&

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Tin plating

Galvani'ation

(oating

Enameling

(opper plating

!ack to beginning

Temporary corrosion protection

The stresses occurring during transport, handling and storage are much greater than those occurring at the

place of use. )uch stresses may be manifested, for example, as extreme variations in temperature, which

result in a risk of condensation. Especially in maritime transport, the elevated salt content of the water and

air in socalled seasalt aerosols may cause damage, as salts have a strongly corrosionpromoting action. The

following are the main temporary corrosion protection methods&

1. Protective coating method

2. Desiccant method

3. VCI method

!ack to beginning

1. Protective coating method

The protective coating method is a passive corrosion protection method. The protective coating isolates the

metallic surfaces from the aggressive media, such as moisture, salts, acids etc..

The following corrosion protection agents are used&

)olventbased anticorrosion agents

*ery high %uality protective films are obtained.

+nce the anticorrosion agent has been applied, the solvent must vapori'e so that the

necessary protective film is formed.

epending upon the nature of the solvent and film thickness, this drying process may take


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as long as several hours. The thicker the film, the longer the drying time. "f the drying

process is artificially accelerated, there may be problems with adhesion between the

protective film and the metal surface.

)ince protective films are very thin and soft, attention must always be paid to the dropping

point as there is a risk at elevated temperatures that the protective film will run off,

especially from vertical surfaces.

)ince solventbased corrosion protection agents are often highly flammable, they may only

be used in closed systems for reasons of occupational safety.

-aterbased anticorrosion agents

-aterbased anticorrosion agents contain no solvents and thus do not re%uire closed

systems.

rying times are shorter than for solventbased anticorrosion agents.

ue to their elevated water content, waterbased anticorrosion agents are highlytemperaturedependent risk of free'ing or increased viscosity/.

The advantage of this method is that the protective film is readily removed, but the

elevated water content, which may increase relative humidity in packaging areas, is

disadvantageous.

(orrosionprotective oils without solvent

(orrosionprotective oils without solvent produce only poor %uality protective films. Good

%uality protection is achieved by adding inhibitors. )ince these corrosionprotective oils are

fre%uently high %uality lubricating oils, they are primarily used for providing corrosion

protection in closed systems engines etc./.

ipping waxes

The protective layer is applied by dipping the item to be packaged into hot wax. epending

upon the type of wax, the temperature may have to be in excess of 0112(. 3emoval of the

protective film is relatively simple as no solid bond is formed between the wax and metal

surface. )ince application of dipping waxes is relatively complex, its use is limited to a few

isolated applications.

!ack to beginning

2. Desiccant method

Introduction

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4ccording to "5 66 789, the purpose of using desiccants is as follows& :desiccant bags are intended to

protect the package contents from humidity during transport and storage in order to prevent corrosion, mold

growth and the like.:

The desiccant bags contain desiccants which absorb water vapor, are insoluble in water and are chemically

inert, such as silica gel, aluminum silicate, alumina, blue gel, bentonite, molecular sieves etc.. ue to the

absorbency of the desiccants, humidity in the atmosphere of the package may be reduced, so eliminatingthe risk of corrosion. )ince absorbency is finite, this method is only possible if the package contents are

enclosed in a heat sealed barrier layer which is impermeable to water vapor. This is known as a climate

controlled or sealed package. "f the barrier layer is not impermeable to water vapor, further water vapor

may enter from outside such that the desiccant bags are relatively %uickly saturated, without the relative

humidity in the package being reduced.

esiccants are commercially available in desiccant units. 4ccording to "5 66 789&

:4 desiccant unit is the %uantity of desiccant which, at e%uilibrium with air at ;9 < ;2(, adsorbs the

following %uantities of water vapor&

min. 9.1 g at ;1= relative humidity

min. >.1 g at 71= relative humidity

The number of desiccant units is a measure of the adsorption capacity of the desiccant bag.:

esiccants are supplied in bags of 0?>, 0?9, 0?;, 0, ;, 7, @, 0>, 9; or @1 units. They are available in low

dusting and dusttight forms. The latter are used if the package contents have particular re%uirements in

this respect.

Calculation of reuired num!er of desiccant units

The number of desiccant units re%uired is determined by the volume of the package, the actual and desired

relative humidity within the package, the water content of any hygroscopic packaging aids, the nature of the

barrier film water vapor permeability/.

Aormula for calculating the number of desiccant units in a package "5 66 787/&

n " #1$a% & #V & ! ' m & c ' A & e & (VP & t%

n number of desiccant units

a

%uantity of water absorbable per desiccant unit in accordance with the maximum admissible

humidity in the package&

admissible final

humidity;1= 71= 61= >1=

factor a 9 > 8 @

e correction factor, relative to admissible final humidity in =&

admissible final ;1= 71= 61= >1=



humidity

factor e 1.B 1.8 1.>6 1.>

* internal volume of the package in m9

b absolute humidity of enclosed air in g?m9

m mass of hygroscopic packaging aids in kg

c factor for the moisture content of hygroscopic packaging aids in g?kg

4 surface area of barrier film in m;

-*Cwater vapor permeability of barrier film under anticipated climatic conditions in g?m;d, measured

to "5 69 0;;, Ct. 0 or Ct. ; d D day/

t total duration of transport in days

The following sample calculation reveals the location of the greatest potential risks&

4 German manufacturer is to export a packaging machine to a customer in !ra'il. The machine is packaged

in wooden box of the following dimensions&

engthinternal & 8.11 m

-idthinternal & ;.86 m

#eightinternal & 9.11 m

This gives an internal volume #V% of& 8.11 m F ;.86 F 9.11 m D 68.86 m9.

The area #A% of the barrier layer is calculated on the basis of the area of the internal sides of the box&

; F 8.11 m F ;.86 m/ D [email protected] m;

; F 8.11 m F 9.11 m/ D 7;.11 m;

; F ;.86 m F 9.11 m/ D 0>.61 m;

Total D B8.11 m;

The packaging machine is secured by bracing with > pieces of s%uared pinewood lumber. These are located

inside the climatecontrolled package. The lumber is air dry, its water content is 06= Dfactor for

moisture content of hygroscopic pac)aging aid #c% D 061 g?kg.

The dimensions of the s%uared lumber are ;.81 m F 1.;1 m F 1.;1 m F-F#/. 4t an approximate density

of pinewood of 611 kg?m9, the mass #m% is as follo*s+

> F ;.81 m F 1.;1 m F 1.;1 m D 1.>7@ m 9

1.>7@ m9 F 611 kg?m9 D 9;7 kg of lumber



The following assumptions were also made&

4dmissible final humidity was stated at 71=. #a% thus D > g and #e% D 1.8

4n aluminum composite film is used as the barrier layer, the *ater vapor permea!ility #(VP%of which is1.0 g?m;d.

The absolute humidity of the enclosed air #!% is 09.@ g?m9 at ;12( and @1= relative humidity

(orrosion protection should last for a total of 011 days #d%.

-hen these values are inserted in the e%uation, the following result is obtained&

n D 0?> g F [68.86 m9 F 09.@ g?m9/ H 9;7 kg F 061 g?kg/ H B8 m; F 1.8 F 1.0 g?m;d F 011 d/]

n D 0?> g F 8B>.B6 g H 7@>11.11 g H >8B.11 g/

n D 0?> g F 61186.B6 g

n D @97> desiccant units

The calculation demonstrates that a total %uantity of 61186.B6 g of water vapor is present inside the

climatecontrolled package or diffuses through the barrier layer. 4 total of @97> desiccant units would have

to be placed in the box to absorb this %uantity of water vapor, which is not a practical proposition. Taking a

closer look at the details of the calculation reveals the greatest potential risks&

* F b D 8B>.B6 g D the water vapor present in the enclosed

air

m F c D ,--,./, g D the water vapor bound in the hygroscopic packaging aids

4 F e F -*C F t D >8B.11 g D the water vapor which diffuses through the barrier layer

over the entire period of protection

"t is clear from the above that the hygroscopic packaging aids in the climatecontrolled package are

responsible for the greatest potential risk, so it would be advisable to locate these outside the barrier layer.

#owever, any screws, bolts or nails which pass through the barrier layer must then be appropriately sealed.

The re%uired %uantity of desiccant would conse%uently change as follows.

n D 0?> g F 8B>.B6 g H >8B.11 g /

n D ;7> desiccant units

This number of desiccant units may easily be accommodated in the box in %uestion.

-hen calculating the re%uired number of desiccant units to "5 66 787, care must be taken to assume that

all the water present in the climatecontrolled package must be absorbed by the desiccant. "t is accordingly



assumed that, as in the present example, the s%uared lumber will dry to a water content of 1=. "n reality,

however, this is not the case as at a relative humidity of 71= agreed admissible final humidity/, the water

content of pinewood is still approx. @= and this water is not released from the lumber. #owever, this fact is

not taken into account in the calculation, which means that the calculated number of desiccant units is

actually too high. +n the basis of the above example, this would make the following difference&

drying to a water content of 1=& 061 g?kg F 9;7 kg D 7@>11 g of water

drying to a water content of @=& @1 g?kg F 9;7 kg D ;6B;1 g of water

7@>11 g ;6B;1 g D ;;>@1 g of water are released on drying from 0@= to @=.

The re%uired number of desiccant units may then be calculated as follows&

n D 0?> g F 8B>.B6 g H ;;>@1.11 g H >8B.11 g/

n D 0?> g F ;7066.B6 g

n D 71;> desiccant units

4s a result, the number of desiccant units re%uired would fall by 79;1 units. The %uantity of desiccant units

is nevertheless still so large that they are virtually impossible to accommodate. The fact still remains that

the hygroscopic packaging aids remain the greatest potential risk in the climatecontrolled package.

0arrier films

!arrier films are available in various forms, for example as a polyethylene film or as a composite films with

two outer polyethylene layers and an aluminum core. The composite film performs far better with regard to

water vapor permeability -*C/, achieving -*C values of below 1.0 g?m;d/. "n the composite film, the

barrier layers are arranged so as to bring about a considerable reduction in permeability in comparison with

a single layer.

"n accordance with current "5 standards, water vapor permeability is always stated for both ;12( and

712(. 4ccording to information from the manufacturer, it may be concluded that water vapor permeability

rises with increasing temperature and falls with increasing thickness. This problem occurs most particularly

with polyethylene films, while aluminum composite films are largely insensitive to rises in temperature.

Placement of desiccant !ags

The desiccants should be suspended from strings in the upper part of the climatecontrolled package to

ensure good air circulation around them.

"t is essential to avoid direct contact between the desiccant bag and the package contents as the moist

desiccant would promote corrosion.

"t is advisable to use numerous small bags rather than fewer large ones, as this increases the available

surface area of the desiccant and so improves adsorption of the water.

"n order to ensure the longest possible duration of protection, the barrier film must be heat sealed

immediately once the desiccant bags have been inserted.



esiccant bags are always supplied in certain basic package si'es which, depending upon the desiccant unit

si'e, may contain a single bag of @1 units/ or up to 011 bags of 0?> unit/. The basic outer package should

only be opened directly before removal of a bag and must immediately be heat sealed again.

Comparison of advantages and disadvantages of the desiccant method

Advantages

esiccants provide excellent corrosion protection to both metallic and nonmetallic items

3emoval of the desiccant on delivery to the receiver is straightforward, unlike the removal

of protective films in the protective coating method. The package contents are immediately

available.

5o particular occupational hygiene re%uirements apply as the desiccant is nonha'ardous.

Disadvantages

Clacement of the desiccant bags and heat sealing of the barrier films are relatively labor

intensive.

The slightest damage to the barrier layer may negate the effectiveness of corrosion

protection.

(alculating the re%uired number of desiccant units is not entirely simple and it is easy to

overcalculate. #owever, too much protection is better than too little.

#umidity indicators inside the package are not very reliable as they are only valid for

certain temperature ranges.

!ack to beginning

3. VCI #Volatile Corrosion Inhi!itor% method

ode of action and use

"nhibitors are substances capable of inhibiting or suppressing chemical reactions. They may be considered

the opposite to catalysts, which enable or accelerate certain reactions.

Inlike the protective coating method, the *(" method is an active corrosion protection method, as chemical

corrosion processes are actively influenced by inhibitors.

"n simple terms, the mode of action see Aigure 0/ is as follows& due to its evaporation properties, the *("

substance applied onto paper, cardboard, film or foam supports or in a powder, spray or oil formulation/

passes relatively continuously into the gas phase and is deposited as a film onto the item to be protected

metal surfaces/. This change of state proceeds largely independently of ordinary temperatures or humidity

levels. "ts attraction to metal surfaces is stronger than that of water molecules, resulting in the formation of





a continuous protective layer between the metal surface and the surrounding atmosphere which means that

the water vapor in the atmosphere is kept away from the metal surface, so preventing any corrosion. *("

molecules are, however, also capable of passing through preexisting films of water on metal surfaces, so

displacing water from the surface. The presence of the *(" inhibits the electrochemical processes which

result in corrosion, suppressing either the anodic or cathodic halfreactions. Inder certain circumstances,

the period of action may extend to two years.

Aigure 0& $ode of action of *("

The mode of action dictates how *(" materials are used. 4t item to be protected is, for example, wrapped in

*(" paper. The metallic surfaces of the item should be as clean as possible to ensure the effectiveness of the

method. The *(" material should be no further than 91 cm away from the item to be protected.4pproximately 71 g of active substances should be allowed per 0 mJ of air volume. "t is advisable to secure

this volume in such a manner that the gas is not continuously removed from the package due to air

movement. This can be achieved by ensuring that the container is as well sealed as possible, but airtight

heat sealing, as in the desiccant method, is not re%uired.

The *(" method is primarily used for articles made from carbon steel, stainless steel, cast iron, galvani'ed

steel, nickel, chromium, aluminium and copper. The protective action provided and compatibility issues must

be checked with the manufacturer.

5.!.& The use of watermiscible, watermixed and waterimmiscible corrosion protection agents, corrosion

protection greases and waxes, volatile corrosion inhibitors *("/ and materials from which volatile corrosion

inhibitors may be released e.g. *(" paper, *(" films, *(" foam, *(" powder, *(" packaging, *(" oils/ is

governed by the German Technical 3egulations for #a'ardous )ubstances, T3G) >06 :3estrictions on theuse of corrosion protection agents which may give rise to 5nitrosamines during use:.

Comparison of advantages and disadvantages of the VCI method

Advantages

)ince the gas also penetrates holes and cavities, these areas also receive ade%uate



protection.

The period of action may extend to two years.

The wrapping need not be provided with an airtight heat seal.

+n completion of transport, the packaged item need not be cleaned, but is immediately

available.

Disadvantages

The *(" method is not suitable for all metals. "t may cause considerable damage to

nonmetallic articles plastics etc./.

$ost *(" active substances may present a ha'ard to health, so it is advisable to have their

harmlessness confirmed by the manufacturer and to obtain instructions for use.

Documents

Classification of Corrosion Protection Methods