Marcus P. Corrosion mechanisms in theory and practice

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increased in order to speed up coating aging or corrosion. In these tests that are

also used for screening of existing coatings, a reference sample with well-known

properties is included to allow estimation of the quality. Outdoor exposures have

to be used for a final verification of the quality, often in parallel with the actual

use of the newly developed coating product. Cyclic weathering tests have been

developed. Examples are the Hoogovens Cyclic Test (HCT) [1], the Hoogovens

Atmospheric Corrosion Test (ABC) and the Volvo Indoor Corrosion test. The

conditions in these tests are much more representative of the actual situation of

practical use of the coatings. Still results are to be validated with actual exposition

under practical conditions. When evaluating the coating systems, the total system

should be taken into account.

First of all, in order to achieve good protective action of the applied layers,

it is important to obtain excellent adhesion of the coating to the base metal. For

this reason the substrate surface must be cleaned very well before further surface

treatment and application of the surface layer.

Cleaning is often performed in two stages. First, the organic impurities such

as oil, grease, and paint are removed from the surface. Then solid inorganic material

such as rust, mill scales, and other corrosion products can be removed. The organic

impurities can be removed in various ways:

With organic solvents

With strongly alkaline solution

With emulsion baths

By steam cleaning

The solid inorganic material can be removed by:

Mechanical treatment including brushing, grinding, polishing, and sandblasting

and shot peening with various kinds of shot, such as metallic particles,

corundum, and glass beads

Heat treatment with flames or induction heating followed by fast cooling to obtain

scaling

Chemical picking with strong acids

Pickling in acid will probably be used less intensively in the coming years

because of environmental problems. In many cases, optimized mechanical treatment

(e.g., micropeening, an optimized form of shot peening with glass beads) will then

be the substitute [2].

In most cases, after cleaning a further surface pretreatment, which may involve

metallic and/or nonmetallic layers, is necessary in order to obtain good adhesion and

thus protective properties. Also, the quality of the application procedures determines

the final protective properties [3–18]. Of course, the structure of the polymeric

network, the chemical composition, flaws, and the ease with which the coating is

damaged on mechanical impact are also relevant for the final effectiveness of the

protection. In fact, we have to take into account the properties of the whole system.

THE COMPOSITION OF COATING SYSTEMS

Organic coatings consist of four basic constituents: binder, pigments and

fillers, additives, and solvents. Therefore the properties depend strongly on the

684 de Wit et al.

actual recipe and the procedures and precautions taken during application of

the layer.

Organic coatings may protect metal structures against a specific otherwise

corrosive environment in a relatively economical way. The efficiency with which

protection is provided is determined by a number of properties of the total coated

system, which consists of the paint film(s), the metal substrate, and its pretreatment.

The composition of the paint film itself has a major influence on the corrosion

protection provided by the coatings. Therefore the major constituents of organic

coating systems are given here in a very concise form. More elaborate information

can be found in the literature [12,13].

Binder

The binder forms the matrix of the coating, the continuous polymeric phase in which

all other components may be incorporated. Its density and composition largely

determine the permeability, chemical resistance, and ultraviolet (UV) resistance of

the coating.

A continuous film is formed out of a recently applied liquid coating through

either physical curing, chemical curing, or a combination of these. An example of the

physical curing process is the sintering of thermoplastic powder coatings. Before

application, this type of paint consists of a large number of small binder particles.

These particles are deposited on a metal surface using special application techniques.

Subsequently, the paint is baked in an oven to form a continuous film by sintering.

Chemical curing involves film formation through chemical reactions. This may

be oxidative curing, where oxygen from the atmosphere reacts with the binder

monomers, thus causing polymerization. Another example is reactive curing, where

a polymer network is formed through polycondensation or polyaddition reactions.

This may be the case with multicomponent coatings, where the binder reacts with

cross-linkers.

Most often, both physical and chemical curing take place, as is the case with

the film formation of thermosetting powders. At elevated temperatures, physical

sintering of the particles takes place, followed by chemical reactions between

different components in the powder. Another example is film formation of

solvent-based reactive coatings, such as the common house paints. In this case, the

solvent (physically) evaporates from the curing film, causing the binder molecules

to coalesce and start chemical polymerization reactions.

Pigments

Pigments may be added to a coating for two reasons. (a) They can provide color

to the coating system, which is an important issue for aesthetic reasons. (b)

Pigments may be used to improve the corrosion protection properties of a coating.

This improvement may be obtained, for example, by incorporation of flake-

shaped pigments parallel to the substrate surface. If a sufficiently large pigment

volume concentration (PVC) is used, the flakes will hinder the permeation of

corrosive species into the coating by elongating their diffusion pathways, resi-

stance inhibition.

Furthermore, so-called anticorrosion pigments may provide active protection

against corrosion phenomena. These pigments may dissolve slowly in the coating

Organic Coatings 685

and may provide protection by covering corrosion-sensitive sites under the coating;

by sacrificially corroding themselves, thus protecting the substrate metal; or by

passivating the surface.

Blocking pigments may adsorb at the active metal surface. They reduce the

active area for corrosion and form a transport barrier for ionic species to and

from the substrate. An example of this type is a group of alkaline pigments (lead car-

bonate, lead sulfate, zinc oxide) that may form soaps through interaction with

organic oils.

Galvanic pigments are metal particles that are non-noble relative to the

substrata. On exposure, these particles (zinc dust on steel) corrode preferentially,

while at the original metal surface only the cathodic reaction occurs.

Passivating pigments reconstruct and stabilize the oxide film on the exposed

metal substrate. Commonly, chromates with limited water solubility are used

as passivating pigments (zinc chromate, strontium chromate). In aqueous

solutions they may cause anodic passivation of a metal surface with a very stable

chromium- and oxygen-containing passive layer.

Fillers

The main function of fillers in organic coatings is to increase the volume of the

coating through the incorporation of low-cost materials (e.g., chalk, wood dust).

They may also be used to improve coating properties such as impact and abrasion

resistance and water permeability.

Additives

The term additives refers to a large group of components with very specific prop-

erties, which typically are added to a paint in very small quantities. The functions

of the different additives may be quite diverse. Examples are thickeners, antifungal

agents, dispersing agents, antifoam agents, anticoalescence agents, UV absorbers,

and fire-retarding agents.

Solvents

The solvent has the function of reducing the viscosity of the binder and other

components and enabling their homogeneous mixing. Furthermore, the reduced

viscosity makes it possible to apply the coating in a thin, smooth, continuous film on

a specific surface. The roles of the solvent in a coating prior to and after application

are quite contradictory. In the liquid state, prior to application, paint should form a

solution or a stable dispersion or emulsion of binder, pigments, and additives in

the solvent. In other words, all solid components should remain more or less

homogeneously distributed in the liquid phase. This requires high compatibility

between solvent and components and the presence of repulsive forces between

components to avoid clustering. In contrast, after the paint has been applied, a major

attractive force between the components is necessary for the formation of a

continuous film. The interaction with the solvent should decrease to enable the

solvent to evaporate from the curing film. In order to achieve optimum storage and

application properties, a correct choice of additives is vital. Correct material

686 de Wit et al.

selection for coating formulation is often a complicated operation, where elaborate

practical experience is needed.

In some specific cases, organic paint may be mixed and applied without the

presence of solvents. These paint systems are referred to as solvent free. Examples

of this are low-viscosity two-component epoxies and powder coatings. The appli-

cation and curing of the powder coating were mentioned earlier. The epoxy coatings

may be mixed and applied without the use of a solvent, as the two components

typically have low viscosity. Mixing and application of these coatings are often

done at elevated temperatures to reduce the viscosity as much as possible.

For environmental reasons, waterborne coatings are being used increasingly.

The binder may be either dispersed or dissolved in the water phase. Dispersion

coatings show lower gloss and less good adherence than the dissolved types such

as alkyds, ureum and melamine, phenol, and acrylic-type coatings.

Also, high-solids coatings are used to minimize the loss of organic solvent to

the atmosphere. These coatings consist of a larger amount of solid particles than

normal organic solvent–based coatings. In order to reach an acceptable viscosity

despite the high content of solid particles, the molecular weight of the binder

molecules is lowered. Of course, this will also influence other properties of the

coating such as sagging.

COMPLEX COATING SYSTEMS

Pretreatment: Conversion Layers

The corrosion protection of metallic substrates by simple organic layers is often not

good enough because of, e.g., poor adhesion. Conversion layers are then applied. For

these layers the substrate metal provides ions that become part of the protective

coating after (electro-) chemical reaction of the substrate with a reactive medium.

Well-known conversion layers are phosphate layers on steel and zinc, chromate

layers on zinc and aluminum, and anodized layers on aluminum, the latter mostly

without an organic topcoat. Anodizing is electrochemical treatment of a metal

(mainly aluminum) while the metal itself is the anode. In this way a reasonably thick

oxide layer (passive layer) is formed, with the thickness ranging from 1 to 30 μm,

depending on the process conditions. Anodizing is performed in sulfuric acid,

chromic acid, boric acid, oxalic acid, and other organic acids. Most important are

anodizing processes in sulfuric acid and in chromic acid.

Pretreatment layers are used for a variety of reasons:

To improve the adherence of the organic layer

To obtain electrically insulating barrier layers

To provide a uniform grease-free surface

To provide active corrosion inhibition by passivating the metallic substrate or by

reducing the rate of the oxygen reduction reaction

For an overview of this extensive field we refer to the literature [19–21]. Two

important examples will be treated here very briefly, and some recent developments

will also be mentioned.

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Chromate coatings can be produced on aluminum and its alloys, magnesium,

cadmium, and zinc. They offer good protection and improve the adhesion of the

organic layer [22–26]. The corrosion resistance provided by chromate is based on

the following three properties [27]:

1. Cr(III) oxide, which is formed by the reduction of Cr(VI) oxide, has poor

solubility in aqueous media and thus provides a barrier layer.

2. Cr(VI) will be included in the conversion coating and will be reduced to

Cr(III) to repassivate the surface when it is damaged, thereby preventing

hydrogen gas from developing.

3. The rate of the cathodic oxygen reaction is strongly reduced.

Chromate baths always contain a source of hexavalent chromium ion (e.g.,

chromate, dichromate, or chromic acid) and an acid to produce a low pH. Typical

pH levels range from zero to around 3. Most chromating baths also contain a

source of fluoride ions, which effectively attack the original (natural) aluminum

oxide film and thus expose the bare metal substrate to the bath solution. Fluoride

also prevents the aluminum ions (which are released by the dissolution of the

oxide layer) from precipitating by forming complex ions. The fluoride concentration

is critical. A low concentration does not result in a conversion layer at all because

of failure to attack the natural oxide layer, and a high concentration results in poor

adherence of the coating due to reaction of the fluoride with the aluminum metal

substrate.

During the reaction hexavalent chromium is partially reduced to trivalent

chromium, forming a complex mixture consisting largely of hydrated (hydr)

oxides of both chromium and aluminum:

+ H

+ 6e → 2Cr(OH)

+ H

The formation of the chromate layer according to this reaction preferentially occurs

along grain boundaries and other heterogeneities at the aluminum substrate.

Chromate coatings are generally amorphous, nonporous, and gel-like when

initially formed. If necessary, they can be readily dissolved in nitric acid at this

stage. As the coating dries, it slowly hardens and becomes hydrophobic, less soluble,

and more abrasion resistant. Up to 20% (Alodine 1200S) of unreacted hexavalent

chromium is also included in the coating, which provides good corrosion resistance

as described earlier.

Chromate conversion coatings are based on two types of processes: chromic

acid processes and chromic acid–phosphoric acid processes. The reaction for the

formation of a chromic acid–based conversion coating is given by the following

overall equation:

+ 30HF + 12Al + 18HNO

→ 3Cr

+ Al

+ 10AlF

+ 6Cr(NO

)

+ 30H

The formed oxide, given as Cr

, is probably better described as amorphous

chromium hydroxide, Cr(OH)

. The color of the formed conversion coating is yellow

to brown and typical coating weights range from 0.3 to 2.0 g/m

. Those layers can

be used as a primer for a paint layer or powder coating but can also be applied

unpainted.

688 de Wit et al.

A possible overall reaction for the formation of a phosphoric acid–processed

coating is

+ 10H

+ 12HF + 4Al

→ CrPO

+ 4AlF

+ 3Cr(H

)

+ 14H

The resulting greenish layer consists mainly of hydrated chromium phosphate

with hydrated chromium oxide concentrated toward the metal. At the conversion

coating–aluminum substrate interface, aluminum oxides and other aluminum salts

are present. The thinner chromic acid–phosphoric acid conversion coatings

(1g/m

) are an excellent base for paint layers. Thicker coatings are often applied

unpainted.

In most countries today extreme care is taken when applying chromate coatings

due to toxicological and environmental hazards. It is possible that legislation will

prevent the widespread use of these excellent layers in the future. A review of key

literature that describes the toxicological and health effects of chromium, chromium

(II), and chromium (VI) can be found elsewhere [28]. As a result of these constraints,

new developments of nonchromate conversion layers have been taking place.

However, it will be difficult to find a good substitute for the chromate layers. A

more extensive review of new developments in corrosion prevention by nonchromate

conversion coatings has been given by Hinton [29].

Treating the metal surface in a weak phosphoric acid solution of iron,

zinc, or manganese phosphate produces phosphate layers. Phosphate layers

offer only limited protection of their own but greatly improve the adhesion of

the organic layers. A good example of a phosphate coating is the treatment gen-

erally used in the car industry. After degreasing the (galvanized) steel substrate

in mild alkaline phosphate baths, the surface is phosphated by a mixed spray-

dip process. A crystalline phosphate layer is formed with a total weight of about

2–5g/m

. The composition and structure of the layer depend on the substrate

and the bath. Two common phosphate compounds are Zn

(PO

)

·4H

O and

Fe (PO

)

·4H

O. After phosphating, the material is given a standard chromate

rinsing procedure.

During the cathodic electrodeposition of the primer layer the phosphated layer

is attacked by the alkalis (pH of about 12 or higher) at the interface [30]. The

phosphate is then treated at 200°C for 30min when the painted metal is baked.

During this treatment the phosphate compounds lose two water molecules. When

the total coating system is in service later, water from the environment permeates the

paint film and reacts with the partially dehydrated phosphate, and it may dissolve

partially or completely in the water phase depending on the pH of the penetrating

water [31]. When anodic and cathodic reaction sites emerge during propagation of

the undercoating corrosion process, the phosphate layer can even completely

dissolve either in the acid anodic medium or in the basic cathodic medium,

especially when NaCl is present [32].

Modification of the actual chromium-based pretreatments has been undertaken

by using trivalent chromium, especially on aluminum substrates. By forming

trivalent chromium compounds on aluminum it has been shown possible to oxidize

part of the film to hexavalent chromium, attaining a corrosion-resistant film

comparable to the normal Cr(VI)-based layer [33].

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Some no-rinse chromate-free formulations based on solutions of chromium

trifluoride, phosphoric acid, and small quantities of polyacrylic acid give good coating

adhesion [34].

Also, molybdates have been used as alternative passivity- and adhesion-enhancing

oxidants, but with limited success [e.g., 35].

Cerium compounds have been found effective in reducing the corrosion rate

of aluminum alloys by inhibition of the cathodic reaction. The cerium can be

incorporated in the passive oxide film to provide a protective conversion coating

[36,37].

Zirconium, titanium, and hafnium compounds have been used in combination

with fluoride as alternatives to chromates with reasonable success but are never as

good as the Cr(VI)-treated surfaces [38,39].

Recent developments to be mentioned are the application of lithium salts,

permanganates, self-assembling monolayers, and finally silanes to promote adhesion

of the organic layers [40–43].

Metallic Layers, Conversion Layers, and Organic Topcoats

The complete coating system in automotive materials is quite complicated. A

schematic overview is given in Figure 1. From this picture it is clear that any

description of corrosion mechanisms in such complicated systems should encompass

the possibility of a variety of defects caused by impact or degradation. A selection

has been made here to describe the initial processes, damage exposing the primer,

damage exposing the zinc coating, and damage exposing the steel.

The environmental factors determining the degradation of car body panels

are salt (from deicing salt and from the sea in marine environments), humidity,

temperature, impact by pebbles, sand, and acid rain. Of course, the most direct cause

of corrosion initiation is physical damage due to impact by any kind of material. The

propagation of any corrosion process depends on the depth of the damage.

During exposure, water permeats an intact paint film and reaches the interface,

as we have seen before. On its way in, the water dissolves organic and inorganic

compounds, which are later deposited on the metal surface, because the whole system

goes through wet and dry cycles and also through mostly anticyclic temperature

cycles. The water displaces the polymer from the metal surface during the wet period,

resulting in poor wet adhesion behavior. Adhesion is sometimes partially restored

690 de Wit et al.

Figure 1 Schematic representation of the general buildup of the coated specimen.

during dry periods. After many cycles, the area of metal surface where the coating has

delaminated, while the water phase at the metal surface incorporates more impurities,

will be considerable. The increasing concentration of dissolved species at the

interface also stems from the huge variety of inorganic water-soluble species present

in the galvanized layer. Oxygen will also diffuse to the zinc layer, which is a rather

good catalyst for the oxygen reduction reaction. However, no real corrosion damage

normally results because of the active corrosion inhibition of the Zn-containing layer.

After damage by impact, this picture changes considerably. In Figure 2, a

schematic picture is given of a coated system after impact resulting only in damage

in the topcoats. Aggressive species can directly reach the primer surface. When water

and oxygen have reached the phosphate layer by diffusion, the cathodic delamination

process can start. Different authors found that the diffusion rate of water in the

typical primers used for car body coatings is high and above the rate for

epoxy-polyamide resins [44–46]. The diffusion rate of oxygen being of similar

magnitude, it must be concluded that the permeation rate through the primer is

not rate determining for the corrosion process. Corrosion proceeds as discussed

for the cathodic delamination process. The corrosion spots spread radially. The pH

increases at the cathodic sites, and the phosphate layer dissolves. Eventually a

blister will form and further damage to the system will result in the mechanism

that takes place at an earlier stage when the coating is damaged to the galvanic

layer, as given in Figure 3.

In that case the phosphate layer provides hardly any protection. The zinc layer

is consumed locally by the anodic reaction, and the steel substrate is exposed. The

corrosion products are not taken up in the paint film. The cathodic delamination

takes place along the zinc-paint interface. The corrosion process continues where the

damaged site reaches the steel substrate as shown in Figure 4. All contaminants

reach the paint-phosphate-metal interface. Zinc dissolves anodically at the tip while

oxygen reduction takes place at the exposed steel surface. At cathodic sites the pH

Organic Coatings 691

Figure 2 Schematic picture of the partially damaged coating system on a car body panel.

Cracks after impact reach the primer. Diffusion of water and oxygen through the primer.

(Picture drawn free after Granata [47].)

increases, leading to loss of adhesion. Also, chloride ions can now easily reach the

corroding surface. The formation of zinc hydroxychloride further damages the

interface by a wedging action. The anodic reaction of zinc lowers the pH. The primer

is not very stable at low pH and is hydrolyzed. Phosphate also partially dissolves,

thus further weakening the paint-phosphate-metal interfaces. It is clear from this

short description that the overall reaction mechanism is very complicated and not yet

unambiguously resolved. Opinions differ considerably and research is going on

[47–48].

692 de Wit et al.

Figure 3 Schematic picture of the partially damaged coating system on a car body panel.

Cracks after impact reach the phosphate and through porosity the zinc layer. (Picture

drawn free after Granata [47].)

Figure 4 Schematic picture of the partially damaged coating system on a car body panel.

Cracks after impact reach the steel substratum. (Picture drawn free after Granata [47].)

APPLICATION TECHNIQUES

Organic coatings can be applied to surface by different methods. The application

depends mainly on the

Purpose of application: protection or decorative

Type of coating, target layer thickness, number of layers

Size and geometry of the object to be coated

Site of application: in a conditioned room, confined space, or in the field

Moreover, environmental and health aspects have a large influence on the

application techniques. A short overview of the different techniques with (dis-)

advantages is given next.

Brushing

Application by brushing is labor intensive and consequently time consuming and

expensive. The layers obtained by brushing depend on the painter and can vary

greatly. Brushing is still indispensable for small areas, repair purposes, and for less

accessible places for parts that are difficult to protect, such as edges, angular parts,

and welds. An advantage of this technique is the very low wastage. Also, it is

generally believed that brushing gives better penetration of paints into pores,

cracks, and crevices [49]. In the case of a solvent-based paints, health risks have

to be taken into account, in particular in confined spaces. However, the technique

is suitable for waterborne and high-solid systems.

Brushing is less suitable for very fast drying products; brush marks remain

visible and the paint film is not homogeneous. It is also difficult to apply more layers

of some types of physically drying paints: the solvent of the last coat layer may

soften the previous layer.

Rolling

Painting by roller is much faster than brushing. It is suitable for large flat surfaces.

On the other hand, it is not suitable in corners, edges, and welds. Additional brushing

is necessary in these cases. Further, the same restrictions apply for the type of

paint as in the case of brushing.

Dip Coating

The object to be coated is dipped in the coating. This method is very simple,

cheap, and can be quick. The quality depends on a number of factors: time of

withdrawal, viscosity, the rate of solvent loss, and the mixing of the coating. The

method can be automated to deal with a large quantity of objects to be coated.

Conventional (or Air) Spraying

The paint is forced from a container to the spray gun by pressurized air or by a pump.

As the paint passes through the nozzle, it is mixed with air and atomized at pressures

of about 2 to 4 bar. Spray guns can be equipped with fluid air supplied by a

Organic Coatings 693