Codes and Standards

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IV. CORROSION AT DISCONTINUITIES IN A PAINT FILM

As noted previously, corrosion of steel associated with paint trims is most troublesome at, or adjacent to, pores, scratches or other bare spots. It is convenient, therefore, to examine the factors related to attack at bare spots. The most important of these factors is the location of the cathodic areas in the corrosion reaction. Possible locations of cathodic surfaces are shown diagrammatical- in Figure 2.

The extent of corrosion at an anodic area will be determined by the magnitude of the current generated by the local reactive corrosion cell It will be governed by Ohm's law;

where I = corrosion current

E = difference in potential between anodic and cathodic surfaces

R = resistance of the circuit

When current flows in a corrosion cell, the initial potential difference E is reduced by what is called polarization. The potential of the anodic surfaces drifts towards that of the cathodic surfaces as a result of an accumulation of corrosion products. The potential of the cathodic surfaces drifts towards that of the anodic surfaces as a result of accumulation of the products of the cathodic reactions. The latter is affected by the rates of evolution of hydrogen as a gas or, more importantly in applications of steel, the rate at which oxygen in solution can react with electrons reaching cathodic surfaces after release by the anodic reaction, In most applications of painted steel the extent of cathodic polarization will determine the rate of the overall corrosion reaction, Anodic corrosion cannot occur at a rate higher than that accommodated by the cathodic reaction.

Figure 3 illustrates the potential shifts that result from polarization, As indicated, polarization limits the amount of current that can flow, it will be reduced further by an increase in the resistance of the circuit.

As a result of polarization the original potential of the anode PA will be reduced by a factor Ap, and the original potential of the cathode PC will shift towards that of the anode by a factor Cp.

As a result, the effective 9otential difference (E) in equation I will become:

(PA - Ap)- (PC + Cp)

and equation 1 becomes:

Let us now examine the factors that determine the magnitude of the resistance R.

These will include, in series, the resistance et the electrolyte or whatever else occupies the discontinuity (D) in the coating (RDt), the resistance of the solution or fi m of moisture outside the discontinuity (RL), and the resistance of the paint coating (C), (RCt).

The resistance of the metallic electron oath is sufficiently low to be neglected.

The factor t in (RDt) and (RCt) takes into account the fact that the resistance of the electrolyte within a discontinuity and the resistance of a coating will increase as the thickness of the coating is increased.

Combining all these component elements, the

resistance factor R becomes:

RDt + RL + RCt and equation 2 becomes:

Now let us examine possible effects of the location of the cathode on the corrosion reaction at the base of the discontinuity.

Location 1 in Figure 2 assumes that both the anodic and cathodic reactions will have to occur at the base of a pore or other discontinuity in a coating. This automatically limits the area that can act as a cathode and, consequently, by increasing the cathode current density, increases favorably the value of the term Cp in equation 3.

Even more importantly, as the dimensions of the discontinuity decrease and the thickness of the coating increases, the discontinuity resistance factor RDt may increase dramatically; especially when, as frequently occurs, the discontinuity becomes clogged with rust (FarO3) which has a very high electrical resistance.

The positive effect of thick coatings is shown by sea water tests of steel covered with a paint of proper thickness, but subsequently found to have many very small pores. The steel showed no visible evidence of corrosion after immersion in see water for more than a year.

What has just been described supports the advantage of increasing the thickness of a paint film, especially if the application involves exposure under conditions of immersion.

The factor RL covering the resistance of the solution or film of moisture explains why corrosion is likely to be more severe in sea water than in fresh water and under conditions of immersion as compared with atmospheric exposure. In the case of the latter, humid atmospheres containing chlorides, sulfur dioxide or other pollutants can promote more corrosion than dry, unpolluted atmospheres.

The rather startling 8500 to 1 range in corrosivities of atmospheres was demonstrated by a test program undertaken by ASTM.6

The factor RCt, the electrical resistance of the coating, becomes important only if the cathode of the corrosion reaction exists underneath the coating, (location 3, Figure 2). In such circumstances, favorable factors will be the thickness of the coating "t" and the resistance of the coating to water absorption and moisture penetration as well as its basic electrical resistance characteristics.

A cathode created under a coating by the passivating action of primers containing inhibitive pigments such as red lead or chromates will have a low potential, Cp, and a relatively large area with low cathodic polarization, Cp in equation 3, Thus, the effect is to increase the corrosion current I. This supports the recommendation that passivating pigments should not be used in paints on steel in services involving continuous or frequent, partial or complete immersion.

As another example, it is possible also to create a cathode under a paint film by migration of copper from an antifouling paint containing cuprous oxide or metallic copper.

Copper ions reaching the steel surface from an antifouling paint can deposit on the steel by cementation and thereby become a powerful cathode to steel at the base of an adjacent discontinuity in a coating. Thus, an anti-fouling paint system based on copper must include an effective anti-corrosive film under the anti-fouling topcoat.

Quite different from the thin invisible oxide films formed on steel by exposure to dry air, mentioned above, are the relatively thick oxide scales formed on steel during high temperature manufacturing operations. This "mill scale" has the composition Fe3O4. It exhibits a potential that in sea water can be more than 500 mV more noble than that of bare steel. Metal exposed at discontinuities in such mill scale becomes the anode in a powerful galvanic cell with resulting severe.localized attack at such anodic areas. The possibility of such effects produced by mill scale under paint coatings and the generally poor adherence of mechanically disturbed mill scale account for the need to remove mill scale from steel in preparation of steel for painting.

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V. EFFECT OF ANODIC PIGMENTATION

A very favorable condition can be achieved if a paint system includes zinc in either an organic or inorganic (silicate) matrix. Since zinc is anodic to steel, an anodic potential in the opposite direction is superimposed on the steel so that the factor in the numerator of equation 3 becomes zero or even negative and consequently the corrosion current I is eliminated. This accounts for the excellent performance of zinc-rich coatings used either as primers or alone for protection of steel in marine and other severely corrosive environments. An essential requirement is that the zinc pigment loading be extensive enough to achieve electrical contact between the zinc particles so that they can function as effective galvanic anodes for the cathodic protection of the steel.

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VI. EFFECT OF CATHODIC PIGMENT

It is unlikely that any paint system would create a cathode at location 2, Figure 2, at the outer surface of the coating; however, this could happen in the case of an an-antifouling paint sufficiently loaded with copper powder or flake to form an effective copper cathode. Dangers from this source have restricted the use of antifouling paints based on metallic copper pigment.

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VII. EFFECT OF GALVANIC COUPLES

The most dangerous location of a cathode is location 4, Figure 2. This would be the case of painted steel in electrical contact with a more noble (cathodic) metal such as a copper or nickel alloy or stainless steel, both being immersed in an electrolyte.

Such a situation would provide a cathode much larger than the very small anodes exposed at discontinuities in a paint film and with a large potential difference between the anode and the cathode (EA - EC), over 500 mV between the steel and the more noble metal.

The resulting galvanic corrosion would result in fairly rapid penetration (pitting) of the steel.

Painting the anodic (steel) member of such a galvanic couple will aggravate rather than minimize galvanic corrosion of the steel. It would be much better to leave the steel bare and tolerate the extent of the broadly spread galvanic corrosion that would result. But the best practice would be to paint both metals in the galvanic couple so as to eliminate both galvanic and normal corrosion.

The next best choice would be to paint the more noble (cathodic) member of the couple and leave the steel bare. Discontinuities in a coating on the cathodic member can be tolerated in view of the small area of cathode that would become involved.

Coatings to be used on cathodic surfaces must be able to tolerate the alkali generated by cathodic reactions.

An interesting form of galvanic corrosion has been encountered in oil production systems in the North Sea. Here, steel drilling and production structures are associated with very large concrete vessels used for storage of oil. The reinforcing steel embedded in the concrete can develop films that make the reinforcing steel strongly cathodic to steel outside the concrete. The galvanic cell generated in this way can accelerate the corrosion of the outside steel. This can be particularly serious if the galvanic effect is concentrated at discontinuities in a paint coating. This could be a factor in deciding whether to use a paint coating as a supplement to cathodic protection and in determining the amount of current required for cathodic protection of the steel in the concrete.

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