CASO PRÁCTICO NÚMERO 1 (25%)

As noted previously, corrosion of steel associated

with paint films 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- ly in Figure 2.

The extent of corrosion at an anodic area will be deter- mined by the magnitude of the current generated by the local reactive corrosion cell. It will be governed by Ohm s law:

Equation 1

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 polariza tion. The potential of the anodic surfaces drifts towards that of the cathodic surfaces as a result of an accumula- tion 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 oc- cur 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.

POSSIBLE LOCATIONS OF CATHODES IN CORROSION CELLS AT BARE SPOTS IN A PAINT COATING ON STEEL ELECTROLYTE 7 1 AN OTHER METAL

PRIMER

(I) At Base Coating

(2) At Surface of Coating (3)At Base of Primer

(4) At Other Metal Surface FIGURE 2

EFFECTS OF POLARIZATION AND RESISTANCE ON CORROSION CURRENTS

I I I il 1 I 1 1

Corrosion Current Corrosion CurrZnt

Limited by Resistance Limited by Polarization and Polarization

FIGURE 3

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 potential difference (E) in equation 1 will become:

(PA -Ap) -(PC + Cp) and equation 1 becomes:

I = (PA -Ap) -(PC + Cp) Equation 2 R

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

These will include, in series, the resistance of the

electrolyte or whatever else occupies the discontinuity (D) in the coating (RDt), the resistance of the solution or film of moisture outside the discontinuity (RL), and the

resistance of the paint coating (C), (RCt).

The resistance of the metallic electron path is suffi- ciently low to be neglected.

The factor t in (RDt) and (RCt) takes into account the

fact that the resistance of the electrolyte within a discon- tinuity 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:

I = (PA -Ap) -(PC + Cp) Equation 3 RDt + RL + RCt

Now let us examine possible effects of the location ot 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, consequent- ly, 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

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discontinuity decrease and the thickness of the coating in- creases, the discontinuity resistance factor RDt may in- crease dramatically; especially when, as frequently oc- curs, the discontinuity becomes clogged with rust (Fe203) 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 corro- sion after immersion in sea 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 immer-

sion.

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 at-

mosp heres.

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 cor- rosion 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 cop- per.

Copper ions reaching the steel surface from an an-

tifouling 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 ef- fective 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 Fe,O,. 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.