Corrosion potential coatings

Corrosion protection measures are divided into active and passive processes. Electrochemical corrosion protection plays an active part in the corrosion process by changing the potential. Coatings on the object to be protected keep the aggressive medium at a distance. Both protection measures are theoretically applicable on their own. However, a combination of both is requisite and beneficial for the following reasons ... 

Table 12.2 Corrosion potentials of substrates of coppter and steel, plated and unplated in same plating solutions. Deposition potential is accompanied by current density (A/m ) in parentheses the plated substrate s coating thickness was 2 5 fim. The final column gives the potential below which hydrogen evolution is possible only in the cuprocyanide is it observed...

In 1979, Leidheiser ( reviewed the use of corrosion potential measurements with regards to the prediction of corrosion at metal-organic coating interfaces. Wolstenholme had last reviewed this literature in 1970 (10). Work in the 1930-1940 s focused on the magnitude of the corrosion potential and how it changed with time (11-14). Negative potentials with respect to uncoated substrates were indicative of corrosion beneath the coating. Positive potentials with respect to uncoated substrates were indicative of the absence of corrosion. 

Anomalous cases were noted in which this generalization did not hold. These very empirical measurements were followed by more thorough studies (IS). Thin paint films with very low electrical resistance show active corrosion potentials which become more positive as the paint film was increased in thickness. Shapes of the potential/time curves were misleading as a guide to ultimate coating protective properties. 

Kendig and Leidheiser (16) electrochemica1ly evaluated thin (9 micron) polybutadiene coatings on steel. They concluded that movement of the corrosion potential in the noble direction was indicative of an increasing cathodic/anodic surface area ratio. Oxygen and water penetrate the coating to produce the cathodic reaction at the metaI/coat ng interface. 

A variety of techniques has been developed to measure the condition of a coating so that some evaluation of its protective ability can be made. Many of these are based on electrochemical measurements [2]. The four techniques used in this study are (1) corrosion potential, (2) AC conductance, (3) tensile adhesion, and (4) weight gain. 

The specific AC conductivity values show a generally decreasing value with decreasing substrate corrosion, with one exception the novolac epoxy cured with an aromatic/cycloaliphatic amine. This is one of the coatings that also did not fit into the trend for the corrosion potential values. 

Of the four techniques studied for evaluating coatings on steel for corrosion control (corrosion potential, conductance, adhesion and weight gain), the most useful was conductance. Corrosion potential did not show a consistent relationship, and weight gain and tensile adhesion showed no correlation with corrosion. 

Another example is the very slight delamination that occurs when a thin copper layer is overcoated with an organic coating such as a photoresist and the system is made anodic. The rate of disbonding is a function of the applied potential and hence the rate of dissolution of the copper beneath the coating. Anodic delamination occurs very slowly relative to cathodic delamination at equal potential differences from the corrosion potential. 

Table 3 - Effect of chemical conversion coatings on corrosion potentials of 7075-T6 A1 alloy.

An epoxy paint for temporary protection of high zinc content 88.3 % relative to dry mass of the coating was investigated on mild steel wire electrodes of 5 mm diameter. The coatings of 27 2 jtim in thickness were studied. The measurements were carried out in 3 % non -- deaerated NaCl solution at room temperature in the frequency range from 1 Hz to 60 kHz using a sine signal of 10 mV amplitude. The measurements were i>erformed in a three-electrode system with the corrosion potential measured vs. the saturated calomel electrode. 

There is a large potential for conducting polymers as corrosion-inhibiting coatings. For instance, the corrosion protection ability of polyaniline is pH-dependent. At lower pH polyaniline-coated steel corrodes about 100 times more slowly than noncoated steel. By comparison, at a pH of about 7 the corrosion protection time is only twice for polyaniline-coated steel. Another area of application involves creation of solid state rechargeable batteries and electrochromic cells. Polyheterocycles have been cycled thousands of times with retention of over 50% of the electrochromic activity for some materials after 10,000 cycles. IR polarizers based on polyaniline have been shown to be as good as metal wire polarizers. 

Coatings. Paint on metals should be intact to reduce corrosion potentials. 

Corrosion resistance of metallic coatings is dependent on the composition and nature of the electrolyte, oxygen concentration, polarization characteristics, ratio of cathodic to anodic area and the surface contaminants. If the corrosion potentials of two metals such as iron and aluminum are close to each other in a particular environment there may be reversal of the galvanic couple. 

Coatings. Paint on metals should be intact to reduce corrosion potentials. Grounding. Grounding cables should be securely attached to the tank and the grounding grid in the foundation. 

Figure 2 shows photographs of LEED patterns for Pd(lll)-( 3 x V3)R30°-I prior to and after removal of about 30 monolayers of Pd siuface atoms. In this experiment, the potential was held close to but not past the anodic dissolution peak that is, the dissolution was carried out at a fairly high rate. The LEED data clearly show that the post corrosion I-coated Pd surfaces remained as well-ordered as they were prior to the dissolution reaction. A layer-by-layer dissolution process is thus suggested. The LEED results, however, do not provide information on whether the corrosion transpires at steps or via place-exchange between the I and Pd atoms. For such information, in-situ STM experiments were invoked. 

Conventional potassium hydroxide electrolytic cells are made from carbon steel. Areas with high corrosion potential are frequently clad with nickel, plastic, or ceramic material. The cathode is constructed of steel coated with a catalyst. The anodes and cathodes of bipolar cells are usually made from nickel or nickel-coated steel. Diaphragms were originally made from asbestos reinforced with nickel nets. Because of the health hazards associated with the use of asbestos, ceramics and polymers are being considered as substitute materials. 

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