Alloys, pretreatment, corrosion resistance

Many studies have shown that surface pretreatment of Fe-Cr alloys has a strong effect on the scale morphology and subsequent oxidation rate For instance, Caplan indicated that several Fe-Cr alloys show improvement in the corrosion resistance due to cold work, with greater than 16% Cr required to show the optimum benefit. Khanna and Gnanamoorthy examined the effect of cold work on 2.25%Cr-l%Mo steels at temperatures between 400°C and 950°C over 4h in 1 atm O2. They found that up to 90% reduction by cold rolling had a negligible effect on the oxidation rate up to 700°C. However, above 700°C there was a general reduction in the kinetics... 

Results from both studies indicate that various pretreatments of alloys can enhance the corrosion resistance of the materials. 

Magnesium CCCs are usually formed in acidic solutions in which Mg surfaces are easily activated. Nitric acid-and chromic acid-based mixtures are common. Because of the high corrosion susceptibility of Mg alloys, CCCs are more often used as a surface pretreatment for paint than as stand-alone corrosion-resistant coatings. 

For aluminium, the choice of pre-treatment will depend on the aluminium (or the aluminium alloy) itself and the corrosion resistance required. Chromium-containing pretreatments have proved to be the most effective, but concern in recent years over the toxicity associated with the chemicals used is now driving forward the development of... 

Organosilane-based coatings have been successfully tested as anti-corrosion pretreatments for aluminium alloys, steel, copper, magnesium and galvanised steel and the results generally demonstrate improved corrosion resistance [26-66]. 

Potentiodynamic polarization measurements are quite appropriate for determination of the pitting susceptibility of aluminum coatings, and/or the corrosion current density/ corrosion rate of coated steel products in general. ASTM G 102, Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements, describes the calculation of corrosion rates and other information from electrochemical measurements. Another example of the use of DC electrochemical methods to examine the corrosion performance of coated sheet materials is a study by D. A. Jones et al. [48]. The study used polarization resistance measurements to examine the mechanism of steel and coated sheet degradation under conditions of alternate immersion. Jones compared the polarization resistance of samples of low-carbon steel, unpainted galvanized, aluminum-coated, and Zn-Ni alloy coated steel during continuous immersion and alternate immersion. Alternate immersion cyclic exposure produced a thick oxide that led to significant underfilm attack. Jones found that phosphate pretreatment tends to increase the resistance of these materials to underfilm attack. This study is an excellent example of the way electrochemical measurements can be used as a complement to other techniques to elucidate mechanistic information. 

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