Alloys, pretreatment, corrosion

Magnesium Alloy, Processesfor Pretreatment and Prevention of Corrosion On, MUitary Specification MIL-M-3171, Dept, of Navy, Naval Air Engineering Center, Philadelphia, Pa., July 11, 1966. 

The study of corrosion is essentially the study of the nature of the metal reaction products (corrosion products) and of their influence on the reaction rate. It is evident that the behaviour of metals and alloys in most practical environments is highly dependent on the solubility, structure, thickness, adhesion, etc. of the solid metal compounds that form during a corrosion reaction. These may be formed naturally by reaction with their environment (during processing of the metal and/or during subsequent exposure) or as a result of some deliberate pretreatment process that is used to produce thicker films or to modify the nature of existing films. The importance of these solid reaction products is due to the fact that they frequently form a kinetic barrier that isolates the metal from its environment and thus controls the rate of the reaction the protection afforded to the metal will, of course, depend on the physical and chemical properties outlined above. 

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... 

MIEM-3171, Processes for Pretreatment and Prevention of Corrosion of Magnesium Alloys, U.S. Government Printing Office, Washington, D.C., 1974. 

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

In addition, alloys without the optimum levels of chromium can be treated to produce good corrosion behavior, thereby allowing the use of lower Cr material in some applications. New materials, possibly without any Cr, may well require careful pretreatment before placed in service (49). 

The use of carbonic acid for pretreatment results in mild acidity inside the pretreatment reactor, with pH in the range of 3.5-4.0 (4,8). Owing to this only moderately corrosive environment, fabrication of pretreatment reactors could be carried out using commonly used metal alloys. In the present study, it is assumed that pretreatment reactors are fabricated using stainless steel316 L (SS316), which has been found to stand up well in laboratory experiments on carbonic acid. 

A number of techniques have been developed to convert corrosion-prone, clean surfaces to less reactive ones. Three common conversion processes are phosphating, anodizing, and chromating. These processes remove the inconsistent, weak surface on metal substrates and replace it with one that is strong, permanent, and reproducible.58 Figure 15.18 shows the effect of various pretreatments on the durability of aluminum alloy-epoxy joints subjected to aging in water at 50°C. 

Chromate conversion coatings are used widely on aluminum alloys as a pretreatment for painting, though in some applications, where noncondensing atmospheric exposure is expected, they may be used as the primary means of corrosion protection. Chromate conversion coatings are used on magnesium, cadmium, and zinc, and on galvanized steel to suppress the formation of white rust. 

Prevention of General Corrosion. Proper selection of materials. In design, a metal or alloy that forms a stable passive film should be recommended. A surface pretreatment in oxidized solutions has been adopted for stainless steels and is recommended in many circumstances. The most popular process a 300 min immersion in a 20 vol% nitric acid at 50°C is recommended for some types of stainless steels.21 The environment can be modified in the bulk and should be effective at the interface in adding oxidizing agents, such as nitrite or strong nitric acid, that maintain the passive state on some metals and alloys.8... 

Corrosive reaction streams. In some application environments, the reactive or corrosive nature of one or more of the reaction components in a membrane reactor can pose a great technical challenge to the selection as well as the design of the membrane element Feed streams often contain some Impurities that may significantly affect the performance of the membrane. Therefore, attention should also be paid to the response of the selected membrane material to certain impurities in the reactant or product streams. Care should be taken to pretreat the feed streams to remove the key contaminants as far as the membrane is concerned in these cases. For example, palladium alloy membranes can not withstand sulfur- or carbon-containing compounds at a temperature higher than, say, 500 C [Kamcyama et al., 1981]. Even at lOO C, the rate of hydrogen absorption (and, therefore, permeation) in a pure palladium disk is... 

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. 

J. Zhang, C. Wu, Corrosion protection behavior of AZ31 magnesium alloy with cathodic electrophoretic coating pretreated by silane. Prog. Org. Coat. 66 (2009) 387—392. 

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