Effects of Substrate Microstructure

Because of specific physical property requirements, metal fabricators rely on a "mixed substrate," such as an alloy, rather than a pure metal. An alloy is typically of higher strength than a pure component because of the synergistic effect of the alloying element (e.g., a small amount of copper greatly strengthens aluminum for aerospace applications). The disadvantage of the alloy substrate from a conversion coating standpoint is twofold:

• The conversion coating must be tolerant of all the alloying elements. Formulating a conversion coating that can simultaneously treat all the elements present in an alloy can be difficult, depending on the elements' relative reactivity.

• The alloying elements may impart an accelerated corrosion rate to the metal. For example, when exposed to identical sodium-chloride-containing solutions, high-purity (99.999%) aluminum corrodes very slowly, whereas copper-containing aluminum alloys (e.g., the 2000-series alloys) corrode extremely rapidly (Ref 4). (On the other hand, alloying elements such as magnesium and zinc have very little effect on the corrosion rate of aluminum.) Aluminum used for alloying is typically of 99+% purity, and this aluminum will corrode at a faster rate than higher-purity material due to the presence of impurities, such as iron, that form intermetallic phases (e.g., FeAl3). A few parts per million of iron, copper, or nickel in magnesium and magnesium alloys can also greatly accelerate the corrosion rate (Ref 16).

One of the principal difficulties in attempting to form a uniform conversion coating on an alloy surface is that alloys are seldom of homogeneous composition (i.e., they are not perfectly solid solutions). With the base metal and with themselves, the alloying elements often form second-phase particulates or "intermetallics" within the solid solution matrix (Ref 17). The intermetallics develop during the alloying process (as is found with precipitation hardening of a supersaturated alloy solution) and render the surface inhomogeneous. These intermetallics can form galvanic couples between themselves and the alloy solid solution matrix, and depending on their relative activity with respect to the matrix, they can either cause acceleration of the matrix or their own dissolution rate.

For example, the CuAl2 that is found in aluminum-copper alloys is typically less active than (i.e., lies cathodic to) the solid solution matrix, which enhances the solid solution dissolution rate. In addition to enhancing the corrosion rate, this can have a profound effect on the uniformity achieved with a particular conversion coating process. As has been noted in a previous study (Ref 7), the activity difference between CuAl2 intermetallics and the solid solution results in the application of a nonuniform (in thickness) conversion coating. A much thicker layer is found on the solid solution matrix because of its enhanced activity. The thinnest layers are found on the (Fe,Mn)3SiAli2 intermetallics, which appear to be less active than the CuAl2 phases.

Alloy temper can also affect microstructure, which in turn can influence coating performance. For example, one study found that chromate treatments that were effective in protecting aluminum alloy 7075-T6 were not effective in treating the same alloy in the T73 (overaged) temper (Ref 18).

The pretreatment process, which usually includes cleaning and deoxidizing or desmutting steps, can have a strong effect on the surface microstructure and thus on the formation of a coherent conversion coating. In the case of the copper-aluminum alloys cited above, surface pretreatments involving either acid etching (Ref 4) or final polishing in a basic silicate electrolyte (Ref 7) produce a surface that contains a relatively uniform layer of copper, as detected by spatially resolved AES. The enriched copper found on the solid solution matrix results from either selective dissolution of the aluminum and/or redeposition of the copper that was dissolved from the intermetallics and/or solid solution. The chromate solution "sees" the homogeneous surface and readily forms a chromate conversion coating over the copper-rich interface, leaving the interface intact (Ref 7). As indicated by transmission electron microscopy studies of electropolished and acid-etched samples of high-purity aluminum that were subsequently chromated, the hydrated chromium oxide coating initially deposits on the metal ridges (the cathodic sites, produced by the pretreatment process) while the anodic sites, where the aluminum oxide was continuously attacked by fluoride ion, lie between the metal ridges (Ref 19). These studies illustrate the importance of the pretreatment steps in producing a homogeneous surface layer before application of the conversion coating.

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