Virtually no dimensional change associated with

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virtually no dimensional change associated with coating formation, and coatings could be applied to carefully machined parts. Modern Accelerated Chromate Formulations In the late 1960s and 1970s, the use of accelerants in CCC formulations was initiated. 89 Accelerants increased coating weight and shortened coating time. Many different accelerants were used, including a variety of organic and transition metal compounds. 90 However, the predominant accelerant in modern commercial formulations is ferricyanide, Fe(CN) 6 3– , which is added to acid chromate-fluoride chemistries in concentrations ranging from 2 mM to 5 mM. The original claim was only that the use of the compound increased coating weight. No special claims for enhanced corrosion resistance or paint ad- hesion were directly given. Several studies have been dedicated to characterizing Fe(CN) 6 3– in conversion coatings and determining its role in stimulating coat- ing growth and determining its influence on coating properties. The two primary theories for the action of Fe(CN) 6 3– are the following: —formation of mixed metal cyanide com- pounds 27,91-92 —acceleration of the film-forming Cr 6+ to Cr 3+ reduction reaction by Fe(CN) 6 3–/4– redox mediation 27 Recent Developments Chemistry modifications continue to be made to optimize coatings for special situations such as ap- plication to specific alloy type 90 or reduction of crack- ing as a result of drying. 93 Several new processes have been developed based on Cr 3+ chemistry rather than Cr 6+ chemistry. 94 In these processes, film forma- tion is carried out in an acidic Cr 3+ salt bath. Usu- ally, an oxidizing treatment is applied to the coating after it is formed to improve corrosion resistance. This likely oxidizes some of the Cr 3+ to Cr 6+ and imparts some Cr 6+ -leachability and self-healing to the coating. It may also increase the toxic hazard of the coating during any removal or maintenance operations. Nonetheless, the resulting coatings offer corrosion resistances approaching that of the accelerated CCCs. Self-Healing “Self-healing,” discussed briefly earlier, refers to the distinctive ability of a CCC to heal small chemical or mechanical defects that expose unprotected un- derlying metal. The process involves liberation of la- tent Cr 6+ into an attacking aqueous environment, migration of the Cr 6+ to an incipient defect, and re- duction to insoluble Cr 3+ hydroxide or interaction with the corrosion product gel to stifle further corro- sion. Self-healing was recognized as a component of chromate coating protection in acidic chromate-fluo- ride formulations no later than the early 1950s. 95 The literature does not ascribe self-healing characteris- tics to the earlier alkaline carbonate-chromate or acidic chromate-phosphate-fluoride formulations.
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  • Fall '19
  • pH, Chromates, Chromate conversion coating

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