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and active ingredients could be added to restore effective coating formation characteristics. These coatings performed well as a base for paint and provided good protection for emerging high-Cu aluminum alloys. Acidic Formulations In the 1940s, acid chromate-phosphate-fluoride formulations arose. 86 These chemistries were distin- guished by the fact that they were acid-based (pH <1.8) and produced thin amorphous coatings with excel- lent corrosion resistance. Total phosphate concentra- tions ranged from 0.2 M to 1.0 M, chromic acid ranged from 0.06 M to 0.2 M, and fluoride ranged from 0.1 M to 0.3 M. Coatings formed in a matter of minutes during immersion or during spray at ambi- ent temperatures, although elevating the tempera- ture up to ~50 ² C accelerated the formation rate. Phosphate-based chromate coating formulations of this type are still in use today. They are believed to consist of predominantly amorphous chromium phosphate (CrPO 4 ·4H 2 O) with a small amount of hy- drated chromium(III) oxide (Cr 2 O 3 ). As such, they are distinct from polycrystalline phosphate coatings in structure, chemistry, and performance. These coat- ings have been used primarily as a base for painting, but boric acid (H 3 BO 3 ) additions were introduced in the late 1950s to limit film formation to promote ap- pearance, weldability, and corrosion resistance. The demand for corrosion protection of various metal parts in military systems used in tropical and marine climates during World War II stimulated wide- spread use and increased experience with chromate metal finishing. After the war, this experience was adapted and applied to commercial manufacturing. Acid chromate-fluoride formulations, in which (di)chromate was the primary film-forming agent, came into widespread use in the late 1940s and early 1950s. Typical bath formulations consisted of 30 mM to 70 mM chromic acid (CrO 3 ), 10 mM to 30 mM dichromate, and 10 mM to 20 mM fluoride. 87-88 Solu- tion pH values ranged from 1.2 to 1.8 and contact times required to complete film formation ranged from 2 min to 5 min at 30 ² C to 35 ² C. These pro- cesses produced the familiar yellow to brown coat- ings associated with chromating of aluminum using modern formulations. The coatings could be as thick as 1 ± m or more. Corrosion resistance was high, though not as high as with sealed anodized coatings, and corrosion resistance on different alloys scaled with the intrinsic corrosion resistance of the alloy substrate. In this regard, the corrosion resistance of CCCs varied inversely with the Fe and Cu content of the alloy. These coatings were thin and provided for low electrical contact resistance, which made them well suited for electronics packaging and electrical connector applications. These coatings were also a (2) In alkaline solutions, chromate is the predominant chromium species in solution independent of the form in which it was added.
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CRITICAL REVIEW OF CORROSION SCIENCE AND ENGINEERING C ORROSION—Vol. 59, No. 5 389 minimal impediment in welding operations. An added advantage to this coating process was that there was
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  • Fall '19
  • pH, Chromates, Chromate conversion coating

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