Overall the evo lution of cccs on aluminum has been

  • No School
  • AA 1
  • 22

This preview shows page 9 - 11 out of 22 pages.

ing chromates have come and gone. Overall, the evo- lution of CCCs on aluminum has been characterized by several distinct innovations, some of which are FIGURE 8. AFM image for a freshly polished (0.01 ± m) Al 2024-T3 surface exposed for 3 s to a commercial CCC solution at 5 ² C. Reprinted with permission from The Electrochemical Society.
Image of page 9
CRITICAL REVIEW OF CORROSION SCIENCE AND ENGINEERING 388 C ORROSION—MAY 2003 easily recognized in modern coating formulations. Throughout this evolution, ease and versatility in processing and bath maintenance, and demand for increased corrosion resistance and paintability, have been the primary drivers for process innovation. CCCs of today are easily applied and offer excel- lent corrosion resistance, including a self-healing ability. CCCs are also easily painted. However, they are produced using hazardous chemical ingredients, and environmentally friendly replacement technolo- gies are desired. Matching the ease of application and high performance of chromate coatings using nonchromate formulations has been difficult. While chromate replacement has occurred in some applica- tions, widespread replacement will depend on further advances in chromate-free coating processing and coating performance. Evolution of Alkaline Carbonate-Chromate Chemistries The earliest inorganic conversion coatings spe- cifically for Al alloys, which were just emerging as engineering materials in the early 1900s, were formed using alkaline carbonate-chromate solu- tions. 79-80 In certain processes, coatings were subse- quently sealed in a dichromate solution. 81 Work pieces were immersed in hot (90 ² C to 95 ² C) potas- sium carbonate (K 2 CO 3 ; 0.1 M to 0.5 M)-potassium (di)chromate (2) (K 2 Cr 2 O 7 ; <0.1 M). Contact times ranged from tens of minutes to 4 h. Over this time period, hard polycrystalline coatings up to 6 ± m in thickness were formed. Little quantitative informa- tion on corrosion resistance has been reported, al- though resistance to mildly aggressive aqueous environments was noted. In these chemistries, the primary film-forming reaction was hydrothermal Al oxide film growth, and contributions to film forma- tion due to chromate were secondary. In the late 1920s, researchers discovered that the processing time of alkaline carbonate-chromate formulations could be shortened to 3 min to 5 min by controlling the carbonate:chromate ratio in solu- tion. 82 For the first time, sodium metasilicate (Na 2 SiO 3 ) and NaF were used in millimolar concentra- tions to produce clear coatings. 82 Post-coating silicate treatments were used to increase corrosion resis- tance on Al-Cu alloys. 83 Interestingly, fluoride addi- tions are reported to have been added to solution to complex with Cr and keep it out of the coating so that clear coats could be produced. The use of small additions (millimolar) of phos- phate and inhibiting transition metal oxoanions (permanganate) in the alkaline carbonate-chromate solutions was reported in the early 1930s. 84 In some of these formulations, chromate was not used at all. 85 Baths of these formulations were easier to maintain,
Image of page 10
Image of page 11

You've reached the end of your free preview.

Want to read all 22 pages?

  • Fall '19
  • pH, Chromates, Chromate conversion coating

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern

Stuck? We have tutors online 24/7 who can help you get unstuck.
A+ icon
Ask Expert Tutors You can ask You can ask ( soon) You can ask (will expire )
Answers in as fast as 15 minutes
A+ icon
Ask Expert Tutors