The case of clear chromate and yellow dichromate coatings on medium carbon-plated bolts
Open Access Dissertation
The purpose of this study was to evaluate the relationship between plating thickness and corrosion resistance of zinc coated medium carbon steel bolts under salt spray moisture conditions. Two types of zinc coatings were chosen for the study: the clear chromate zinc and the yellow dichromate zinc. One hundred and eighty 3/8-16 x 5 3/4 inch medium carbon steel bolts were selected for the test and divided into groups of 18. Two types of salt solution were used in the study: the low salt and the high salt. Bolts with various coatings tested with low salt concentration were composed of groups of 18 with different coating conditions. Bolts with various coatings and tested with high salt concentration solution were composed of groups of 18 each.
An electronic balance was used to take the weight before and after for each sample tested. An electronic thickness tester was also used to measure the thickness before and after for each sample. The hypothesis was supported when P-value of the overall test was less than the set alpha value.
The percentage weight loss, thickness loss, and the analysis of variance or ANOVA were the statistics used to interpret the results of the test.
The conclusions drawn from these tests were:
1. Medium carbon steel bolts plated electrolytically with yellow dichromate zinc were more corrosion resistant than the ones plated with clear chromate zinc.
2. The yellow dichromate zinc appeared to be more protective than the clear chromate zinc.
3. Bolt coatings should be at least 0.0002" thick to guarantee a minimum protection on medium carbon steel bolts.
4. Yellow dichromate zinc was chosen to be the best coating on medium carbon bolts.
5. The clear chromate zinc does not provide a good protection for medium carbon bolts beyond 60 hours in the fog chamber of the salt spray apparatus.
Based on the findings of this study, the following suggestions were made:
1. Manufacturers should be aware of some geographical locations with high moisture content and high pollution in the atmosphere and provide thicker coating for fasteners used in their products.
2. Manufacturers should try to coat bolts with yellow dichromate zinc.
3. Manufactured products going to the ultra-dry desert climate of Nevada and Arizona could use fasteners coated with clear chromate zinc provided they are at least 0.0003" thick.
Year of Submission
Doctor of Technology
Department of Industrial Technology
1 PDF file (IX, 137 pages)
© Souleymane Boukari - 1994
Boukari, Souleymane, "The corrosive effects of salt spray on electrolytic zinc-plated mechanical fasteners:" (1994). Theses and Dissertations @ UNI. 797.