Summary for fe zn8 coastal environment Discussion
Fe/Zn8 thick-film passivation in coastal salty environment is the central concern here: Manoj B. reports white powder deposits on screw heads after one year in packed storage, and asks whether the coating is suitable for coastal use and what alternatives exist. The reply says the storage-versus-use conditions are important, but if the screws are actually exposed to a salty coastal environment, Fe/Zn8 is not adequate because 8 µm zinc is intended for moderate exposure, not severe or very severe exposure. The answerer also notes that white rust in storage should be prevented by the chromate passivation layer, and says properly dried, properly stored parts should not normally show white rust within a year. He cautions that switching from thick-film passivate to thin-film with sealer is probably not the first move, and asks about salt spray test results. Manoj adds that they test to DIN EN 9227 and that the city’s humidity and heat are harsh.
February 3, 2016
Q. We are using Fe/Zn8 thick-film passivation colourless screws along with Plastic housing in our switch gear product
One of our customer who is in the coastal area had stored our product in packed condition in stores for 1 year as stock… When he have opened the packaging of use he has observed white powder type of deposits in the heads of all the screws which is used.
Now he is asking for the reason.
My question is:
a) Does Zinc thickfilm passivation work in Coastal salty environment?
b) What are the other alternatives we can use?
Regards
Manoj B
Designer – Pune, India
A. Hi Manoj. I’m getting a little confused about storage vs. usage conditions…
If these screws are used in, and exposed to, a salty coastal environment, Fe/Zn8 is not adequate. That 8µ thickness is for “moderate” exposure, not “severe” exposure or “very severe” exposure. But perhaps they are used within sealed cabinets?
Protection from white rust in storage, however, is indeed a function of the chromate passivation layer. Thick-film trivalent passivates may differ from thin film trivalent passivates with sealers in various ways, but corrosion resistance was fundamental to the development of both approaches. I would not expect properly dried, properly stored parts to get white rust within a year, but it is obviously difficult to ascertain exactly where the problem lies. I doubt that switching from thick-film passivate to thin-film with sealer would be a good early step though. Are you doing regular salt spray testing and, if so, what results have you been getting? Thanks.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
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February 2016
February 4, 2016
Q. Thanks for your quick observations & suggestions.
I am sending the the photo of the screw to you email, the screw is used on thermoplastic housing (which is switch which is used in outdoor applications) the product is IP67 rated.
We actually do salt spray test according to DIN EN 9227
But this is the first time we are facing this problem, the climatic condition of the city is quite harsh
75-95% humidity & 38 °C
Will using a stainless steel screw solve the problem? Or we can use plating itself with some changes?
Manoj Babu
– Pune, Maharasthra, India