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Rusting observed on Siemens grey powder coated box in marine water

Siemens grey powder coated box is kept in marine water atmosphere up to 1 year , but rusting is observed on boxes , why is it happening?

Santhosha.K
adc krone communication - Bangalore, India
2005

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Hello, K. Things rust because environments are corrosive and the protective coatings applied to them were not up to the job. But it is not possible for anyone at this distance to say whether the problem was misapplication, defective coatings on a particular lot, or a design that was not fit for the purpose. If you can be more explicit in your question, hopefully it will be possible to be more explicit with the answer..

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2005

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Dear sir,

I want to elaborate the details of the finish of these boxes . Base material is made out of M.S.sheet with 2 mm thickness .The finish is done by powder coating with pure polyester texture finish .The process of powder coating prior is carried out with hot phosphating 7 tank cleaning.We have found these boxes started rusting at the bottom ,corner edges and inner flats .The traces of rust was seen after a period of 11 months. It is fully exposed to out door environment near sea environment.These boxes are already installed. We need the causes for the failures to take up the corrective action and preventive actions.
whether it is related to pretreatment chemicals?
Powder formulation, baking temperature,

S.Srinivasan
Krone Communications Ltd - Bangalore, Karnataka, India
2005

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2005

I believe that the parts are being processed to your specs, you are only calling for 500 salt spray hrs with a 5-6 rating, I would suggest a better pretreatment, iron phosphate is a little bit weak for the environment that it's going into. It's likely that there are also some process issues that are typical of the pretreatment of parts that are powder coated, increase your specs (salt spray hours and failure rating) to your supplier and see where that gets you. (Hint:it'll get you a price increase.)

Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina

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Assuming the steel was clean (ideally grit blasted) and phosphated correctly, it only leaves the quality of the painting, or the quality of the paint. The fact that the rusting is occurring in the middle of a panel suggests it is the paint film breaking down.
Rusting is electrolysis, and this will only occur if an electrolyte and oxygen are available. Exclude one or more of these and the electrolysis will not happen. Using an impervious paint or more sometimes more practical, a thick coating, can do this. Texture paints are undulating in their profile. In order to produce a bump, a dip is formed, and some of these dips can leave a very thin film to act as waterproof barrier. Therefore, there is always an inherent weakness in using texture paints as a corrosion protective barrier. The solution is to use two or more coats. Use an epoxy primer, which is more waterproof than polyester. The topcoat is then an aesthetic coating UV stable, which protects the all-important primer. This is now well on the way to being an Architectural specification.

Dale Windridge
- Margate, Kent, UK
2005