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Defects in hardchrome plating

adv.    

Dear sir,
Plating thickness of 20 microns is coated over a plunger pipe welded with plates at both end with thickness 8mm dia 270mm length 2m with base metal as MS.The plunger is inserted in to the cylinder and and is lifted hydraulically.
It is used as garage hoist to lift vehicles for car washing (mostly) and servicing.
We found lot of black spots over the circumference of coated plunger because of which is a oil leak from inside the cylinder, each time when the plunger comes out. At a close observation it was found like a small spot rusted in brown color. This if found in one area will spread within a few days to the entire area.
What possibly could be the problem? Wether the coating or corrosion or what?

M.ASHOK
- COIMBATORE, TAMIL NADU, INDIA
2006

adv.