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Defects in hardchrome plating

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Dear sir,
Plating thicknesss 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 cylinderand and is lifted hydraulically.
It is used as garage hoist to lift veichles for carwashing(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
ELGI EQUIPMENTS LIMITED - COIMBATORE, TAMIL NADU, INDIA

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