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The Corrosion Forum: cathodic disbondment on stainless steel prop shafts

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    Its a common fact that overprotection can blow the paint off of propshafts. the protection range for bronze and stainless is -500 -700 mv. has anyone done a study at what point above the 700 mv cathodic disbondment creates a problem. we also see in the feild on vessells using high load copper bottom paints damage caused by the overprotection of zinc. the damage is generally local around the zinc area. is there a way to measure in the feild how much copper is depleted from the paint on the hull
    • CommentAuthorzee
    • CommentTimeFeb 12th 2008
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    The cathodic delamination (disbondment) is due to generation of OH- during cathodic protection. The delamination around zinc in particular is due to dissolution of zinc anodes that creates high pH (OH-) environment. Paint systems exhibit different resistance to disbondment (in alkaline environment: OH-) depending on surface prep, degree of cure and type of resin. For a specific paint system/substrate the potential in which CD takes place can be determined by application of of cathodic potential and measurement of resulting disbondment. We often use this test to evaluate the disbondment characteristics and integrity of a paint coating in salt environment. Call me at 412-788-1263 ext #13 to discuss the details of the test. By the way, you can protect stainless steel at potentials more noble than -500 mv.

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