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Salt Fog Failure

we have recently experienced an 1500 hour salt fog failure on 10 mil polymer coated product. the 1000 hour phase performed well. the failure is denoted by a delamination of the polymer at one edge or sometimes both edges of the scribe mark. the failure exhibits a white deposit at the scribe mark , then a greying of the galvanized coating and then relatively undisturbed galvanize coating. the de lamination occurs at the greyed galvanize area. what is the most probable mechanism involved . previous work only indicates the presence of zinc and oxygen

Thomas R Wasil
steel company - Warren, Ohio
2000

The salt fog reacts with the exposed zinc and will undercut the coating unless correct pretreat or primer is used. If you apply coating and oven cure chromating or dry-in-place chromate processes will work. If you preheat the metal a fine grain crystalline zinc phosphate should work. This will stand up to 250 deg C for short periods before totally dehydrating and loosing structure. R

Roger Bridger
- Croydon, UK
2000

If the scribe cuts through the primer or pretreatment what good is the pretreatment step? Isn't the galvanic process going to happen any way if the scribe cuts through the pretreatment to the base zinc?

Mike McGinness
- Pasadena, Texas
2000

Hi Mike You are right that the zinc will preferentially corrode to protect steel. Unfortunately oxidation of the zinc surface will cause delamination of any applied coating. However the presence of the correct surface pretreatment coating will greatly retard undercutting of the organic film. To give an example: powder coat clean HDG, scribe and expect to get 250-400 hours hot salt spray; apply a hexavalent chromate coating and 1500+ hours will be achievable. R

Roger Bridger
- Croydon, UK
2000