Salt spray analysis in the sheet metal industry.

Hot salt Spray will give you an indication of potential corrosion problems but it is not a definitive test. Often Hot Water Testing is used to check the adhesion of the coating to the substrate and this will help tell you if the system you have applied will perform in the field.

You will not see a dramatic difference using zinc rich primers when comparing liquid with powder coatings. This is due to the basic excellent performance of powder as anti corrosion coating.

Over welds the problem is outgassing and residue of potential contaminants which will weaken the bond between the substrate and coatings. For steel degreasing and iron phosphating will provide a suitable substrate for subsequent powder coating.

Salt spray basically indicates under very limited conditions the corrosion resistance of the pretreatment. In practical terms corrosion resistance should be ranked zinc phosphate (best) then iron phosphate and the chrome pretreatment (least). I practice any of these depend on how clean the surface was prior to applying the pretreatment. The cleaner the surface the better the adhesion of the pretreatment and therefore the better the corrosion resistance.

Also the pretreatment for the most part only becomes problematic when the coating has been penetrated. The pretreatment then determines how well the coating adheres to the substrate. This is what the salt spray is trying to test. Trying to relate the results to real life is almost impossible unless the product is used very close to the sea where the salt spray from the waves reaches it.

As to welds, there are two factors. One the surface cleanliness again. Welds will often have smut or adherent oxides. Unless these are removed problems with any of the systems will occur. Depending on the amount of weld oxide, they can be effected by the pretreatment pH and can also effect the pH of the solutions. Also there can be underlying microstructural changes close to the weld which would cause localised galvanic corrosion. These may have more effect than the pretreatment on any corrosion results

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