January 2014 Spray 23 NAA AD thickness. The porosity and nonuniform thickness causes non-uniform diffusion of materials through the corrosion product layer. The nonuniform diffusion over the surface of the package metal could also cause pitting corrosion under the general corrosion product layer. Holes in tin coatings Tinplated steel is steel sheet metal with a thin layer of metallic tin on both sides of the sheet. The tin layer is not perfect and has holes that expose either the iron-tin alloy layer between the tin and the steel, or the holes expose the base steel. Certain formula compositions will cause the tin layer to pit the steel base through the coating to the package metal under the coating. In some instances, the material passing through the coating could be significantly more corrosive than your formula and cause pitting corrosion. The chemical composition of your formula ultimately will determine whether or not spray package metal pitting corrosion is caused by an inclusion, crystal defect, general corrosion, holes in tin or coatings that become semi-permeable membranes. Please send your questions/ comments/suggestions to rustdr@ pairodocspro.com. Back issues of Corrosion Corner are available on CD from ST&M. Thanks for your interest and I’ll see you in February. SPRAY wherever steel is exposed by a hole in the tin layer. My experience has been that this type of pitting corrosion is not as common as other types of pitting corrosion. Transforming of an internal coating into a semi-permeable membrane Coatings are actually not barriers between your formula and the spray package metal under the coating. Water, ions and other formula ingredients can absorb into the coating, thereby changing the properties of the coating. A saturated coating could become a semi-permeable membrane that subsequently allows only specific formula ingredients to pass freely
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