Weathering vs. Cleaning Wet Storage Stain
Question: Is it necessary to Clean Wet Storage Stain?
Wet storage stain (sometimes erroneously referred to as “white rust”) is the commonly known problem of white surface oxides formed on newly galvanized steel due to stacking items too tightly and/or storing in a humid environment. Wet storage stain occurs when the natural weathering process is interrupted or paused by poor storage conditions which do not allow the part to experience free flowing air or dry cycles. Eventually these conditions lead to an excessive buildup of zinc oxide (powdery to the touch) and zinc hydroxide (tacky or gelatinous to the touch).
Zinc oxide and zinc hydroxide are many more times voluminous than the zinc metal consume from the coating, causing many to believe the appearance of wet storage stain is more alarming than it really is. Although not aesthetically pleasing, the formation of light or medium wet storage stain on the surface does not require removal for the hot-dip galvanized coating to perform as intended. Once the parts are installed, they will become exposed to free-flowing air and natural wet/dry cycles which allow the part to naturally weather. Furthermore, weathered galvanized surfaces which have already formed their normal protective layer of zinc corrosion products (the zinc patina) are seldom attacked.
Specifying removal of light and medium wet storage stain will re-start the weathering process timeline and is only recommended for projects where heightened aesthetics are critical. Where necessary, caution should be taken to not damage the hot-dip galvanized coating with chemical cleaning agents, and the recommended cleaning procedure is available in the AGA publication Wet Storage Stain. An exception is made for cases of heavy wet storage stain which must be remedied due to unacceptable coating thickness or the presence of bare spots.
© 2021 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.