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Stress Corrosion Cracking of Carbon Steel Fasteners in HVAC System

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At the construction site of a new building, several unexpected failures of friction-type fasteners used to support the HVAC system were experienced. The thin sheet metal fasteners all failed in the same area, a bend that is under high stress when installed. Excessive corrosion was present on the fasteners and also occurring on electrical conduit, I-beams, sprinkler fittings, and other metal structures.

An on-site examination indicated that the common factor for corrosion was the presence of fire retardant material, either in immediate contact or in close proximity to the attacked item. In some locations the fire retardant was obviously wet with corrosion actively occurring underneath the fire retardant, Figure 1. To ascertain the probable cause of the ongoing corrosion of the HVAC steel fasteners, a laboratory testing program was carried out.

Electrochemical tests indicated water extracts from the fire retardant were very corrosive. Immersion tests to verify this were conducted. Immersion tests consisted of immersing new, unused fasteners in actual fire retardant material in the as-is conditions, i.e., as scraped off a wet area around an I-beam. The samples were packed in the fire retardant and kept for 92 hours at room temperature, 104F and 140F. Similar tests, at elevated temperatures, were done in tap water, which served as a control.

The laboratory tests were successful in duplicating the corrosion observed at the construction site. The wet fire retardant is very aggressive, far more aggressive than ordinary tap water, causing general and pitting attack which, when combined with the stresses acting on the fasteners, led to stress corrosion cracking.

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