Pack rust

Pack rust is a form a localized corrosion typical affecting joints and crevices in steel components that are exposed to atmospheric conditions. The expression is often used in relation to bridge inspection to describe rust "packing" between parallel steel plates. Pack rust is usually characterized by buckling in one or more parallel metal plates.

If not properly addressed, pack rust can compromise the integrity of a structure, weakening beams and cover plates and over-stressing fasteners. Further,

Treatment
If corroded joints are welded shut or otherwise sealed without treating existing rust, the corrosion process may actually accelerate. Rust cannot be stopped by sand blasting to remove existing paint and applying coatings or sealants alone. Sand blasting will not remove the corrosion products in inaccessible areas. The active corrosion cells (within crevices) include water, oxygen and hydrochloric acid (and sometimes also nitric and sulfuric acids). The corrosion process will not be stopped by caulking or sealing up of the exterior edges of the connections. Rust must first be removed, then a corrosion inhibitor should be applied to stabilize the remaining metal before a protective coating is applied.

Distortion resulting from pack rust can be removed using a rivet hammer with a modified snap and a buffer plate placed between the snap and the buckled plate. To correct the distortion, heat is applied to the buckle with an oxygen fuel torch with a Rose Bud tip. The buckle is heated to just below 800 degrees Fahrenheit and a buffer plate is placed over the heated buckle. The hammering action of the rivet hammer drives out the rust from the buckle and flattens the plate. The heating and hammering is sequenced to prevent distortion. A few buckles are removed in one area and then the operation is moved to another buckle further away to keep from concentrating too much heat in one area.