Atmospheric Corrosion and Weathering Behavior of Terne-Coated Stainless Steel Roofing

Terne (Pb-Sn) coated stainless steel has been utilized as a roofing material for a number of commercial, institutional and residential structures throughout the United States and elsewhere. As the base metal, stainless steel offers exceptional long-term resistance to atmospheric corrosion. While not requiring a protective coating, the application of terne coat is intended to provide the stainless steel with the aesthetic appeal of dull gray, aged metal roofing found on some period structures. Within the recent ten years, several cases of reddish-brown discoloration of terne-coated stainless steel roofing have been documented. These undesirable changes in appearance have occurred in benign rural and more aggressive marine locations. A review of the corrosion literature provides historical evidence that terne coat, and lead, are more prone to corrosion in marine and rural sites than in more urban and industrial areas. This difference in behavior can be related to the absence of atmospheric pollutants, e.g., sulfur oxides, which otherwise enable terne coat and lead to develop a protective sulfate layer. X-ray diffraction analysis has determined that the reddish-brown discoloration found on roofing in some unpolluted atmospheres is lead oxide. This paper reviews the occurrence and progression of discoloration on installed terne-coated stainless steel roofing, and that observed on test panels of the roofing material exposed at the LaQue Center for Corrosion Technology, Inc., marine atmospheric test sites at Kure Beach, NC. In addition to the issue of discoloration, mass loss and metallographic examination have quantified the loss of terne coat due to corrosion and weathering.