ABSTRACT
After reviewing the fundamentals of corrosion and the numerous factors affecting corrosion in building water systems, through case studies this paper explores four specific modes of failure of significant impact on all types of buildings. First will be dezincification of brass, with a focus on valves, and consequent service outages and property damage due to malfunctions and leaks. Next are the problems that plague galvanized pipe, including case studies involving potable water distribution and fire suppression systems, focusing on flaws in pipe manufacture and possibly insufficient galvanizing thickness Third is graphitization of cast iron, in fittings, pumps, water mains and sewer stack pipes, and how this condition can go unrecognized for years, until catastrophic failure results in service outages and extensive property damage. The fourth is pitting corrosion of stainless steel due to the not-so-obvious effects of manganese.
INTRODUCTION
Building water system components are at increasing risk of damage due to corrosion and metal loss as they approach the end of their design life, or when accelerated corrosion develops within the system. The purpose of this paper is to provide practical information on the risk associated with corrosion activity in building water systems, and includes several case studies that detail investigations into corrosion failures.
Corrosion can be defined as the deterioration of a metallic material that results from a reaction with its environment. The corrosion of metals consists of three essential mechanisms: (1) the reaction at the anode; (2) the reaction at the cathode; and (3) the conductive flow of electric charge (a) as ions through the electrolyte, cations to the cathode and anions to the anode; and (b) as electrons moving from the anode to the cathode through the metal. If any of these mechanisms is inhibited, accelerated corrosion is suppressed.
In building water systems, the metal components of the system provide the anodes and cathodes. The electrolyte is water, but far more than H20, it is that simple molecule plus all the substances that are dissolved and suspended in it. Those substances determine whether the water causes accelerated corrosion of certain alloys. Assessing corrosion risk should be made an essential part of prioritizing planning repair and replacement projects. The impact of leaks is one parameter. For example, a leaking ¼” valve on a domestic water pipe in the basement poses far less risk than a perforating 6” fire suppression pipe on the 12th floor. Applying corrosion risk assessment makes for better decisions.
