Selection and Protection of Metals in Building HVAC Systems

Freedman, Arthur J. (Arthur Freedman Associates Inc.) | Hoffman, Richard W. (Hoffman & Feige Inc) | Cotton, Irvin J. (Cotton & Associates Inc.)

OnePetro 

ABSTRACT
An understanding of the characteristics and the handling and protection requirements for the materials used in building water systems is essential to reliability of the building systems. This requires selection of the proper metals and alloys for proper and uninterrupted service through the life of the system. System reliability is also based on effective water treatment and service programs plus ongoing monitoring to minimize corrosion, deposition and microbiological problems that can impact system performance.

INTRODUCTION
An active building has many service water systems in use for HVAC purposes and many different materials in those systems. Piping, tankage and equipment systems in buildings are a major part of this support system in every building. Without the HVAC and utilities equipment and associated piping systems, buildings could not cool, heat, ventilate, humidify, supply water, supply energy, remove waste products, or be protected from fire. In short, the buildings as we know them, could not be. It is critically important to select proper materials and coatings for these applications, effectively treat or protect these systems and evaluate the condition of water-carrying and fuel/steam/process pipes in buildings. The object of these procedures and evaluations is to determine the performance of the systems and to determine the extent of any deterioration that may have occurred, establish the cause(s) of this deterioration, and provide information needed to make informed decisions concerning future operations to protect the building. In this paper, a basic educational summary, we will review properties and corrosion behavior of metals and materials commonly used in buildings, types of corrosion, methods of chemical water treatment monitoring and service programs, and inspection requirements necessary to maintain cost-effective reliability.

BUILDING SYSTEMS MATERIALS
The major water utilities systems in any building consist of cooling systems (i.e., piping, condensers, cooling towers, storage tanks, chillers) and heating systems (i.e., steam system, hot water piping, condensate piping and boilers). The predominant piping material in HVAC systems is mild steel. Copper is commonly used for heat transfer tubes because of its heat transfer properties and its inherent corrosion resistance. Other materials commonly used are: brasses and other copper alloys, cast irons (gray, ductile and malleable) for valves and fittings, stainless steels, and plastics. Table 1 shows general application of materials in building water systems. The following discussion covers general types of corrosion that can impact these materials, and a detailed review of specific material properties and corrosion problems for each metal.

TYPES OF CORROSION
There are a number of different types of corrosion:

Uniform or general corrosion takes place at a generally equal rate over the entire surface and usually is corrosion resulting from acids in a water environment on metals having minimal to no protective properties.

Pitting corrosion is nonuniform, occurs at a localized anodic area, may be sharp and deep, and is an example of an environment offering some protective properties but not complete corrosion inhibition. It is associated with concentration-cell corrosion, galvanic corrosion, and crevice corrosion.

Galvanic corrosion occurs as a result of the exposure of two dissimilar metals in the same environment and is most noticeable when they are directly connected. On the basis of the relative potential of the two metals, the one less noble will corrode (the anode), leaving the other more noble metal (the cathode) intact, thus offering protection for the catho