Masonry Magazine April 1988 Page. 23
Corrosion
General. Awareness of possible corrosion problems in metal-tied masonry walls has increased due to corrosion damage found on reinforcement in concrete highway pavements, bridge decks and some masonry structures. The potential for corrosion problems in masonry has increased as construction and design philosophies have changed and as environmental conditions have changed over the last decades. These changes include use of thinner masonry walls and masonry veneers that are most susceptible to water penetration, increases in atmospheric pollutants, use of accelerators containing calcium chloride, increased use of insulated cavities (resulting in the relocation of the dew point within the wall section) and combinations of different metals in brick veneer wall systems. This list is not all-inclusive; corrosion potential can also be affected by the function of a structure, geographic location, compatibility of construction materials, detailing and workmanship.
Corrosion Protection. In order to provide corrosion protection, environmental factors must be controlled or metals used in construction must be protected. Conventional corrosion protection methods attempt to protect metals embedded in masonry by isolating them with impervious coatings (barrier protection), by using metals that are corrosion-resistant, or by providing cathodic protection in which one metal becomes sacrificial to protect another.
Galvanizing
Galvanizing (zinc-coating) provides resistance to corrosion by two methods. First, the zinc coating acts as a barrier shielding the underlying steel from corrosive action. Second, it acts as a sacrificial element that is consumed before the base steel is attacked. This sacrificial nature protects the base steel at scratches and holidays in the zinc coating caused by fabrication, handling or installation, until most of the adjacent zinc coating is consumed. Studies have shown that the protective value of zinc coating is proportional to its thickness. Thus, for longer periods of protection, a thicker zinc coating is required. Also, when the protective zinc coating is depleted, the corrosion of the base steel will progress as if no galvanizing were present.
Two methods of galvanizing are used to protect metal masonry ties: mill galvanizing and hot-dip galvanizing. Mill galvanizing takes place after steel wire or sheets have been processed to their specified dimensions and prior to fabrication of the tie. During the mill galvanizing process, zinc can be applied in a variety of thicknesses, as shown in Table 4. Hot-dip galvanizing is performed by dipping completely fabricated assemblies into molten zinc until a specified amount of zinc is bonded to the base metal. Hot-dip galvanized coatings are typically thicker than mill galvanized coatings and therefore, provide longer periods of protection.
Hard-Drawn Copper-Clad Steel Wire
Metal ties made from hard-drawn copper wire are available only as unit ties since copper cannot be effectively welded to fabricate continuous joint reinforcement. Due to copper's electrochemical properties, it only provides barrier protection and does not provide sacrificial protection. Therefore, if nicks penetrate the copper cladding, the base steel can experience corrosion.
Stainless Steel
Stainless steel ties are often specified for use in very corrosive environments. Stainless steel ties are specified under ASTM A 167 and are generally made from one of the austenitic stainless steels. Stainless steel resists corrosion well; however, if in contact with carbon steel, a galvanic cell can result and actually increase the potential for corrosion. For this reason, combining stainless steel ties or screws with carbon steel or galvanized steel components is not recommended.
Fusion-Bonded Epoxy
Epoxy coating is a relatively new process used to provide corrosion protection for metal ties. The process has been adapted from epoxy-coated reinforcement bars used successfully in concrete systems with severe environmental exposures. Epoxy coating provides protection by acting as an impervious barrier. The epoxy coating is bonded to the base steel by a heat-induced chemical reaction through which a chemical and mechanical bond is formed. The combination of the two types of adhesion helps to prevent cracking of the coating due to handling, installation or stress reversals. The epoxy coating is not sacrificial like zinc; therefore, nicks and holidays in the coating can lead to corrosion of the base steel.