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Rapid Set® Eisenwall® — TECHNICAL BULLETIN #3
Relative Corrosion Performance of Stucco Reinforcement — January 6, 2011
A recent increase in the occurrence of rusting and corrosion in metal lath and corner reinforcement has led CTS Cement Manufacturing Corporation to take an in-depth look into the relative performance of these products. A University of California-backed study has found that the corrosion resistance of the metal wire used in stucco reinforcement is directly related to the thickness of the wire's zinc coating.
Most steel wire is protected from corrosion by application of a zinc coating, either by traditional zinc galvanizing (electroplating) or hot-dip galvanizing. The purpose of the zinc coating is to prevent the corrosion of the underlying steel. Zinc is less resistant to corrosion than steel, so the coating is eroded away first, delaying any corrosion to the steel. As long as there is zinc still present, the steel in that area will be protected. The length of time or degree to which the zinc coating can protect the steel, therefore, is directly related to the amount of zinc present, which is determined by the thickness of the coating.
Several samples of zinc-plated steel corner reinforcement were tested for resistance to corrosion using a neutral salt spray test method in accordance with ASTM B-117-09. The samples were rated for performance according to the degree of corrosion present. The same samples were also tested to determine the thickness of their zinc coating per ASTM B-487. The results indicate that the resistance to corrosion is directly related to the thickness of the zinc plating. The samples with the thickest coating performed best, and samples with the thinnest coating performed worst. Structa-corner reinforcement manufactured by Structawire Corp. exhibited the thinnest zinc plating and worst corrosion resistance performance. In some cases the thickness of the coating was measured as less than half the thickness of the leading brand. For this reason CTS recommends against the use of Structawire products and their use will void our warranty.
Some stucco reinforcement manufacturers have experimented with alternative metal coatings containing chromium. Chromium has a higher resistance to corrosion than steel, therefore these coatings are not sacrificial and can only protect the steel when intact. While these coatings perform well under laboratory conditions, when scratched in real world use, they actually accelerate the corrosion of the underlying steel in the damaged area. CTS does not recommend the use of stucco reinforcement with these coatings and their use voids our warranty. CTS is also concerned with the use of stainless steel corner reinforcement, as the contact with dissimilar metals will initiate corrosion, and therefore is not recommended.
To best guard against corrosion, avoid water intrusion, ensure proper embedment of the wire reinforcement, and use quality galvanized, class three hot dipped galvanized, or plastic-nose corner reinforcement. In coastal or high moisture environments, class three hot dipped galvanized or plastic-nose corner reinforcement is required.