Corrosion

Technical Bulletin: Minimizing Corrosion in Stucco Wall Systems

March 24, 2015

To reduce the risk of corrosion of metal lath and corner reinforcement in Stucco Wall Systems, careful selection of materials, proper application of details and cement stucco coats, proper embedment and proper building design must be maintained. Corrosion cannot occur without the presence of water, therefore, proper details of avoiding horizontal surfaces, and timely repair of cracks must be maintained. Acrylic finishes, while designed to avoid water intrusion, act to contain moisture ingress and keep the wall from drying, sometimes enhancing corrosion.

CTS Cement Manufacturing Corporation recommends the use of plastic or stainless steel corner aid when acrylic finish is used over Rapid Set® Eisenwall® cement and Stucco Mix basecoats, or in high moisture/humidity environments. This practice is also specified by the following associations and manufacturers:

  • Technical Services Information Bureau — Technical bulletin, July 2008; “Plastic nose beads are recommended for cement plaster with an acrylic finish coat and a wire nose is recommended for cement-based finish coats. Cement finish tends to flake off of the plastic nose. Acrylic finish coats can hold moisture longer and have been known to cause rust when used in damp coastal locations”.
  • Northwest Walls and Ceiling Bureau — Article, February 20, 2006; “The type of finish chosen will have an impact on the corner bead or corner aid that should be used. In particular, one should be concerned about the type of nose on the corner bead. Acrylics tend to work best with a PVC or plastic nose type of corner aid. The acrylic bonds to the plastic and it eliminates the rusting that can occur with wire nose beads and an acrylic finish coat. With traditional cement finish stucco, the nose should be wire. Cement will stick to the PVC but not bond. It will eventually pop off the corners. Also consider the PVC is flexible and the cement finish is not.”
  • Omega Products International — Technical Bulletin, April 2008; “Selection and Installation Recommendations for Corner Aid to Reduce the Risk of Rusting: Omega Products recommends the use of PVC, zinc, or anodized aluminum external corner aid products when one or more of the following conditions is present: High humidity environments, High salt environments, Acrylic finish coats utilized.”

Performance and corrosion resistance of wire lath and corner aids depends on the quality of the steel and the quality and thickness of the galvanizing or zinc coating, as well as proper embedment of the metal lath and accessories. If using metal corner aid (and when selecting all metal lath), be sure to select high quality products from a reputable manufacturer. CTS has conducted corrosion testing on metal lath and corner products available in the market and found they are not all of equal quality; corrosion performance can vary widely.