It is my opinion that it is incorrect procedure to specify or install steel corner beads with solid metal flanges on exterior walls of portland cement plaster, or where interior walls may be exposed to water. An exception would be: if the device has a plastic nose and the flanges are of open wire or expanded metal design. I have observed, on many occasions, that corner beads with galvanized steel nose and solid metal flanges develop serious problems when installed in conjunction with cement plaster that is exposed to moisture. Some types of observed problems will be described below. The problems develop when steel corner beads are installed as reinforcement of vertical and horizontal arisses of outside corners of exterior plaster walls; at tops of parapet walls; on plastered balcony railings; on planter boxes; at tops of retaining walls and other construction features. Some architects consider steel corner beads to be appropriate reinforcement for any or all of the features and situations described above. I disagree with that premise, unless the bead has a plastic nose and open-flange design.
Corner beads installed at exterior portland cement plaster corners may be appropriate in locations where moisture in the form of rain or melted ice or snow is rare, such as in Saudi Arabia, Israel and other parts of the mid-East. Corner beads may be appropriate in conjunction with cement plaster in locations where moisture in the form of rain, wash water or melted ice or snow will not contact the devices.
Some types of problems which may affect steel corner beads exposed to water do not occur where cornerite (typically CornerAid) has been employed as corner reinforcement and has been installed correctly. (See my article on CORNERITE.) A brief description of problems encountered with corner beads which were exposed to weather, on cement plaster jobs, is provided below.
Solid metal flanges of corner beads often introduce several problems on a completed project, to be described here.
a. caliper of the decorative layer of stucco over the solid flange necessarily is quite thin and not strong enough to resist stress because of rapid drying of the very thin lamination over the metal base;
b. the relatively thin decorative coating over steel flanges may delaminate from the solid metal wings and expose the metal to additional corrosion;
c. as flanges rust, production of rust results in development of considerable pressure due to the change in structure. That pressure would be especially adverse to the relatively thin coat of stucco on the flange and could be expected to cause the coat of stucco to release from the metal flange. Witness the manner in which rusting rebar in concrete sometimes cracks concrete. Note that only a thin coat of stucco normally is applied over the flanges of the metal trim. Oftentimes, that coating may range in thickness from only about one-sixteenth inch to about one-eighth inch. As the flanges of the metal trim rust, that relatively thin overlay may be ruptured and delaminate from the metal wing. That action would exacerbate the corrosion problem. All concerned must realize that rain water no longer can be considered to be relatively pure water, but often carries chemicals in the droplets of water, which are corrosive;
d. very thin fissures frequently open at the interface between the metal bead and adjacent plaster after application, as the metal expands and contracts with changes in temperature;
e. a very fine crack may develop in plaster along the outer edge (away from the bead) some time in the future, again because of differential expansion and contraction of the metal device, as well as movement in the building;
f. a rust stain may form along the nose of the metal bead, because of exposure to moisture and air, and subsequently create a stain on adjacent plaster;
g. the relatively thin coat of stucco that overlies the solid metal flange may actually separate from the wing of the corner bead and fall away; and
h. a corrosion chamber typically occurs behind the bead and solid flanges of the corner bead because of inability to insert plaster behind that device, through the solid flanges.
In addition to normal rainfall, the environment near an ocean or industrial area typically is especially adverse to metal, and, unless metal is protected against salt air and chemical-laden atmosphere, corrosion should be expected.
Formation of rust on metal beads is accelerated during the process of application of cement plaster. As plaster is placed, troweled and floated, the surface of metal trim installed at arisses is scratched and abraded. As the blade of the trowel is scraped over the galvanized surface of the corner bead, the protective coating of galvanizing suffers loss of value. Additionally, the galvanized coating is abraded by grains of sand in the plaster. A result is that the protective coating is removed, or scraped too thin to provide necessary protection from corrosion. Hence, the delayed formation of rust, which stains the arris or corner of the building, plus plaster below the corroding metal.
It is my opinion that exposed steel corner beads installed at exterior locations in cement plaster walls could rust, eventually, under normal circumstances, in any climate. Unfortunately, abrasion of galvanizing from the surface of steel would accelerate corrosion. Exposure of steel to moisture, air and alkaline environment should be expected to induce chemical reaction.
Cornerite, generally known as "CornerAid", when installed correctly, is completely embedded in cement plaster as reinforcement at outside orners, and thus is protected from exposure to moisture and air; two factors which, in combination, cause steel to rust.
In summary, I recommend specification and installation of cornerite rather than corner beads as reinforcement of outside corners of portland cement plaster exposed to moisture, unless the corner bead has a plastic nose and open flanges.