Practical Guidelines for the Inspection and Repair of Hot Dip Galvanized Coatings
Information courtesy of: HOT DIP GALVANIZERS ASSOCIATION SOUTHERN AFRICA
Appearance
The ability of a hot dip galvanized coating to meet its primary objective of providing corrosion protection should be the chief criteria in evaluating the coating's acceptability.
The basic finish requirements of the hot dip galvanized coating are that it be:
relatively smooth,
continuous,
free from gross imperfections,
free from sharp points (that can cause injury), and
free from uncoated areasC.
C To be essentially free from uncoated areas is best described in SABS 763 4.3.2 b). " The area of an individual bare spot or thin area shall not exceed 5mm2 . The combined area of bare spots or thin area shall not exceed 25mm2 per metre of length or per square metre of surface of an article."
The above points are of particular importance when a subsequent organic coating (duplex coating) is to be applied. Smoothness and lack of roughness achieved by mechanically wiped products, such as continuously galvanized sheeting or wire, are not to be used as the criteria for accessing batch hot dip galvanized products. Roughness and smoothness are relative terms where the end use of the product must be the determining factor in setting tolerances.
The hot dip galvanized coating should be continuous to provide optimum corrosion protection.
Handling techniques for hot dip galvanized articles may require the use of chain slings, wire or other holding devices to immerse the material into the galvanizing bath if suitable lifting fixtures are not provided on the item. Chains, wire and special jigs used to handle the items may leave a mark on the hot dip galvanized item. These marks are not always detrimental and reason for rejection. Should these marks, be greater than 5mm per chain mark and expose the bare steel, suitable repair should be carried out using the procedures indicated in ISO 1461. See also "Coating Repair Procedures".
Differences in the lustre and colour of hot dip galvanized coatings do not affect corrosion resistance and the presence or absence of spangle has no effect on coating performance. The well-known spangled effect found on some hot dip galvanized surfaces is simply a factor of primary crystallisation. It is chiefly dependant upon the zinc bath chemistry, the rate of cooling, the method of pickling, the steel chemistry, and the thickness of the coating. In fact, dull grey or patchy matte grey hot dip galvanized coatings give service lives equal to or greater than bright or spangled coatings. Variations in coating appearance or finish are important only to the extent that they will affect corrosion performance or the intended use of the article. The primary function of a hot dip galvanized coating is corrosion protection. Specific requirements beyond the standard set out in ISO 1461, shall be communicated to the galvanizer in writing or discussed at the contract review, prior to the work being hot dip galvanized.
The information supplied may include -
a) The composition and any properties of the metal that may affect the quality of the hot dip galvanized coating.
b) Identification of significant surfaces. See page 15B for definition.
c) A visual standard should be established if a special finish is required.
d) Any particular treatments that are required or not required before or after the process.
e) A variance in coating thickness. See notes below table 3.
f) The acceptable method of repair, if this is found to be necessary - see also "Coating Repair Procedures"