Practical Guidelines for the Inspection and Repair of Hot Dip Galvanized Coatings

Information courtesy of: HOT DIP GALVANIZERS ASSOCIATION SOUTHERN AFRICA

Thickness and Uniformity

The thickness of the hot dip galvanized coating is the primary factor in determining its life under given service conditions. The thicker the coating, the better corrosion protection it offers. For most atmospheric conditions, the relationship between corrosion protection and coating thickness is approximately linear, i.e., the service life is doubled if the coating thickness is doubled.

The factors which influence coating thickness are a combination of several variables. The galvanizer can alter zinc temperature, time of immersion and rate of withdrawal from the molten zinc. These factors can be used to marginally alter coating thickness, particularly with reactive steels. The formation of the protective iron/zinc alloy layer, is a diffusion process. As with all diffusion processes, the reaction proceeds rapidly at first and slows down as the layer becomes thicker. Higher bath temperatures and longer immersion times will generally produce thicker alloy layers.

The thickness of the outer zinc layer is largely independent on immersion time. Coating thickness depends on the rate of withdrawal from the zinc bath and the extent of drain-off. A fast rate of withdrawal causes an article to "carry out" more zinc. This results in a heavier coating, although the distribution of the zinc layer may be uneven. Wiping of the coating during withdrawal materially reduces its thickness and thus its protective value, and is avoided except where such treatments may be required for the smoothness demanded by conditions of service such as for pipe and conduit.

Tubes hot dip galvanized in accordance with EN 10240 utilise steam blowing internally and an air ring externally, to remove excess zinc. Some variation in internal coating condition is acceptable within 50mm of the tube ends. With fabricated articles, local differences in the drain-off, because of the shape of the article and the angle at which different surfaces leave the bath, may result in some variation In coating thickness.

TABLE 2.

* Local coating thickness is defined as the mean of the measurements taken within a specific reference area. Mean coating thickness is the control sample number average average of the local coating thickness values from each reference area.

• Thickness legend - 3 ≤ t < 3 = thickness less than 6mm but greater and equal to 3mm

• Where only one reference area is required according to size of the article, the mean coating thickness within that reference area shall be equal to the mean coating thickness given in the above table.

Other factors influencing the coating thickness may be beyond the control of the galvanizer.

The chemical composition of the steel plays a major role in determining the thickness and appearance of the final coating. Certain steel compositions tend to accelerate the growth of the iron/zinc alloy layer so that the hot dip galvanized coating may have a matte finish with little or no outer zinc layer. This coating also tends to be thicker than the typical bright hot dip galvanized coating. The galvanizer's control over this condition is limited. Steels containing the elements, phosphorus in excess of 0.03%, or silicon from 0.05% to 0,12% and above 0,3% or combinations of both elements, are particularly prone to heavier coatings consisting of mostly iron/zinc alloys. Due to the iron/zinc alloy growth in these coatings, extending to the outer surface, the final appearance of the coating can be dark grey in colour distributed evenly or unevenly over the surface. The surface condition of the steel before hot dip galvanizing also affects the thickness and smoothness of the final coating. Steels which have been abrasively cleaned, or left to weather for some time prior to hot dip galvanizing, can produce coating thickness' double those produced on steels that have been only chemically cleaned.

The mass, shape and amount of cold working of the item being hot dip galvanized also affects coating thickness and uniformity. When a fabricated article has both heavy and light sections, a difference in coating thickness between the sections may result. The immersion time will vary according to the relationship of the surface area of an item to its mass. The galvanizer has little control over this situation. Combining heavy and light sections may also result in unacceptable distortion (refer to HDGASA wall chart, "Design for Hot Dip Galvanizing").

Since the time to the appearance of first rusting of the base steel is usually determined by the thinnest portion of the coating, an evaluation of galvanizing quality must take into account both the minimum thickness of the coating and its distribution. Specifications for hot dip galvanizing recognise that variations in coating thickness are inherent in the process. The minimum thickness of the zinc coating is generally specified as an average thickness of the specimens tested and/or a minimum thickness for any individual specimen.

When measurements are made to determine the distribution of thickness of the hot dip galvanized coating, 5 or more coating thickness readings should be taken in each reference area. Reference areas should be taken approximately 100mm from the ends of the article to avoid end effects. Usually the end of an article which leaves the bath last will carry a thicker coating. This is particularly so towards the edge, where, at the time of drainage, the last few drops of zinc tend to agglomerate as a result of surface tension.

The minimum coating requirements specified in ISO 1461 for different material thickness' and classes of work are summarized in table 2 and table 3. For comparison purposes, the above tables compare thickness equivalents to the old SABS 763 specification. Table 4 indicates the minimum coating requirements specified in EN 10240 for different classes of coating.

The specification does not stipulate a maximum upper coating thickness limitation. Excessively thick coatings on threaded articles are undesirable. In order to ensure effective tensioning, the coating thickness on fasteners should not exceed a maximum of 90pm, this applies particularly, to high strength bolts and nuts.

TABLE 3.

* Local coating thickness is defined as the mean of the measurements taken within a specified reference area. Mean coating thickness is the control sample number average of the local coating thickness values from each reference area.

• Thickness/diameter legend - 6 ≤ a < 20 = diameter less than 20mm but greater and equal to 6mm.

• Where only one reference area is required according to size of the article, the mean coating thickness within that reference area shall be equal to the mean coating thickness given in the above table.

Variance in coating thickness. A requirement for a thicker coating (25% greater than the standard in table 2) can be requested for components not centrifuged, without affecting specification conformity.

NOTE: Where steel composition does not induce moderate to high reactivity, thicker coatings are not always easily achieved. Thicker coatings are more resistant to severe environmental conditions, but can be more brittle and may require special handling. The efficacy of corrosion protection of a hot dip galvanized coating (whether light or dull grey) is approximately proportional to coating thickness.