Repair a Galvanized Coating - Hunter Galvanizing

Coating Life

The immediate environment in which it is located determines the corrosion rate of any metal. Corrosion rates of hot-dip galvanized coatings in different environments have been extensively researched and documented worldwide and have been found to corrode between ¹/₁₇ and ¹/₈₀ slower than uncoated steel.

Coating life is directly proportional to the coating thickness of the zinc, and as corrosion rates in specific environments are known, the life expectancy of the hot-dip galvanized coating can be estimated.

The following information regarding general atmospheric conditions is supplied as a guide only. Specific microclimates within a general area will influence the rate of corrosion further and must also be taken into consideration. These issues can be discussed further with Hunter Galvanizing staff.

Warm, dry atmosphere

The anticipated life of galvanized coatings in an arid dry environment is long term. Protection may continue indefinitely as zinc stability is very high resulting in minimal coating loss. Zinc coatings on steels of thicknesses of greater than 6mm may remain stable for 100 years or longer.

Rural areas

Coating life may be affected by aerial spraying of fertilizers or insecticides. In dry form these elements will pose little threat, however in solution, fertilizer and insecticides will attack galvanized coatings. Long-term protection can be expected if not subjected to hostile chemicals with a coating loss of about 1-3 microns per year. Based on steel thickness greater than 6mm it is plausible to expect coatings to remain effective 25 to 85 years.

Industrial areas

In light industrial areas, hot-dip galvanized coatings will generally perform well with coating loss average of 3-5 microns per year. Based on steel thickness greater than 6mm and subject to exposure to adverse contaminants, 15-30 years may be achieved. High levels of sulphuric gases and chemicals located in some heavy industrial areas will increase coating loss to about 5-8 microns per year. In this environment, a duplex coating system of hot-dip galvanizing and paint may prove beneficial.

Coastal areas

Corrosion rates are higher in the presence of salt air. Hot-dip galvanized coatings perform well in comparison to other protective systems; however, duplex coating systems of hot-dip galvanizing and paint provide the optimal protection. Coating loss will average between 5–15 microns per year subject to the proximity to the ocean and levels of rainwater to wash marine salts from the coating surface. Life span of coatings may range between 5-10 years based on steel thickness greater than 6mm.

Coating Defects

Ash Marks

Zinc oxide, better known as zinc ash, is the result of immersing a base metal into molten zinc which forms on the kettle surface during the galvanizing process. When withdrawing the product from the zinc bath, paddlers will skim away this ash leaving an almost mirror type finish to the zinc surface for the work to protrude through. However due to shape, size and complexity of fabricated steel items, zinc ash may become trapped or stick to the coating surfaces. This zinc ash in most cases will leave a yellow to light brown stain and may appear dull or built up in some areas of the product.

Acid Leeching

Overlapping surfaces that have been stitched welded are susceptible to acid leeching. This is the result where pre-treatment liquids become trapped and crystallise leaving anhydrous residues during the galvanizing process. Overtime water is absorbed from the atmosphere and weeps out to the immediate surface area. The resulting rust like stain is usually reddish/brown in colour and is outside the control of the galvanizer.

Bare Spots

In most cases bare spots are the result of some type of contaminant that lays within the steel surfaces that prevents the metal from being cleaned. This can range from a number of things such as cutting fluids, oils, welding slag, adhesives and paints to name a few. If left untreated, the presence of such residuals will create what is known as bare spots, or an uncoated galvanized coating. Depending on their size, bare spots may be treated with a zinc enriched epoxy coating, however if the area is small enough this can be left untreated as the cathodic protection provided by hot dip galvanizing will take effect.

Dross Pimples

Dross is a byproduct from the galvanizing process and is formed through a metallurgical reaction when free iron particles come in contact with molten zinc. The rouge iron particles are mainly the result of pre-treatment process residue which is transferred to the zinc kettle during the galvanizing process. Dross will usually display as a pimple like finish on the galvanized coating and will not affect the cathodic properties of the coating.

Drainage spikes

During the withdrawal stage of the galvanizing process, molten zinc runs to low points on the product while suspended under a headframe. Pending steel thicknesses, some sections will take longer to solidify than others often creating sharp drainage spikes. These sharp points are removed during the fettling stage.

Passivation Staining

Passivation is a process of immersing freshly galvanized product into a chemical solution for a short period of time. This in return will give the pure zinc outer layer a protection film to help prevent white rust, wet storage stain or light white oxidization from occurring. This process can often leave a yellow/greenish stain and is more noticeable on thicker steel sections. This colouration is no means for rejection and will become less visible over time with weathering.

Paint Markers

Quiet often paint markers are used during storage for steel merchants or the fabrication stage to identify steel section, cut sizes or quantities of product available. These markers are often oil based, which can have an adverse effect of the finished galvanized coating. Paints and inks can be difficult to remove during the ‘Pickling’ pre-treatment process, resulting in sections of the finished galvanized product being uncoated where the paint has simply burnt off throughout the galvanizing process. Hunter Galvanizing recommend using water based pen markers, or other identifications such as welded numbers, stamped sections or even stamped identification tags.