Galvanised Steel - Causes and Defects in Galvanised Coatings

Topics Covered

Background

Defects in Galvanised Coatings

Ungalvanised Weld Areas

Dark Staining Adjacent To Welds

Dull Gray or Mottled Coatings

Dross Pimples/Inclusions

White Storage Staining

Ash Staining

Striations and General Surface Irregularities.

Runs, Drainage Spikes and Puddling

Bare Patches

Rust Staining

Delamination

Black Spots

Spangled Coatings

Background

There are a number of common types of defects arising from the hot dip galvanising process. An explanation of the causes of defects and variations in appearance follows.

Defects in Galvanised Coatings

Ungalvanised Weld Areas

Coating misses on weld areas are caused by the presence of welding slag on the welds. All welding slag must be removed by the fabricator prior to despatch to the galvaniser.

Dark Staining Adjacent To Welds

Preparation chemicals entering unsealed overlaps or through poor quality welds boil out of the connection during galvanising and cause surface contamination and coating misses during galvanising.

Also, anhydrous fluxing salts left in the connection will absorb atmospheric moisture and leach out onto the adjacent galvanised surface. Leaching of these salts will eventually reach equilibrium. Affected area should be washed clean to remove slightly corrosive leachate.

Dull Gray or Mottled Coatings

Reactive steels will generate thicker galvanised coatings that are duller than standard coatings. These coatings have longer life because of their greater thickness and their appearance is a function of steel metallurgy and generally beyond the control of the galvaniser.

Dross Pimples/Inclusions

Dross is formed in the galvanising process in the form or zinc-iron crystals (approx 95% zinc - 5% iron) with a higher melting point than the metal in the zinc bath. Dross trapped in the galvanised coating may give the coating a rough or gritty appearance. The presence of dross inclusions in the coatings is not detrimental to the coating’s performance as the corrosion resistance of zinc dross is identical to that of the galvanised coating.

White Storage Staining

After galvanising, items stored or stacked in wet, poorly ventilated conditions will react with atmospheric moisture to form bulky white zinc hydroxide deposits on the surface of the galvanised coating.

Ash Staining

Zinc ash is formed in the galvanising process as the work is immersed in the zinc. The ash formed is skimmed off the surface of the molten zinc prior to withdrawing the work from the galvanising bath. Sometimes, ash is trapped inside inaccessible areas and sticks to the outside of the coating as the work exits the bath. Ash may leave a dull surface appearance or a light brown stain after removal. It does not affect the performance of the galvanised coating.

Striations and General Surface Irregularities.

Ridges and lines thicker than the adjacent galvanised coating are caused by different rates of reaction of the zinc with the steel surface due to stress areas on the steel surface or the presence of weld areas or weld metal with modified metallurgy to the parent metal. This phenomenon is most commonly encountered on pipe and tube products. Coating performance is unaffected.

Runs, Drainage Spikes and Puddling

These defects are unavoidable in the hot dip galvanising of general items and are acceptable as long as they do not interfere with the assembly of the function of the item or present a safety hazard in handling or service.

Bare Patches

Uncoated areas on the surface of galvanised work are due to poor surface preparation, inadequate pretreatment in degreasing, pickling and pre-fluxing. These areas must be repaired using a recommended repair method or the item regalvanised if the defect is of sufficient size.

Rust Staining

Uncoated steel in contact with galvanised coatings will accelerate corrosion of the coating and stain the coating brown in the area of contact. This can be removed by wire brushing.

Delamination

Very heavy galvanised coatings (over 250 microns thick) may be brittle and delaminate from the surface under impact and require more careful handling in transport and erection. Thin, cold rolled items with very smooth surface finish and manufactured from reactive steel may also give rise to coating delamination.

Black Spots

Scattered black spotting is due to residual galvanising flux crystalising on the surface of the work and is generally due to poor rinsing after galvanising or flux contaminated rinse water. This defect is usually encountered from galvanising baths using the ‘wet’ galvanising process where the flux is on top of the molten zinc. Excess aluminium in the galvanising bath can also give rise to this defect.

Spangled Coatings

Some hot dip galvanised coatings exhibit a high level of ‘spangling' caused by zinc crystal patterns on the surface. This phenomenon arises with galvanising alloys produced in particular smelting processes and these alloys are commonly used for hot dip galvanising. There is no difference in coating performance.

 

 

Source: Industrial Galvanizers Corp.

 

For more information on this source please visit Industrial Galvanizers Corp.

 

Date Added: Oct 1, 2002 | Updated: Jun 11, 2013
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