QUALITY AND INSPECTION
Coating thickness measurement
The thickness of zinc coating can be measured by destructive or non-destructive methods. However, for most purpose non-destructive instruments are adequate. The magnetic Instruments available are of two types one measures the magnetic attraction between a permanently magnetized needle and the base steel or iron, and the other works on a Magnetic principle. Non-destructive tests can be carried out at any stage in the life of a galvanized article to establish the thickness of the remaining zinc coating.
Coating finish
Table x summarizes variations in finish which may be observed. The variations are often caused by surface features of the steel itself, and the acceptability of a coating should usually be judged primarily on its long-term performance and corrosion resistance.

Table x,Summary ofvariations in galvanized finish


appearance

   acceptability of protection

dull grey coating (all alloy, no free zinc)

acceptable

rust stain

acceptable (easily removed by stiff brushing)

general roughness

acceptable unless otherwise agreed

lumpiness and runs

acceptable unless otherwise agreed

pimples

acceptable unless dross contamination is heavy

bulky white deposit (wet storage stain)

acceptable(provided coating thickness remains in compliance with BS EN ISO 1461)

flux staining

not acceptable

bare spots

generally not acceptable,damaged area can be repaired following guidance

given in the standard.



QUALITY AND INSPECTION

Dull grey coating
Silicon is sometimes added to steel as a deoxidant during production and this speeds up the reaction between the steel and the molten zinc. When the galvanized article is removed from the bath, but still remains hot, the reaction may continue and convert all or part of the surface zinc layers to zinc-iron alloys. These are dark grey compared with the light grey of pure zinc. Although after a period of exposure the difference in grey colour becomes less pronounced.
Zinc-iron alloy coatings on reactive steel are thicker, and hence longer lasting, than those on rimmed or aluminum-killed steels. Corrosion resistance, thickness for thickness, is at least as good as unalloyed zinc,and may be better in acidic industrial atmospheres. Zinc-iron alloys are more abrasion resistant than zinc but thick coating have a greater tendency to flake handled roughly and appropriate care must be taken.


Rust stains
Sound galvanized steel with many years of corrosion-free life still remaining can sometimes be rust stained or discoloured This may give an incorrect impression that the coating has failed and may occasionally be, visually unacceptable. lt maybe the result of one or more of the following factors: Direct contact of galvanized articles with unprotected or inadequately protected steel (e.g. Galvanized steel section fastened with unprotected electroplated or painted steel bolts.
- Deposits of iron and steel dust and swarf from other operations or sources on the galvanized surface
- Water draining from unprotected or poorly protected steelwork (e.g. from damaged areas on painted steelwork.)
- From cleaning residues in welds. During cleaning, acid may penetrate into the weld area via pin holes or other gaps in the welding.
- Rusting of areas welded after galvanizing and subsequently left unprotected or inadequately protected.

To avoid rust staining, all parts of a structure should receive comparable effective corrosion protection where possible nuts and bolts and other fasteners should also be hot dip galvanized. Welds should be continuous possible to minimize the retention of cleaning residues and should also be slag-free. Structure should be designed to avoid run-off water from other metals on galvanized steel. Where welding after galvanizing is necessary, welded areas should be thoroughly cleaned and renovated.
Discolouration and staining from most external sources have no effect on the life of the coating. However, affected areas may be cleaned to improve the appearance of the structure. Generally, wire brushing or the use of a scouring powder will remove the stain and leave a sound galvanized Coating.




General roughness
BS EN IS0 1461 demands that a galvanized coating shall be 'smooth' but points out that smoothness is a relative term and that coatings on fabricated articles should not be judged be the same standards as those applied to mechanically wiped products such as galvanized sheet, tube and wire.
An uneven coating is usually due to excessive or uneven growth of the alloy layers because of the composition or surface condition of the steel. An uneven coating is often thicker than a conventional coating and, therefore, has a longer life but on rare occasions it may be unsatisfactory or it may Interfere with the intended use of the article.








Lumpiness and runs Lumps and runs caused by uneven drainage of zinc from an article when it is removed from the bath may occur due to shape or thinness of the component and are not harmful to the life of the coating.









Pimples Pimples are caused by inclusions of dross (a pasty zinc/iron alloy residue that forms in the galvanizing bath) in the coating. These may arise from iron salts carried over on the work from the cleaning tank and unable to escape from the surface of the coating, Contamination may also arise from agitation of the dross layer at the bottom of the bath. Dross has a similar corrosion rate to that of zinc and its presence in the coating as finely dispersed particles is not objectionable.





Wet storage stain
Wet storage stain is the white corrosion products and dark stains which may be seen on the surfaces of newly galvanized articles when they have been cosely stacked and stored or transported under damp or wet conditions. Where wet storage stain has formed, the coating beneath may be stained dark grey or black.

To prevent wet storage stain zinc coated articles must be transported and stored under dry and well ventilated conditions. lf stored outdoors, the surfaces should not be in close contact: free circulation of air is necessary to prevent condensation and retention of moisture. Nesting or close packing must be avoided as capillary action can attract water into closely contacting surfaces, Articles should not be stored in direct contact with the ground Barrier coatings, such as clear lacquers, may be used to preserve a bright finish where appearance is paramount. Heavy deposits of wet storage stain should be removed. This can usually be achieved with a stiff bristle brush or light abrasives. Chemical methods should be used as a last resort and require thorough rinsing with fresh water after use.





Flux staining
Where flux is used during the dipping process, flux residues may adhere to the surface after immersion and pick up moisture to form white corrosion products. Although this is a surface effect, flux stains may be detrimental to the life of the coating and should be removed.








Bare spots
Due to the sacrificial action of zinc, small localised flaws up 5mm maximum width are usually self-healing and have little effect on the life of coating. Articles with uncoated areas due to faulty processing will be detected by the galvanizer on inspection and retreated but, occasionally bare spots can also be caused by rolling defects in the steel such as laps and fold, laminations and non-metallic impurities rolled into the surface.




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