ContentsQuality assurance
Galvanizers Association always insists upon the highest quality standards from its member companies, who are required to process work to BS 729 and/or to BS EN ISO 1461. The requirements of these standards ensure that the zinc coating is continuous and of the required thickness.
Quality Assurance for industry has been enhanced by the introduction of the BS EN ISO 9000 series of standards -'Quality Systems'. Already the majority of hot dip galvanizing plants are registered and others will be doing so in the near future.
Coating weight or thickness measurement
The nature of the process ensures that, in most cases, if the coating has formed, it will automatically be of sufficient weight to meet the requirements of either BS 729 or BS EN ISO 1461. There are a number of inspection techniques which can be used when necessary.
A definitive check of the weight or thickness of a zinc coating can only be made by destructive methods, i.e. a 'strip and weigh' test or by preparing and measuring thickness on a microsection. However, for most purposes non-destructive instruments are adequate. The magnetic instruments available are of two types - one measures the magnetic attrction between a permanently magnetized needle and the base steel or iron, and the other works on a magnetic induction 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. Magnetic testing of coating thickness is covered in BS EN ISO 2178.
Coating finish
Table 5 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.
Dull gray or dark gray coatingSilicon 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 gray colour becomes less pronounced.
Zinc-iron alloy coatings on reactive steel are thicker, and hence longer lasting, than those on rimmed or aluminium-killed steels. Corrosion resistance, thickness for thickness, is a 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 coatings have a greater tendency to flake if handled roughly and appropriate care must be taken.
The dark grey coating surface may develop staining after a relatively short period of exposure, even in mild, damp conditions. This is only a surface effect and does not indicate seroius deterioration of the coating: the galvanized coating remains and continues to protect the steel.
Staining and discolouration by rustSound 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 hav failed and may occasionally be, visually unacceptable. It may be the result of one or more of the following factors: 1. Direct contact of galvanized articles with unprotected or inadequately protected steel (e.g., galvanized steel sections fastened with unprotected, electroplated or painted steel bolts). 2. Deposits of iron and steel dust and swarf from other operations or sources on the galvanized surface. 3. Water draining from unprotected or poorly protected steelwork, e.g. from damaged areas on painted steelwork. 4. From cleaning residues in welds. During cleaning, acid may penetrate into the weld area via pin holes or other gaps in the welding. 5. Rusting of areas welded after galvanizing and subsequently left unprotected or inadequately protected. 6. Water running off other materials, notably metals such as copper and certain hardwoods (e.g. oak). This effect may occur whenever water can dissolve materials from one surface and deposit them on the galvanized steel.
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 (see Section 8). Welds should be continuous wherever possible to minimise the retention of cleaning residues and should also be slag-free. Structures should be designed to avoid run-off water from other metals onto galvanized steel. Where welding after galvanizing is necessary, welded areas should be thoroughly cleaned and the zinc coating restored.
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 roughnessBS EN ISO 1461 and BS 729 demand 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 by 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 runsLumps 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. Sharp points of excess solidified zinc are not acceptable as they may present a hazard during handling. Fabrications with areas where spikes have been knocked off, exposing bare steel require remedial repair of the coating which is explained overleaf.
PimplesPimples are caused by inclusions of dross (a pasty zinc/iron alloy residual 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. But, major dross inclusions tend to embrittle the coating and the galvanizer will avoid this.
Wet storage stainWet storage stain is the white conrosion product and dark stains which may be seen on the surfaces of newly galvanized articles when they have been closely 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. If stored outdoors, the surfaces should not be in dose 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.
Bamer coatings, such as clear lacquers, may be used to preserve a bright finish where appearance is paramount. A range of proprietary products are available, some formulated for specific applications. 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 and dirt stainingWhere flux is used during the dipping process, flux residues may adhere to the surface after immersion and pick up moisture to fonm white corrosion products. Although this is a surface effect, flux stains may be detrimental to the life of the coating and should be removed.
Dirt may be picked up on the surface of the coating from the site, tnuck beds or from contact with other articles. These are readily washed off to reveal a sound coating underneath and are not, therefore, hammful.
Bare spotsDue to the sacrificial action of zinc, small-localized flaws up to 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 folds, laminations and non-metallic impurities rolled into the surface.
Blisters
Blisters in a galvanised coating are caused when acid entrapped in surface flaws during pickling being expelled due to the hear in the galvanizing bath. They usually arise from surface defects in the steel. Since they represent minor disturbances of coating uniformity, they do not significantly affect corrosion resistance.
The presence of silicon in the base steel significantly affects the structure and properties of the resulting galvanized coating.
Certain silicon levels can be responsible for excessively thick galvanized coatings which are often brittle and dull in appearance.
The effect of silicon content on galvanized coatings in shown in the graph. In general, a peak in zinc pick-up occurs between 0.05 and 0. 1% Si followed by a trough at around 0.15 to 0.20% Si after which the reaction rate increases again. Galvanizing temperature and immersion time also affect the location of the peak and trough of the graph.
Since the life of the coating is proportional to its thickness, heavy prey coating, provided they are sound and continuous, are beneficial.
Hot dip galvanizing coating thickness may be tested by the following:
1. Non-destructive testing (General method)
2. The weighing test
3. The stripping test
4. Microsections
5. Preece test
6. Peeling and flaking
1. Non-destructive testing:
There are a number of simple magnetic gauges which can be used to give a quick approximate measurement of coating thickness. Fairly reliable results are obtained on smooth surfaces such as sheets. but they are less reliable on the rougher surfaces of structural steel or castings. It is advisable to have standards of known thickness against which periodic checks can be made and these standards should be of the same basis material as that to be inspected. The advantage of these instruments is that they can detect local variations in coating thickness which are not perceptible by other methods. Their big disadvantage is that they can give very different results in different hands.
2. The weighing test:
For large articles, or where comparatively heavy weights are concerned, weighing the work before and after galvanizing is a simple and fairly accurate expedient. If weighed before pickling, allowance must be made for the loss in weight due to pickling. In some cases the calculation of surface area may be tedious, but it is justified where batches or repeated runs of one type of article are encountered.
3. The stripping test:
Where more reliable average values are required or where compliance with a specification is necessary, coating weights may be determined by means of the stripping test given in BS EN ISO 1461 & ISO 1460.
4. Microsections:
Where laboratory facilities are available, thickness can be measured by a microscopic examination of a polished and etched section through the coating. This is a direct and accurate method of determining coating thickness and gives additional information about the extent and nature of the alloy layers comprising the coating. It is not a production test, however, and would only be used in special circumstances.
5. Preece test:
The copper sulphate (Preece) test is concerned only with uniformity of a coating. The test gives only a limited amount of information regarding the quality of the zinc coating and discretion should be exercised in its application.
6. Peeling and flaking:
Any tendency towards flaking of the coating can be determined by a few light blows with a small ball-headed hammer such as a 1 /21b peening hammer The coating should show no signs of peeling. Thin materials such as sheet or wire may be tested by the bending or wrapping tests included in the appropriate British Standards for these products.
Flaking is often associated with bad preparation of the steel for galvanizing, but other causes are unevenness of coating. critical ratio of thickness of alloy layer and zinc layer, or excessive growth of alloy layer through slow cooling. The last two conditions can sometimes result from stacking work immediately after galvanizing. In the case of excessive growth of alloy layer the coating will become dull grey.
Observance of the following recommendations will minimize distortion:
1. To design and build assemblies and sub-assemblies in suitable modules for single dipping.
2. Use sections of near equal thickness at joints.
3. Use symmetrical sections where possible.
4. Bend members to the largest acceptable radii.
5. Accurately preform parts to avoid force or restraint during joining.
6. Continuously weld joints if possible using balanced welding techniques to reduce uneven thermal stresses.
7. Observe rules in 'Guide to design & Fabrication of Components for Galvanizing'.
