Galvanised steel
| Infobox on Galvanised steel | |
|---|---|
| Example of Galvanised steel |  |
| Facts | |
| Origin | - |
| Stowage factor (in m3/t) | - |
| Humidity / moisture | - |
| Ventilation | - |
| Risk factors | See text |
Galvanised steel
Description / Application / Shipment / Storage / Risk factors
Galvanising is the process on a substrate of metal, normally steel. The demand for high quality rust preventive steel sheeting and coated strip is great. Coatings bases upon zinc are popular, because they provide the additional benefit of a sacrificial element in areas where the steel might be exposed. The galvanising of steel sheet can be defined as the application of a zinc coating over the surface of the steel to protect it against exposure to the atmosphere and consequent rusting.
There are two methods of deposit, namely hot dipping or electrolytic. Normally the former is a much heavier coat but may be to some degree irregular in thickness, whereas electrolytic is even and probably much thinner although the actual coating weight can be controlled. With the method of electrolytic galvanising, the plating is dull grey and unattractive in appearance as opposed to the bright perspective of the zinc surface produced by the hot dip method.
When first coated the product has a bright, shiny surface but this will dull brown by exposure to air; this dulling action is not detrimental. During the manufacturing process base metal must be cleaned and fluxed. The cleaning is normally by acid and it is therefore essential that prior to coating the cleaning medium is removed in totality. If it is not, the residual acid may well attack the zinc coat from the underside. Again, in all probability where there is residual acid, the acid of the zinc will be discoloured and could be confused with that generated by contact with sea water.
It is possible to recover the base metal by immersing the product in hydrochloric acid and then re-coating, and this could well be a feasible option where one is dealing with component parts, e.g. galvanised steel conductor pylons, but it is extremely doubtful if this is an economical option where sheet or standard merchant bars are involved.
Deterioration after galvanising
On leaving the production line the applied zinc surface is bright – in the case of the hot dip method – however, as the plating cools the surface of the zinc is attacked by the atmosphere and becomes covered with a very thin layer of basic zinc oxide. The formation of this protective layer takes place in two stages, and does not itself protect the zinc from oxidation. It is, however, progressively converted into basic oxide by combining with the carbon dioxide and moisture present in the atmosphere. The forming of this protective coating, as it develops further, tends to cause the zinc surface to take a dull appearance. Given time, the surface also attracts dust particles from the atmosphere so that the material looks very dull and dirty.
The efficiency and lasting properties which will provide long-term protection to the steel beneath will depend upon the environment. Zinc coatings will deteriorate faster in marine atmospheres and those areas where air pollution is prevalent than under climatic conditions, where the air is purer and drier. Under certain conditions where the atmosphere is not affected by a high degree of pollution, and the plating surfaces are situated in air streams where they are subjected to periodical fresh water wetting and drying, it has been known for the plating to survive for 30 years or more.
White rust
An aggressive liquid, such as sea water or chemical dilution, when moisture is involved e.g. moist air, causes a thick voluminous white dust to form on the surface of the plating and this effect is generally referred to as ‘white rust’. If such a condition continues, the galvanised coating will in due course be consumed and eventually the atmosphere will reach the surface of the steel beneath the zinc and red rust will appear. The development of white rust on galvanised surfaces seems to favour an atmosphere where the air is highly moisture laden and the ambient air is still. Such conditions can prevail in an unventilated ship’s hold, and is responsible for the heavy white rust deposits and damage whereby structural material has to be re-galvanised after delivery.
Type of packing
Galvanised sheeting in coil form, and also plates stacked one on top of the other to form a bundle, are wrapped in kraft paper, after which a metal envelope of galvanised waste sheeting is applied. This is strapped up as a single unit and secured with flat metal strapping bands.
The galvanised metal wrappers are sometimes new and bright in appearance, but often affected by drip marks through having been in contact with liquids before being used to cover the coils or packages. Wrappers are often affected by white rust, and there may be evidence of sporadic areas of oxidation having developed at some time.
During the transport from the factory, by lighter or lorry, and in storage awaiting shipment, even bright new wrappers can deteriorate considerably in appearance. Further deterioration can and will take place during the ocean voyage. In view of this, merchants taking reception of such goods are highly suspicious of the condition of the contents of the coils or packages, owing to the condition of the wrappers. Claims are often entered and surveys are held. In some instances there is ample evidence to support the fact that moisture has penetrated the wrappers, gained access to the actual goods so initiating the development of damage to the material. In some instances, the suspicion aroused by the condition of the wrappers has proven to be false as the contents are found to be in good order and condition.
Ambiguity surrounding the cause of damage
In many cases the aspect and nature of the damage gives rise to doubts as to what the basic cause of the damage might be. Upon removal of the wrappers, the edges of the plating are bright and the entire unit shows no stains or blemishes of any description. When the coil is unrolled, or the stack of sheets turned over in the case of packages, the galvanising is seen to be heavily oxidised with large voluminous areas of white rust. Quite often, all surfaces throughout the coil or packages are affected. In other instances, defects on plate edges are relatively minute, and bear no relation to the enormous centralised damage found within the unit being examined. A special feature of this type of defect is that, in general, the damage is mainly confined to the surface centre of the plating and may not extent completely to the edges.
Conditions referred to above, have more than likely originated from the fact that when, after manufacture, the sheet is wound into coil, or the sheets have been stacked in the case of packages, air is entrapped between the plating. When this takes place, if the material possesses a high temperature, the air entrapped between the plating is capable of holding more moisture than it can when the goods cool. The moisture held by the entrapped air, upon cooling, precipitates moisture onto the surface of the plating. The corrosion product will be mainly zinc oxide as there is insufficient air for an adequate supply of CO2 to create basic oxide.
Effects of sea air
It is appropriate to point out that galvanised surfaces are more readily and severely corroded when salt particles are present in the air, or when they make contact with seawater. With regard to salt particles in the air, it is to be emphasized that the quantity of salt present in the air at sea is very small. In a ship’s hold, even under conditions of forced ventilation, the amount of salt crystals in the air does not amount to very much. Nevertheless, analysis of white rust from galvanised surfaces which have made a voyage in a ship’s hold, have shown faint traces of salt. These salt traces are responsible for intensifying the corrosion of zinc for two reasons.
1) The salt attracts more moisture to the surface of the plating than would otherwise be the case were it not present.
2) The moisture attracted on account of the presence of salt acts with the salt particles to form an electrolyte on the surface of the zinc, which together form electrolytic cells producing, in addition to white rust, an oxychloride of zinc. It is to be noted that zinc oxychloride is not hygroscopic, but zinc chloride is highly hygroscopic. The presence of salt particles does not therefore intensify the damage to the extent expected.
The faint traces of salt derived from the atmosphere are never likely to be confused with the large quantities of salt which would be present through contact with sea water. There would consequently be no confusion in distinguishing between damage arising from the two causes. Damage through contact with sea water also leads to the development of white rust, and also red rust, as the zinc coating, where contact is made, will be penetrated. Therefore, in addition to white rust, iron red rust, zinc oxychlorides and sea salts will be present. In order to determine whether or not sea water is involved, samples should be submitted for spectrographic analysis, whereby it can be determined beyond all doubt that the elements to their approximate values, always present in sea water, are involved. Silver nitrate applied to galvanised surfaces causes the test area to turn black, and for this reason is unreliable as a test for the presence of chlorides.
Possible causes of damage
Damage to galvanised steel sheeting can be caused by the following:
- Poor preparation of the surface of the steel prior to applying the galvanised coating.
- After pickling, washing of the plating may not remove all of the acid from the surface of the plate.
- Water used for washing or cooling contains impurities.
- The formation of white rust may develop if wetting from fresh water, rain and/or cargo sweat takes place. This may be particularly so if the goods reside in a still atmosphere where possibilities of evaporation of the moisture are very poor.
- White rust will develop rapidly if the goods come into contact with sea water.
- Galvanised surfaces will deteriorate much more rapidly if they are permitted to reside in a marine atmosphere.
- If the material possesses a high temperature when it is packed in a humid atmosphere, moist air entrapped between the turns of plating, or sheets, may precipitate moisture when the material cools, with resulting deterioration of the zinc coating.
- Galvanised goods should not be stowed in the same compartment as fertilisers, or in a compartment which connects via some of ducting, e.g. ventilation trunks, with another in which fertilisers are stowed. Fertilisers contain chemical ions which, when in contact with galvanised surfaces, and in humid ambient conditions, result in an aggressive medium for the promotion of the deterioration of zinc coatings.
- Transportation on board ship should be carefully considered, especially with regard to ventilation of the compartment where the goods are stowed.
Galvanised Wire
This product is manufactured with cold drawn steel wire to which the zinc coating is applied in much the same way as the hot dip method of galvanising steel sheets. The wire is first annealed, then pickled, washed and fluxed, before being finally passed through a bath of molten zinc. Thereafter, the wire passes through wipers to regulate the thickness of the zinc coating. Such material is extensively used for fencing in the form of plain or barbed wire and also for the manufacture of wire rope, springs and nets, etc.
Galvanised wire is normally shipped in the form of loose coils which are on many occasions unpacked and unprotected. The principal points with regard to the deterioration of galvanised surfaces (see above) also apply to galvanised wire.
It is virtually impossible to ship bright, galvanised, unwrapped wire coils, and deliver them in the same bright condition at final destination. The actual oxidation process which takes place when galvanised surfaces are exposed to the atmosphere, causes a dull appearance to develop. Add to this the environment experienced on a sea voyage and there is no doubt that the wire will be dull when delivered. Sea air circulated through a ship’s hold contains salt particles which are to some degree incompatible with galvanised surfaces and tend to advance the development of zinc oxide. Receivers contend that when marketing the goods, the dull appearance imparted to the galvanising – through a natural, unavoidable and even desirable oxidation process – prevents a satisfactory sales price being obtained against the market price of locally manufactured material which has a brighter and more appealing aspect. This is, of course, a commercial consideration because the dull condition of the galvanising does not amount to real damage. Such goods should be kept dry at all times, and every endeavour should be made not to expose the material to moist still air. Direct transhipment into the sea carrier is desirable but not always possible. If pre-shipment quay storage cannot be avoided a well ventilated space should be provided. In such circumstances, the material must be kept clear of the quay floor by placing canvas tarpaulins beneath and over the top of the parcel or parcels awaiting shipment. Quayside storage prior to shipment should be kept to a minimum, as an accumulation of dust on the wire can absorb moisture and contribute to the evolvement of a dull aspect to the zinc coating.
As the main objective is to deliver the cargo in good order and condition it has to be handled with care, so that marks, scoring damages to the zinc coating and contact with dirt is avoided.
Any apparent imperfections in galvanised wire should be taken seriously. Whether the wire is dull or bright in appearance must be considered. White powdery voluminous deposits in spots or patches, referred to as white rust, must be viewed as damage and likewise, with regard to visible areas of red rust. Scratches, score marks, dirt and dust, also bending, twisting and/or kinking of windings are inadmissible. They must be regarded as damage or, in the case of dust and dirt contamination, as a condition which will eventually lead to the development of damage.
Moist wooden dunnage can damage galvanised surfaces and it should be ensured that any wooden dunnage used is dry. This dunnage should be covered with a canvas tarpaulin or strong kraft paper. The transport of this type of wire can be much more successful if the goods are wrapped.