Preservation of cold rolled sheet by means of temporary protectives
From the ICMEESA Archive
Problems encountered and the factors involved in the development of a successful protective formulation.
The protective formulation is particularly important when shipment is over long distances and the steel is held in stock for extended periods, often under unsatisfactory conditions.
This article discusses the problems encountered and the physical, chemical and microbiological factors involved in the development of a successful protective formulation.
Problems
Unsatisfactory protective coating can lead to the following defects:
- The surface of the metal is difficult to clean prior to phosphating and painting. There may be a film of oxidised or varnished oil which is not removed by conventional alkali cleaner or solvent wipe.
- There may be extensive rusted areas or pepper and salt rust spotting which etch the surface so that, even when removed, they are difficult to cover up with the thinner modem paint and plating coats.
- There may be black or brown stains which are impossible to remove without recourse to pickling or acidic cleaners.
- Scratches may be seen, particularly where the oil film is thin or the oil is of low viscosity.
- The coating may be too dark and opaque to show the surface of the underlying metal which makes selection of the steel for particular uses difficult.
- The preservative may develop unpleasant odours.
With the high price of steel and the need to reduce wastage and cut costs of cleaning, it is important that the correct choice of protective coating be made. The cost of protection is generally less than one tenth of a percent of the cost of the steel – not a high price to pay in the light of removal costs and rejects.
Conditions for transit and storage
The sheet steel is produced, coated and stored under good conditions, in the absence of rain and condensation. However, it may be packed under relatively high humidity and be subjected to colder conditions during transport. These can lead to a humidity chamber effect and condensation of moisture within the coil or stock. It has been found that packs of sheets sent across the Equator, in summer and winter, can contain several hundred milliliters of condensed moisture which can penetrate a protective coating mechanically.
Overseas shipments are subject to the effects of sea water in ship holds and to a salt-laden atmosphere.
Road transportation in bad weather can lead to problems with rust due to penetration of rain under tarpaulins.
Fig. 1: Cold rolled steel in storage.
Bundles of sheets can collect dust and dirt in transit and, although outer sheets may be rejected, corrosion can be started on edges, which can penetrate into the sheets.
Upon receipt, steel is often stored on damp floors or under leaky roofs.
Mechanical handling methods are designed to prevent serious damage to edges, but coil and stacks may be lifted a dozen times before use and movement between laps cannot be completely avoided. Other than on a pallet, an oil will show considerable movement of laps which are in any case under tension and pressure. The same can be said for bundles of sheets, especially when several bundles are lifted together. Uneven flooring in a truck or works can create pressure areas and scratching.
Oil coatings tend to drain over time and the upper half of a coil may have little protective oil after several months in storage. Steel can be received in good condition but can deteriorate in storage. The outer wrap may be of good appearance but there may be serious corrosion through the mass. But how is this observed other than by re-coiling on receipt?
Methods for protectives application
Normally, a felt spreader roller with oil drip feed is used at temper mills. Some works have mechanical sprays for protective coating. Recently, the use of electrostatic sprays has been found to provide more even coatings on very high-speed mills. Roller coating has been used, but the steel must be flat to give a uniform coating.
Some works apply two side coatings but, in the main, the protective is applied to one side, allowing printing of the uncoated side during coiling and stacking to give complete coverage. Obviously, over economy in oil usage and poor shape in the steel can seriously affect the printing action. A patchy coating gives rise to possible differential aeration corrosion under long-term storage.
In recent times, experiments have been conducted with high-viscosity oil, mechanically agitated as a fine emulsion in hot water. The oil plates out as a very thin uniform coating and the excess moisture is blown off with air. This method allows heavier oils to be used economically.
Methods for removing protectives
Normally, the coating oil is left on the steel through the slitter and press shop. The fabricated parts are cleaned by hot alkali cleaner followed by rinsing prior to phosphating. Excessive oil coating is sometimes removed by a solvent wipe. In some cases, a cold cleaner based on alkalis may be used but this is only suitable with easily removable coating oils. Solvent emulsion cleaners have value, but only where the parts are used without painting.
In the automotive industry, the coil may be fed through a soluble oil emulsion to remove excess oil prior to carrying out deep drawing. This method removes excess oil without giving inter-shop corrosion problems.
In the case of corroded metal, the oil and corrosion are removed by either an acid detergent cleaner wipe or by alkali cleaning followed by pickling. Such metal may need mechanical polishing to remove pits before further use.
The cost of removal can be more than the cost of coating the steel. Most paint adhesion problems go back to the incomplete removal of coating oils and drawing compounds.
Coating compositions
Two main types are used: soluble oils and compounded mineral oils. Soluble oils are based on petroleum sulphurate which gives good corrosion protection and are normally applied neat to the metal. The reason for soluble oils is the mistaken idea that they are easy to remove with water.
Unfortunately, in storage, the coupling agents tend to be lost, making the oil film no longer water soluble. At this stage, the sulphurated film is very difficult to remove completely, and may by this time have suffered microbiological breakdown, resulting in staining.
The regularly used mineral oil products are additive with lanolin or lanolin substitutes, sulphurated and petrolatum. The lanolin is not very stable to oxidation and can become very hard and sticky with consequent problems over removal. Antioxidants help with this problem, but only to a limited extent. Lanolin also tends to be water absorbent and can give rise to patchy stains. Formulations with animal and vegetable components are susceptible to microbiological attack from moulds, fungi and bacteria, the result of which can be staining.
Both categories may contain paraffin or solvent to assist in thin coating, but the solvent can migrate with temperature fluctuations, resulting in rust. Choice of solvent is also critical in respect to flash point.
Water-based coatings such as those with nitrite, borate and organic amines have value to the steel works as temporary interstage coatings but, in normal circumstances, a coating oil is applied before shipping.
Properties of effective protectives
The following factors should considered in the formulation of a good coating oil.
Viscosity
This should be in the range of 200 – 300 seconds Redwood 1 at 25°C to give the most satisfactory coating by felt roller or spray. One liter should be sufficient for about two tonnes of average sheet steel.
Lower viscosities tend to give films that drain rapidly, and higher viscosities run the risk of patchy coatings unless excess is used.
Spredability
The difficulty in complete coating of both sides of the sheet in commercial practice makes it important to design oil with an active spreading action onto non-wetted areas. This involves the inclusion of polar ingredients into the oil. An assessment of this factor can be made by placing known volumes of oils from syringes onto a clean temper mill surface and noting the speed of increase of diameter of the drops with time. The angle of contact of oil to steel can be measured, but the practical approach allowing for the surface roughness as produced is a better indication of spreading action.
Covering capacity and price
This is a combination of viscosity, spreadability, scratch resistance and method of application. Suffice to say that, although the cost of the oil coating is less than one tenth of one percent of the cost of the steel, an attempt to save by using a cheap, less satisfactory oil can lead to heavy reject losses. This is especially true when the product is found defective in protection after being shipped over long distances. Product which is satisfactory on receipt can be stained after months in storage. This is where a performance specification is necessary to serve as a guide to the cost of protection.
Drainage resistance
When coils and sheets have been in storage for a time it will be noted that oil flows out. On inspection, uppermost areas will have very little or no oil film and may show rusting or scratching, while the lower areas are still fully protected.
An argument often heard is that high-viscosity oil will not run off or drain as much as thin oil. This is only true in the very short term. Even a high viscosity oil moves, albeit slowly and, on long storage, most non-additive oils drain away. There is a reduction of the rust preventive properties of an oil film as the thickness decreases. This may be proportional to thickness for substantial films, but at a threshold minimum oil layer, most of the protection ceases.
The best solution is to introduce an element of thixotropy into the oil, i.e. when static, the oil is thick and relatively immobile but when sheared or sprayed, the oil behaves as if it were thin. The effect does not have to be very pronounced to give films that will resist flow during storage times of one or two years. The evaluation is simply the suspension of various oil coated steel panels for extended periods, removal of the oil bead along the lower edge if any, and weighing at intervals.
Oxidation resistance
Oxidation of the oil and additives can lead to staining due to the generation of acids, an cleaning problems due to formation of polymeric, gum-like materials.
It is important to select a mineral oil base with good oxidation stability, backed up by an oxidation inhibitor. The use of an acid absorber, i.e. an alkaline ingredient in the formulation, can assist in neutralising any acid components.
Lanolin can create cleansing problems due to oxidation as can many fatty components, though synthetic lanolin has a superior resistance to oxidation. Oxidation resistance is best determined by placing oil coated panels of steel in an oven at elevated temperature e.g. 70°C and examination of the metal periodically for stains and for cleanability.
Colour/transparency
Many oils are dark in colour and semi-opaque, making surface inspection difficult. A satisfactory oil should be light in colour and transparent to show up the surface readily. The favourite argument that dark translucent oils contain lots of good additives does not hold in today’s world.
Scratch resistance
Unless very special precautions are taken in packing, coils and stacks of sheets are subject to movement in handling with pressures of perhaps approaching 15 000 kP in some regions. A good protective oil must also have film strength i.e. be a lubricant to overcome or reduce scratching. This property has been evaluated by placing two oiled plates in contact under a load and moving one plate against the other, cleaning off the oil and examining the surfaces for scratches.
Flash point
This is an obvious factor, but particularly so because of the complex electrical equipment around the process equipment which, if burnt, can put the production unit out of action for months. Normally, a figure in excess 250ºC is required.
Fingerprint suppressant
Operatives in steel works wear gloves when handling thin metal, but this is not always the case when sorting sheets and packing coil. The salt from hands can go to work under the oil film to produce heavy stains which can show through thin paint films, especially pastel colours. It is therefore necessary to include a fingerprint suppressant, generally a glycol solvent.
Aromaticity and health
Although not true in South Africa, most European countries specify a maximum aromatic content in oils used for processes where there is human contact. Continued contact with high aromatic oils over years can lead to cancer of the skin.
Anti-dermatitic properties
Solvent containing formulations can lead to defatting of the skin and, in turn, to possible dermatitis. Paraffin is often used as an ingredient to thin oils, especially when the supplier’s back is turned. This can have a skin defatting action. It is good practice to avoid solvents or to avoid skin contact. Many gloves will soak up oil and can therefore pose the more serious problem of continuous contact with solvent.
Non-staining to steel
Protective oils should not of themselves cause steel staining but, unless close quality control of additives performed, minor ingredients can cause problems. An example is residual acid value in additives and traces of copper and other metals from storage and handling. Bactericidal agents, particularly of the chlorine type, can also stain. The test is to leave ample panels immersed in the oil hot and cold over time and to compare with untreated panels.
Contact corrosion resistance
Metal surfaces in contact through oil film can develop stains far more rapidly than single-coated panels due to the closer proximity of surfaces of differing composition, and the wide variation in oxygen levels as a result of varying thicknesses of oil and the distance from the edges.
This is evaluated by making a small stack of oiled panels under weight and stoving in an oven to accelerate the action, if any.
Removability
Little difficulty is normally experienced with removal of the oil film when fresh, using a conventional alkali metal cleaner or perhaps emulsion cleaner. After ageing, however, it can be a different story due to effects of oxidation and polymerisation of the oil.
The standard method of assessment is the “Volkswagen Test” in which an oiled panel is immersed in a 3% Hoechst cleaner at 95°C for fifteen minutes without agitation. The panel is rinsed in clean water and should be free of residual oil.
Bactericidal action
Bacteria, molds and fungi can live on mineral and vegetable oils, often in unlikely places such as inside aircraft fuel tanks. Brown and black staining of oil sheets, unpleasant odours and rust spots can also be caused by organic growth. Excherichia Coli can cause gastroenteritis and the consequences of human contact in a production plant are quickly apparent. This infection probably arose from a human source, and if one considers the contamination of oil in a holding tank, steel can be at risk without any obvious effects for some weeks. Frequent checks for bacteria should be undertaken unless the oil contains a bactericide.
The oil should be naturally resistant to bacteria but a bactericide must also be used as a safeguard. Many bactericides can be corrosive and only two commercially available products meet the requirements. It has been found that alternation over a three-month period of supply is the best long-term preventive action.
Droplet corrosion resistance
A wrapped, 7-tonne coil can contain several cubic centimeters of water through condensation. This water will collect at edges and lower areas. It is necessary to ensure that the formulation of the oil will resist action of free surface water. Some oils, especially soluble oils, will pick up the water and can carry this to the metal surface to cause staining. Many lanolised oils will absorb water slowly until it penetrates the film. An oil may resist humidity but may fail to protect in areas awash with water.
Pressing and forming lubrication properties
These are not normal requirements for a protective coating, but many customers prefer to do simple pressings and forming operations with the existing oil film. This helps to protect parts during treatment and only one final cleaning operation is required prior to phosphating, painting and others. A good protective must be a good metal working oil. It is curious that many of the formulations that give excellent rust prevention can also work as good metal working oils. Results obtained in cup tests can be just a little inferior to polythene sheeting, the normal standard of assessment of drawing oils.
Future trends
With the shortages of mineral oils, it is possible that water may be used as the carrier for the protective coating, but with careful control to avoid wrapping water into the laps of steel.
One interesting development is the use of wax emulsions which can be applied to the steel as a uniform film to rapidly split out to leave a continuous wax coating impervious to water when dry.
Such coatings are easily removed with a hot alkali cleaner. Tills type of approach is already in use for metal pressings and up to six months’ indoor protection is easily possible.
Water solutions of organic-inorganic blends can be used as coatings provided that adequate drying is available. These formulations have the property of inhibiting the points where rust would start, e.g. residual salts from the rolling mill and pickling operations. These coatings have rust inhibition both wet and dry. If an outer coating of solvent wax rust preventive is sprayed onto edges as a sealing medium, then good protection can be obtained with water rinsing for removal.
Some of the new rust preventive additions have high resistance to bacterial attack and have a platelet structure which acts as a fish-scale protective layer. Such products, when blended with mineral oil, also show strong thixotropic properties with little tendency to drain. These additives can be made water dispersible with little loss of protective property.
Conclusion
Protective coating formulations can be produced with all the necessary properties for the coating of steel sheets. The disadvantages can be avoided with saving in cost of cleaning and rejects. Many of the factors can be applied with advantage to the protection of other steel components in storage and transit.
Contact Mariana Jacobs, ICMEESA, Tel 011 425-0585, icmeesa@icmeesa.org.za
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