Painting Before Fabrication
When steel is stored for any length of time prior to fabrication, it should be protected against corrosion.
A typical example of this situation is found in the shipbuilding industry where ship’s plate is bought in large quantities, and delivered to the shipyard for storage prior to use. Unless corrosion is prevented, long periods of storage may lead to deterioration of the plate, perhaps to the point where it steel structure, such as an oil/gas platform.
It has become common practise to prepare the surface of such steel and apply a protective coating as it is received at the place of storage, and prior to the storage period. In particular, fixed shot-blasting machines (often known as ‘wheelabrators’) may be used, followed by the application of a ‘pre-fabrication’
primer, or even a substantial part of the final coating system.
Any coatings that are used should be:
(a) Tough, to withstand handling during the fabrication stage (b) Capable of being overcoated after fabrication
(c) Nontoxic when welded, if welding is to be used as the method of fabrication
It is also useful if the coatings are rapid drying, in order to permit handling fairly soon after the coatings are applied.
The two main advantages of coating prior to fabrication are:
1. Steel is protected during storage
2. Surface preparation and coating methods are easier and faster (therefore more economical) whilst the steel is in relatively small and simple units
The main disadvantages of coating prior to fabrication are:
• Coatings may be damaged during fabrication
• Deterioration of coatings and/or surface contamination acquired during storage or fabrication may mean that the coatings have to be fully or partially removed before subsequent painting operations
Storage and Handling
Steel to be stored on site in a partially painted condition, should be dealt with so as to avoid deterioration in handling, transport and storage by employing the following rules:
• Allow adequate time for full hardening of coating.
• Use nylon slings and ropes for lifting (not metal)
• Plan to reduce movements to a minimum.
• Use special supports and lashings on vehicles.
• Allow good ventilation of components when stacked.
• Inspect at monthly intervals and deal with any deterioration as it arises.
• Maintain good discipline during erection to avoid damage and contamination.
Painting at Site
In a situation where structural steel arrives at site in a partially painted condition, consideration should be given to several factors:
• Is the coating clean and in good condition?
• Is there any handling damage?
• Is the steel stored properly prior to fabrication.
The same consideration apply to the steel after fabrication has taken place, but prior to full painting. It is essential also at this stage to remove all traces of weld flux and spatter, and to effectively clean the weld areas.
Coatings used for this twostage type of coating process should necessarily have an indefinite overcoating period.
Access (via scaffolding/cradles etc)
A major coating consideration, particularly at the maintenance stage, is the provision of access for the surface preparation and coating application procedures.
This can be a major cost factor in any coating project, and can also be a major hindrance if the design of the access is poor. A particular problem arises with the preparation and painting of the scaffold support points, which may be numerous. Another problem may be caused by mechanical damage whilst the scaffold is removed.
If cradles are used for painting suitable areas, problems may be generated by difficulties with coatings overlapping (or not overlapping) or by lack of access to some areas.
The safety of all access points is also a major consideration. For the first time in 1982 in the North Sea, full time scaffolding Inspectors were employed on major coatings projects.
16 SURFACE PREPARATION - ALTERNATIVE METHODS
Steel Surface Defects
It is not widely appreciated that ‘surface preparation’ does not just mean the removal of all millscale, rust and contaminants, but suitable ‘dressing’ of the steelwork to remove all surface defects that could break through the paint film or prove difficult to protect adequately by painting.
It should be remembered that where defects are exposed by blastcleaning and subsequently removed by grinding, it is necessary to reprepare the immediate area in order to retain the surface profile.
Surface Laminations and Shelling
The commonest type of surface defect on steelwork is surface laminations, generally caused by the rolling process. It is important that all such defects are removed by grinding as no painting system, however, thick, can effectively protect them.
In the case of small shelling and surface laminations, even if these do not project above the surface, they may later curl upwards and penetrate the painting system.
Cracks and Crevices
Any form of crack or deep crevice will form a danger to the protective treatment, as it cannot be effectively filled by the painting system. It will contain impurities, and gather entrapped moisture and air, then form a galvanic cell leading to a painting system failure. All such cracks and crevices should be ground out, unless too deep for such treatment, in which case, they should be filled by welding and then ground smooth.
Inclusions
All forms of surface inclusions, such as rolled in millscale should be removed by chipping and the surface ground (with weld filling if necessary).
The Painting Specification
To obviate any possible dispute as to whether surface defects should be removed, it is strongly recommended that a clause be included in the painting specification covering the dressing of surface defects.
Manual Cleaning
Manual cleaning is the slowest and least satisfactory method of surface preparation. Normal tools used are wire brushes or scrapers or chipping hammers. The process is slow, laborious and costly, with the end result still far from satisfactory. It is impossible to remove all rust and millscale by this method.
A further complication these days can be the reluctance of labour to engage in arduous manual tasks.
Manual cleaning should only be used when weather or some other factor precludes the use of any other process.
Power Tool Cleaning
The principal tools used are the grinder and the rotary wire brush. Whilst quicker than hand tools, the work is very laborious and expensive.
The biggest drawback to the use of power tools is the burnishing effect that arises on the metal. This polishing effect, inherent with power tools, is the most unsatisfactory feature of this method. A polished surface seriously affects paint adhesion and should be avoided at all costs. In addition, no amount of polishing will remove the rust from the bottom of pits etc.
A more sophisticated type of power tool is the needle gun. This tool consists of a number of hardened steel rods that are vibrated against the surface. It is slow in operation and suffers from the same defect as other power tools as it has a burnishing effect when producing a relatively clean surface.
Blast Cleaning: Equipment and Materials Abrasives
The degree of surface roughness and the rate of cleaning depend primarily on the characteristics of the abrasive grit used.
Although the blasting abrasives in general use range widely from crushed walnut shells, glass and crushed slag, to various metallic shots and grits, and even ceramic grits, there are only three main types of grit that find general acceptance for the gritblast preparation for painting. These are:
• Chilled Iron Grit
• Crushed Slag
• Ceramic Grits
Attention is particularly directed to the fact that, despite the widespread use of the term ‘sand blasting’, sand is not listed as a grit blasting abrasive. Sharp sand or flint is indeed a cheap and highly effective abrasive, but increasingly cannot be used (nor any abrasive containing free silica) on factory premises throughout the world, due to the very real danger of Silicosis.
Permission to use sand is very occasionally given for ‘site work’ in the open air, but only when the operators and other personnel are carefully protected from the dust created and the Local Factory Inspector approves the site and blasting conditions.
Chilled Iron Grit
This is by far the most widely used abrasive for surface preparation in a coatings application facility.
Chilled iron grit is available in a variety of grades and to a specific minimum hardness. It is an excellent general purpose abrasive, due to its relatively high density which gives high particle energy, and its slow but effective rate of breakdown which maintains sharp cutting edges on the grit particles.
Crushed Slag
While chilled iron grit is used extensively for grit blast preparation in works or on site where grit reclamation and recirculation can be practised, it is too expensive an abrasive to be used where grit reclamation is not possible, as on many site jobs.
With the nonavailability of sand, certain crushed slag from metallurgical processes have been made available as relatively cheap expendable abrasives. Copper slag and Aluminium slag are common. While quite effective grits for 'once only' use, by reason of their rapid breakdown to dust, they are not generally suitable for grit reclamation and reuse.
Ceramic Grits. (Aluminous Oxides and Silicon Carbides)
These are relatively expensive grits, but their use is often justified by special considerations.
Due to the retention of sharp cutting edges on the particles in use, their cutting action is particularly effective, especially on hard base materials which may resist effective blasting by chilled cast iron grit.
Additionally, this effective cutting action is shown at blasting pressures considerably lower than normally employed for other abrasives. These ceramic grits are particularly well suited to blast preparation of thin metal surfaces, which show ‘buckling’ or distortion if blasted with chilled iron grit at conventional blast pressures.
Finally, as these ceramic grits are essentially inert to normal corrosive influences, they can be safely used to grit blast stainless steel or nonferrous material surfaces, without causing rust staining or discolouration.
Shot
Types of blasting abrasive that are rounded in shape are known as shot. Their common use during the development of blastcleaning techniques leads to the common misuse of the term 'shotblasting'.
requirements of today's high performance coatings. One use of shotblasting may be to 'workharden' a metal surface by 'peening', a process which can reduce the incidence of stresscorrosion cracking.
Chemical Analysis of typical Copper Slag Abrasive
A typical analysis of Copper Slag abrasive may show the chemical content to be similar to the following table:
Notice that there is very little copper in any form, since the slag is the by-product of copper extraction from ore. Notice also that most of the contents are oxides of one metal or another.
Compressors
Compressed air is a common source of power for blasting machinery, paint spray equipment, power tools etc. It is favoured on site because it is relatively safe, being less dangerous than, say, electricity. In order to produce quantities of compressed air, it is necessary to use a compressor. Normally driven by a diesel motor, a compressor draws in atmospheric air, pressurizes it, and feeds the air into a pressure vessel (known as a receiver). The air is then held in the receiver until demanded by the equipment in use.
The production of compressed air gives two problems to the surface preparation process. These are:
• Any change in atmospheric pressure may result in the release of water vapour from the air.
• Because compressed air in the receiver is stored by pressurizing an oil reservoir, there is a possibility of oil vapour being retained by the air as it is released.
Both of these factors require that adequate vapour traps are fitted to blastcleaning equipment in order to remove the contaminating oil and water.
Compressors are rated for:
• Air pressure, measured in pounds per square inch (p.s.i.) or bar.
Air pressure is normally set at a maximum of 100 p.s.i. For portable compressors (in the U.K.) and this pressure, if successfully maintained, is capable of producing an efficient blastcleaning operation.
• Capacity, measured in cubic feet per minute (c.f.m.) or litres per minute.
The capacity of a compressor will determine the quantity of air it is able to deliver at its working pressure.
For blastcleaning purposes, it is better to have a large capacity compressor working below its maximum level rather than a smaller compressor that is working at or near to its maximum level.
Blast Cleaning Cabinets
It is sometimes desirable to blastclean individual items in an enclosed space so that other trades can continue to work in the immediate vicinity. If this is a regular requirement, many factories will buy or build a blastcleaning cabinet. The size of typical cabinets may vary from the very small 'cupboard' where blasting is done from outside the cabinet, with hands inserted through holes in the side to the relatively large blasting room. The more sophisticated blast rooms may have a rail system to transport large items into the room, and will have grit recovery and recycling systems. In general, the blast cleaning apparatus is similar to that used for on site blasting.
The most complex of the blastcleaning cabinets are designed for large quantities of steel to be blastcleaned on a regular basis, such as all plate received by a shipbuilding yard. These machines, often known as 'Wheelabrators', are designed to work on a continuous basis, and include a conveying system that will carry items through the cabinet continuously.
It is usual for these cabinets to use a system of rotating vanes to propel the abrasive, from which the term wheel-abrator has been adapted for general use. These cabinets too have an abrasive recovery and recycling system, and are capable of very high rates of cleaning.