There are several general stages that are followed to successfully prepare a surface for adhesion, coating, welding or other procedures.
1. Surface Condition Assessment
Before any preparation can begin, it is important to assess the condition of the surface. Standards can assist with this assessment, such as with BS EN ISO 8501-1, which provides details of rust grades ranging from A-D for steel surfaces. These grades may help determine if surface preparation will be successful and what techniques are going to be required. Surface profilometry may also be beneficial to identify if the existing surface can provide the required mechanical key for subsequent processing steps, although in most cases, some form of surface preparation will generally be required.
2. Remove Old Coatings
Surfaces generally require any old coatings to be removed before a new coating is applied. Applying a new coating over an old, failed coating will allow pre-existing problems like bubbling, flaking or peeling to continue beneath the covering layer. Stripping coatings back until there is a clean surface, such as white metal, will help to mitigate problems like corrosion and lengthen asset lifespans.
3. Remove Oils, Chlorides, Acids and Other Surface Contaminants
Old coatings are not the only substance that will need to be removed from a material’s surface. Many surfaces in industrial settings come into contact with oils, grease and other lubricants, all of which need to be removed so as not to compromise the bond strength between the substrate and the new coating.
Chlorides can also accumulate on a material’s surface as they travel through the air. This is particularly prevalent near the sea or other marine environments. Chlorides can cause rates of oxidation in metal to increase, meaning that corrosion damage will occur sooner; this is known as chloride induced corrosion.
Because these types of surface contaminant can be invisible to the naked eye, testing may be required to determine their presence.
4. Remove Loose Parts of the Surface
The surface of the material itself will also need cleaning of any loose parts that may flake or crumble. This can be achieved by techniques such as abrasive blasting, which will efficiently remove rust, mill scale and other loose parts from the surface of the substrate.
5. Profile the Surface
Once any loose materials and other substances have been cleaned from the surface it is important to profile the surface itself. New coatings may require a new surface profile than the original coatings. A correctly profiled surface, tailored to the coating process / material, will offer improved adhesion and mechanical bonding.
6. Dry the Surfaces
Coatings work best with surfaces that are dry. Wet surfaces can cause pinholes to develop during the curing process as moisture evaporates from between the surface and the coating, forming small holes. Although some coatings use a second layer to cover these pinholes, moisture can also negatively impact drying times. Flash corrosion can also occur where moisture is present on bare metal surfaces, this corrosion can continue beneath the surface of a newly-applied coating. Humidity can also be a factor, so it is worth checking whether a coating can be applied at the environment’s humidity level. The requirements are usually documented in the relevant coating and application standards.
Following these six general rules will improve the chances of successfully applying a coating, using an adhesive or creating a good weld.
Surface preparation ensures that a material is ready to receive a coating, be adhered to another material or other similar surface-related applications. A dirty, oily, flaking or otherwise compromised surface can greatly reduce the effectiveness of a new coating, cause adhesion failure or otherwise undermine a process.
Surface preparation works to ensure the best-possible mechanical bonding, adhesion or weld quality, while also helping to mitigate against later problems, such as corrosion and mechanical damage.
Surface preparation can take the form of chemical or mechanical methods, including the use of solvents, abrasive blasting materials, heat, acids and jets of water. There are also more traditional techniques using hand or power tools.
Abrasive blasting, sometimes termed grit blasting, [MR1] is generally deemed to be the most efficient and versatile surface preparation method and, as such, can be broken down into different types. However, solvent cleaning is another vital surface preparation technique depending on the requirements of the preparation and the condition of the material being prepared.
Here we detail different surface preparation methods:
Abrasive Blast Cleaning
This method of cleaning uses an abrasive jet of particles in a compressed air stream which impacts on the surface, removing impurities, mill scale, old paint and rust. Widely used in shipbuilding and repair , it is particularly effective in removing oxide corrosion. It is also widely used in the aerospace and medical sectors prior to the application of thermal sprayed coatings, and many other industries.
Abrasive blast cleaning can also be performed with centrifugal impellers that use revolving radial bladed wheels to throw abrasive material is the surface to be prepared. The force of impact varies according to the size of the wheels and their velocity.
A wide range of abrasives are available, including both non-metallic and metallic varieties like aluminium oxides or metallic grit such as chilled iron, as well as other plastic, ceramic and natural materials appropriate for the substrate materials and application requirements. The size of the abrasive particles can impact the quality and rate of cleaning, with fine grades being fine for relatively new surfaces and coarser grades being better for more heavily corroded surfaces. Higher purity grades of alumina blast media are also used in high value manufacturing processes such as aerospace and medical applications. In some instances, a mix of abrasive grades may be used. The abrasives can be recycled using separator screens and re-used.
Blast cleaning produces dust and debris that needs to be removed from the surface. This can be achieved with mechanical brushes or air blowers, sweeping, vacuum cleaning or washing with water or solvents.
There is a range of different levels of abrasive blast cleaning, from light blast cleaning through to white metal blast cleaning, which is the highest grade of abrasive blast cleaning . The requirements are defined in the appropriate industrial / application standards:
1. Light Blast Cleaning / Sweep Blast Cleaning / Brush Off
This light form of blast cleaning is used to remove loose coatings, mill scale and rust as well as roughen up surfaces ahead of a new coating being applied. This will not remove tightly adhered materials and is used where the expected life expectancy of a coating is short, such as with antifouling of ship hulls. It is also used for applications in mild atmospheres and non-corrosive environments where damage is likely to be light.
2. Industrial Blast Cleaning
Industrial blast cleaning will remove 90% of tightly adhered matter, although shadows, streaks, and stains from mill scale, old coatings or rust are allowed to remain. This type of abrasive blasting is best used where existing coatings are thin, well adhered and compatible with the new coating to be applied.
3. Commercial Blast Cleaning
Commercial blast cleaning will remove 100% of tightly adhering matter, but allows for shadows, streaks and stains to remain on 33% of the surface. This method provides a high, but not perfect, standard of cleanliness and is commonly used for items in non-corrosive environments.
4. Near White Blast Cleaning
Near white blast cleaning will remove all tightly adhered material as well as removing shadows, streaks and stains from at least 95% of the surface area. Less expensive than white metal blast cleaning, this technique is frequently used for high performance coatings over steel that is to be exposed to severe environmental conditions. It is commonly specified for use on offshore platforms, in shipyards and in other marine environments.
5. White Metal Blast Cleaning
This highest form of abrasive blast cleaning will remove all shadows, streaks or stains as well as all visible coatings, dirt, dust, grease, mill scale, oil, rust, oxides, corrosion products and other foreign matter. This method is used for instances where components are serving in demanding environments such as high temperatures and pressures, as well as in corrosive environments. Being the most efficient form of abrasive blast cleaning, this process is used where the consequences of coating failure justify the added expense, such as with nuclear reactors, chemical tanks, submarines, or critical turbines.
Wet Abrasive Blast Cleaning
Water can be introduced into an abrasive blast stream to help reduce dust, typically when removing lead-based paint or water-soluble contaminants. The conventional method is to use the same pressure as with dry blasting with the addition of the water being introduced behind the nozzle so that it is atomised and accelerates through the nozzle alongside the air and abrasive substance. Alternatively, the water can be added in controlled amounts at the base of the blast pot before mixing with the air and abrasive as they pass along the blast hose. A lower pressure system is also available whereby the water is injected into the air stream. The water then shrouds the abrasive/air mixture to prevent dust from escaping from the stream. However, the low pressures mean that fine abrasive particulates can remain on the surface and will need to be washed away. In some instances, inhibitors are used as part of the wet abrasive process to prevent the cleaned surface from rusting. In these instances it is worth checking that any leftover traces of the inhibitors will be compatible with any coating that is to be applied after. Alternatively, light dry blasting can remove any surface rusting following wet abrasive cleaning. It should be noted that the substrate must be dry prior to coating.
Hand and Power Tool Cleaning
Surface preparation can also be done using hand or power tools. Hand tool cleaning, such as with a wire brush or sander can be used to remove loosely adhered mill scale, rust or old paint. However, hand cleaning tends to leave a layer of tightly adhered coatings or rust on the surface. Power tools, such as rotary wire brushes, sanding discs or needle guns offer more effective and less laborious cleaning, including for power tool cleaning to bare metal. While these methods don’t provide the same level of cleaning as abrasive blasting, in some instances, they can be the only acceptable alternative methods.
Flame cleaning uses an oxy/gas flame that is passed over the surface, with the heat causing a differential expansion between the surface and rust scales. This causes the scales to flake off, but is not very effective at removing all of the rust and often needs to be applied alongside other techniques, such as hand tool cleaning to remove all loose rust or particles. Flame cleaning can also cause damage to coatings on the reverse side of the object being treated.
This technique involves immersing your material (usually steel) into a bath of acids that dissolve or remove mill scale and rust without attacking the exposed steel surface. This can be 100% effective, but is usually only used ahead of hot-dip galvanising for structural steels.
Ultra-High Pressure Water Jetting
Although combinational pressure fresh water cleaning is used to remove salts, fouling, loose coatings, paint or other contaminants following other surface preparation methods, ultra-high pressure water jetting is gaining in popularity as a surface preparation method in its own right. Using pressures of over 1,700 bar (25,000psi), ultra-high pressure water jetting is able to remove high percentages of soluble salts from steel surfaces without generating spent abrasives, which can be costly to dispose of.
The higher the pressures used, the less water is needed. The process warms the surface being treated, which allows the water to dry quickly without generating sufficient heat to cause thermal stresses in steel surfaces. As well as removing soluble salts, this environmentally friendly process also lifts rust, old paint and coatings from a surface. Small amounts of abrasive can be introduced into the water stream to create a rougher surface profile, but this can also increase operating costs.
As mentioned above, abrasive blasting cannot be used for all surface preparation requirements and, when it comes to removing oil, grease, dirt, and other random staining or grime, solvent-based cleaning is required. Also known as chemical cleaning, this method uses organic solvents or detergents. The simplest form of solvent cleaning is to use soap and water with a rag to wipe the surface down. However, this is also the least effective as the rag can end up just smearing the substance across the workpiece. Soap can also inhibit coating adhesion and other processes, so needs to be thoroughly washed off.
There are a number of extra considerations to be taken account of following surface preparation, depending on the exact nature of the surface and applications.
Welds and Cut Edges
Welds and cut edges can contain imperfections that create variable surface profiles, uneven surfaces or sharp projections that can cause problems for coating applications. Weld quality inspections do not usually consider coating requirements, meaning that welds can contain pinholes, projections, excessive undercutting, spatter or residual slag that can be detrimental to coating quality. Welds, cut edges and other unit areas can be treated to ensure coatings adhere in sufficient thicknesses or ground or filed down to improve the surface.
Surface cleanliness is important at all stages of coating, including on painted surfaces prior to the application of further coats. It is important to clean surfaces just before painting as they can accumulate dust, concreting grout leaks, blast cleaning products, or the products of bolting and welding.
Residual Blast Media
In some cases, residual blast media embedded in the substrate material can be detrimental to coating performance. It is important to select the appropriate blast media / substrate combination and blasting parameters to minimise the effects of embedded grit on coating performance in service.
Surface preparation is seen as being the most important factor for the success of a coating system. The performance of coatings are influenced by the ability of the coating to adhere to the substrate material. Surface preparation not only cleans away undesirable contaminants like mill scale or grease, but also introduces a suitable profile for the coating.
In the majority of cases, grit blasting is carried out manually, but is increasingly being mechanised to ensure process control and reproducibility to ensure performance in service.
Adhesive bonding requires a clean, dry surface and surface cleaning provides this, allowing for high bond strengths and durability. Different substrate materials require different surface preparation methods, from a simple brushing to more extensive abrasive cleaning. However, even where not much preparation is required, the smallest of activities can make a big difference to the consistency and effectiveness of a bond.
Surface preparation is an important step for a range of materials ahead of coating, welding, and other processes. There are a range of different techniques that range in effectiveness and cost. Generally speaking, the more complete the cleaning, the more expensive the technique. In some cases, several surface preparation techniques need to be combined, such as through the use of solvent cleaning to remove oils ahead of abrasive cleaning, followed by jet washing to remove left-over dust particles.
Whichever technique is chosen, surface preparation is important for the application of coatings and for effective adhesive bonding.
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