Surface PreparationSubstrate surface preparation is an integral part of any coating deposition process. This includes removing unwanted impurities and residues, as well as conditioning the surface to achieve good adhesive bonding. By taking time to adequately prepare the surface, both coating quality and endurance will be optimised. Inadequate surface preparation is the single greatest cause of surface treatment failure. The selection of the appropriate cleaning and surface preparation techniques is dependent on the nature of the surface and the intended coating or surface treatment technique. The different methods to prepare surface are:
These procedures are further explained in the following paragraphs. 1-Manual cleaningDifferent tools are used to wipe away rust, oxide scale and foreign materials from the surface. It is often not completely effective and, to obtain a good finish, the costs can be very high. 2-Mechanical cleaningPneumatic and electrical devices are used to clean the surface. It provides considerable improvement over manual cleaning but, used alone, the surface condition is often not good enough to provide adequate coating adhesion. It must be followed by chemical cleaning as below. 3-Flame cleaningA slightly oxidising flame is applied to the metallic surface. Surface water and other volatile contaminants are eliminated, but tenacious deposits are not removed, and distortion of the surface might occur. Alone, this technique cannot provide sufficient cleaning. 4-Blasting dry/wetA stream of sand, wire, metal or glass particles (grit/shot) is applied at high velocity under controlled conditions in a closed blasting chamber Depending on the required finish, the velocity, type of abrasive media, etc, can be adjusted. Normally, the process is used only to clean the surface, achieve a specified roughness (ie 'blasting'), repair some imperfections or prevent fatigue cracks propogating into the base material (ie 'Shot peening'). In the case of thermal spraying, it is often the only surface preparation that is required. 5-Grease removingThis is performed with solvents and detergents and eliminates contaminants from the base surface. It is used before applying the coating and, in combination with the other surface preparation methods, helps to ensure coating adhesion. 6- Alkaline bath/Electrolytic bathAlkaline: A combination of ingredients such as surfactants, saponifiers, emulsifiers, stabilisers, etc. They are physically and chemically active and reduce interfacial surface tension. Electrolytic: It is a modification of an alkaline bath. An elctrical current is used to produce gassing on the surface, which then helps to remove contamination. 7-Acid surface preparationThis is acid cleaning or pickling. Usually, sulphuric acid is used, together with an oxidising inhibitor to stop rust formation in the least covered zones and to minimise the formation of toxic vapours. Afterwards, the part must be washed with a hot water solution of neutralising characteristics to eliminate acid residues. It is especially important to control concentration, temperature and immersion time of the process. There are multiple variations of this treatment, with different acids, oxidising inhibitors and immersion times to achieve a specific condition. In Electrolytic cleaning, the metal is made the anode or cathode and is submerged into an electrolitic acid solution. 8- Molten salt cleaningA salt bath is used in order to removal scale. Procesess can be classified as oxidizing, electrolytic, and reducing. If complete scale removal is required, an acid pickling must be applied after the salt bath process. 9-Ultrasonic cleaningHigh frequency sound waves pass through a cleaning agent, creating minute gas bubbles which enhance the cleaning process by a mechanism of cavitation. It is probably the most efficient cleaning process. Because of its high capital investment required, it is often uneconomical. Return to the
Guide to Surface Engineering |