一周年庆典 STEEL PREPARATION

BMA004

Prior to surface preparation, the steel surfaces must be prepared, both as concerns the steel works and the removal of contaminants. This is essential, as poorly prepared steel surfaces will have a detrimental effect on the length of the service life of any coating system applied.
In certain circumstances the surface preparation will be part of the steel preparation. This is the case when steel structures to be painted are, e.g. metalized, galvanized or phosphatized. The surface preparation, which may be abrasive blasting or acid pickling is carried out as part of the process prior to the application of the paint system. As the mentioned protection processes commonly are carried out either at a different location or quite some time before the application of the paint (or both), it will always be necessary to prepare the steel surfaces at least by washing/cleaning.
The same, but to a lesser extent due to their function as temporary protection, is valid for prefabrication primers (shop primers).
Steel works—steel quality
Preferred rust degrees of raw steel are A and B according to ISO 8501-1 (please note that non-corroded steel may show impressions from the hot-rolling very similar to corroded pits). Any parts of cast iron must have a non-porous surface. Steel which has been wheel-abraded (cleaned by centrifugal blasting) and given a prefabrication primer (shop primer) should be without any pitting corrosion and with the prefabrication primer in good condition.
Steel works—design
No openings, holes, etc. in, e.g. structural members should be less than 50 mm in diameter and distances between structural members should also be above 50 mm as dictated by the possibilities for proper application of the paint. Furthermore, structural members and details should be designed in such a way the water traps giving raise to stagnant pools, are avoided.
Pipe work should be designed so no turbulence will occur. Drainage should be provided when the construction gives rise to water traps. Sharp edges should be avoided. Direct metallic contact between to metals with a great difference in potential is not advised unless the metals are insulated from each other.
The construction should in general be designed in such a way that it will not obstruct the surface preparation or the paint application according to the prevailing specification.
In the case of one-coat paint systems, the in-service performance is extremely dependent on optimum application of the paint film. A design which is as application friendly as possible, i.e. with fewest possible structural members—will play a very important role in achieving the full benefit of such systems.
Steel works—preparation
All welding seams must have a surface finish, which ensures that the quality of the paint system will be maintained in all respects. Holes in welding seams, undercuts, cracks, etc. should be avoided. If found, they must be remedied by welding and/or grinding. The welds are should be prepared according to WELD REPLICA NACE RP 0178 (Grade D) as a minimum.
All sharp edges should be ground to a radius of curvature of minimum 3 mm so that the specified film thickness can be built up. Rolled profiles, etc. from the steel mills normally have acceptably rounded edges.
All weld spatters and laminations must be removed. Repair by means of welding/grinding should be carried out where necessary. The use of Anti-Spatter agents is generally NOT recommended.
Edges of thick steel cut by gas cutting may be so hard that approx. 0.3 mm must be removed by grinding before a suitable surface profile can be obtained during abrasive blasting.
If certain small defective areas cannot be remedied immediately, these areas should be taped off for later repair and their positions noted. However, sufficient time should be allowed at the steel survey so that any repair work may be carried out immediately.
Before a survey the surfaces must be cleaned and dried to allow adequate inspection.
Removal of contaminants—oil/grease
Oil and/or grease on surfaces which are to be prepared for paint application can be easily detected by the “water-on-goose” method. This consists of sprinkling water onto surfaces, which are suspected of being contaminated with oil or grease. If the water forms small droplets on the surface, it is highly certain that there are contaminants of this nature present. Such oil and grease contamination will need to be removed.
This is best done with a water-based, alkaline degreaser, followed by fresh water hosing (cold or hot). Such alkaline degreasers will also remove soluble salts, which may be present on the surface. Care should be taken to ensure that the pH-value of such alkaline degreasers is not too high for degreasing of zinc or aluminum, however, degreasers with pH-values up to 12 or 14 are fully acceptable to be used for iron or steel.
In industrial plants details are often degreased in alkaline baths, as this is more efficient than degreasing through spray application of the degreaser. Such baths are usually hot and even boiling. Alkaline are highly caustic and care should be taken in handling such degreasers. Especially when hot.
Emulsifiable solvent-based degreasers may also be used. This class of degreasers are concentrated mixes of strong solvents with a high degree of solvency towards oil and grease, emulsifiers and (sometimes) water. These degreasers should also be followed by fresh water hosing, and hosing down with hot water or steam leaves the least remnants of oil on the surface. Some emulsifiable degreasers are specially developed for cold water hosing.
Important: Please note that washing oil/grease contamination with solvents or thinners and a clean rag will only spread the contaminations over a larger area and should be avoided.
Removal of contaminants—soluble salts
Soluble salts, if present on a substrate at time of application of paint systems, will later cause osmotic blistering and premature breakdown of the paint system applied.
Surfaces to be painted should be hosed down with fresh water and allowed to dry prior to application of the paint system. It should be noted that the time between fresh water hosing down and application of the first coat in the paint system should be as short as possible in order to minimize the danger of re-contamination of the washed surfaces. If the time between coats is unduly long, the surfaces should be checked for any contamination with soluble salts before application of the subsequent coat(s). If contamination is found, the surface should be hosed down again.
Checking surfaces for contamination with soluble salts is best done in accordance with ISO 8502-6.
If it is suspected that there are soluble iron salts on the steel surface this is best checked in accordance with ISO 8502-1.
Removal of contaminants—miscellaneous
Dust, f.i. on internal tank areas, are best removed through vacuum cleaning. Checking if unacceptable amounts of dust are present on the substrate is done according to ISO 8502-3.
Micro-condensation or ice formation on surfaces may sometimes be extremely difficult to see with the naked eye. If it is suspected that there is either micro-condensation or ice formation on substrates, this is easily checked by means of water indicating paper. The paper is pressed to the surface for a period of 30 seconds (glove must be worn to avoid reaction with moisture on the fingers) and checked for colour changes indicating moisture on the surface (the heat from the hand will melt any ice).

joettts

There is a question,
What's proper preparation for SGCC material ,before painting ?
please advice !
thanks !

coil

晕啊，水平有限，看不懂啊，哪位大侠翻译下？