Surface profile (anchor pattern) and degree of cleanliness are the two important factors to take into consideration when preparing surfaces for a coating. Both are critical to the performance of any coating system. Whilst often mistaken as being one in the same, it is very important to know the difference between these two measurements.
Coating manufacturers have long recognized the necessity of proper surface preparation for their coatings to succeed. Inadequately prepared surfaces will cause costly, premature failure of the coatings. Consequently, coating manufacturers specify how the surfaces are meant to be prepared prior to applying their products. If their specifications are not adhered to they cannot guarantee the performance of their products. With this in mind, a considerable amount of research has been undertaken by coating manufacturers to establish the type of surface their products will need to insure the positive, uniform bonding of their coatings to the surface for the required length of time.
Surface Profile/Anchor Pattern
A surface profile (anchor pattern) can be produced by propelling abrasive particles with compressed air at a high velocity, through a blast nozzle, on to a surface. The abrasive particles cut into the steel to form peaks and valleys on the surface. The resultant profile (peaks and valleys) provides an anchor pattern for the applied coatings to grip onto.
Size, type and hardness of the abrasive, blast nozzle pressure, distance and angle of impact affect the depth of profiles. Different abrasives create different profiles; reinstating the importance of abrasive selection!
Profiles are measured in mills, microns, or millimeters. The most common term used in the RSA is microns when referring to paint thickness as well as surface profile. Typically, specifications will state a micron profile average due to the wide range of abrasive particle sizes within a given abrasive supply. For example, an average profile of 50 microns will actually show a mixture of profile readings ranging from 25 to 75 microns. Precise profiles are simply not possible as there is no practical method of reproducing a perfect 50 micron profile throughout a surface due to varying particle sizes and varying surface hardness’s on any given surface.
Once the correct abrasive has been selected, it is important to establish strict blasting techniques. Any deviation in nozzle pressure, stand-off distance, or angle of attack will affect profile results. Reduction of air pressure and an increased stand-off distance may cause smaller profiles. Severe nozzle angles may produce a skimmed blast pattern, rather than defined peaks and valleys.
Different types of blasting media create different types of profies.
There are various types of instruments for measuring profiles. It is essential to utilize a measuring device to check and document profile conformance.
DeFelsko Surface Profile Gauge DeFelsko Testex Tape Gauge
Degrees of Cleanliness
In addition to stipulating a profile, a coatings data sheet will dictate a required degree of cleanliness. Four general grades of cleanliness have been established by professional industrial organizations for abrasive blast cleaning. The four grades range from a quick blast of loose materials to 100% removal of all contaminants. The degrees are usually understood as white metal blast, near white metal, commercial blast and brush-off blast. Cleanliness of a blasted surface is therefore a set of visual standards to be cross-referenced with its relevant visual guide.
|Comparison of International Codes|
|Degree of Cleanliness||SSPC||NACE||SIS|
|White Metal Blast||SSPC-SP5||NACE NO.1||SA-3|
|Near White Metal Blast||SSPC-SP10||NACE NO.2||SA-2 ½|
|Commercial Blast||SSPC-SP6||NACE NO.3||SA-2|
|Brush Off Blast||SSPC-SP 7||NACE NO.4||SA-1|
- White Metal Blast Cleaned Surface: A white metal blast cleaned surface, when viewed without magnification, shall be free of all visible oil, grease, dust, dirt, mill scale, coating, oxides, corrosion products and other foreign matter.
- Near White Metal Blast: A near white metal blast cleaned surface, when viewed without magnification, shall be free of all visible oil, grease, dust, dirt, mill scale, rust, coatings, oxides, corrosion products, and other foreign matter. Random staining shall be limited to no more than 5% of each unit area of surface (a square 76mm x 76mm), may consist of shadows, slight streaks, or minor discolorations caused by stains of rust, stains of mill scale, or stains of previously applied coating.
- Commercial Blast: A commercial blast cleaned surface, when viewed without magnification, shall be free of all visible oil, grease, dust, dirt, mill scale, rust, coatings, oxides, corrosion products and other foreign matter. Random staining shall be limited to no more than 33% of each unit area of surface (a square 76mm x 76mm), and may consist of light shadows, slight streaks, or minor discolorations caused by stains of rust, stains of mill scale, or stains of previously applied coating.
- Brush Off Blast: A brush off blast cleaned surface, when viewed without magnification, shall be free of all visible oil, grease, dirt, dust, loose mill scale, loose rust, and loose coating. Tightly adherent mill scale, rust, and coating may remain on the surface. Mill scale, rust and coatings are considered tightly adherent if they cannot be removed by lifting with a dull putty knife after abrasive blast cleaning has been performed.