Surface preparation is the essential first stage treatment of a steel substrate before the application of any coating, and is generally accepted as being the most important factor affecting the total success of a corrosion protection system. The performance of a coating is significantly influenced by its ability to adhere properly to the substrate material. Residual millscale on steel surfaces is an unsatisfactory base to apply modern, high performance protective coatings and is therefore removed by abrasive blast cleaning. Other surface contaminants on the rolled steel surface, such as oil and grease are also undesirable and must be removed before the blast cleaning process.
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This was straightforward enough until the industry demanded a new specification for a cleanliness grade that could cut costs by replacing White Metal in situations where near-white was good enough.
The two versions were not equal: Sa 2. ISO declined to shoe-horn the new specification into their system as Sa 1. Despite the differences, the grades of cleanliness are generally thought to be compatible. They reflect similar permissible levels of stains and tightly-adhered rust, mill scale and coatings, and can be summed up with a chart:. ISO is a visual reference and does not explicitly state percentages. The specifications specifically mention stains, streaks and shadows, but they are practically the same: a residue showing a difference in color but of no discernible thickness.
Tightly adhered material refers to anything that cannot be peeled off with a dull putty knife. White Metal is expensive to achieve, especially on maintenance jobs, and typically reserved for critical applications where the cost of failure is catastrophic. Near white is good enough for service in most severe environments. Commercial is less expensive and suitable for non-corrosive atmospheres and service environments.
Brush Off will save the owner the most money in the short run, if he can get away with it. When choosing a coating, the owner weighs the costs of blasting and painting against the risk of a premature coating failure. If the worst-case scenario is that he has to repaint in 5 years instead of 7, he might save money by cutting back from Commercial to Brush-Off. Visible deposits of oil, grease and dirt must be spot cleaned prior to abrasive blast cleaning.
The standards specify numerous methods for solvent cleaning. The most common method — and the least effective — is washing with soap, water and a rag.
A dirty rag will also smear grease and oil: care must be taken to wipe, fold, repeat, and replace often. For large surfaces, pressure washing with soapy water is recommended, although soap residue will inhibit coating adhesion and should be rinsed off. Brush Off is specified to remove loose rust, mill scale and coatings, and uniformly roughen up a surface in preparation for a new coat. Tightly-adherent materials are permitted to remain.
Brush Off is specified where the expected life of the coating is short, such as ship hull antifouling, or in mild atmospheres and non-corrosive service environments, such as the exterior of a tank, in a rural location. Industrial is specified for conditions when the existing coating is thin, well-adherent and compatible with new coating. It is the most recent abrasive blasting standard, and not widely specified.
ISO has no corresponding specification. Commercial Blast Cleaning specifies that all tightly-adhering matter must go. Commercial is specified when a high, but not perfect, degree of cleanliness is warranted.
Near White is typically specified for high performance coatings over steel exposed to severe environmental conditions, such as chemical spills and fumes, high humidity, and proximity to salt water. It is commonly specified for off-shore platforms, shipyards and other marine environments.
White Metal is the highest grade of abrasive blast cleaning. No shadows, streaks or stains are permitted. When viewed without magnification, the surface shall be free of all visible oil, grease, dust, dirt, mill scale, rust, coating, oxides, corrosion products and other foreign matter.
White metal is specified for steel serving under high temperatures, high pressures, and corrosive environments, and in cases where the catastrophic consequences of coating failure justify the extra expense, such as nuclear reactors, turbines, chemical tank linings, submarines, etc. ISO is a pictorial standard containing reference photographs that illustrate what each blast specification looks like on a variety of rust grades and initial conditions. It comes in a hardcover A5 format that can be directly compared to the surface.
Judging percentages of stains is an imprecise art that often causes rulers to come out. Preparing a job standard is a best practice for avoiding disputes:. Surface preparation standards provide a basis for a service level agreement between blasters, contractors, inspectors and project owners. Knowing the standards are important for any abrasive blaster, and is a necessity for certified blasters. The specifications contain detailed methods and practices for preparing surfaces. They are frequently updated, so refer to them by name, date, edition number, and supplement when discussing project standards.
We hereby confirm receipt of your message and will contact you shortly. What you need to know to choose the right nozzle for your abrasive blasting application. Know the facts: there is no such thing as dustless or dust-free blasting in surface prep. A guide to calculating a bid that is designed to meet your target profit margin. Yes, I agree to cookies.
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Surface Prep Standards Explained - SSPC/NACE & ISO 8501