Surface Preparation Standards SSPC/NACE ISO 8501

Understanding Abrasive Blasting Cleaning Standards SSPC/NACE and ISO 8501

Comparing the two predominant abrasive blasting cleaning standards, ISO 8501 and SSPC/NACE, can be challenging. Although they accept roughly the same levels of cleanliness, they classify them in opposing ways.

ISO 8501

ISO 8501 was published in 1988 by the International Organization for Standardization after combining the contents of the 1967 Swedish Standard SIS 055900 with the German DIN 55928. ISO 8501 is a pictorial standard that illustrates the appearance of various rust grades at different cleaning levels, including textual descriptions of the cleanliness levels. ISO 8501 lists the cleanliness levels in order of increasing work required.

• Sa 1: Light Abrasive Cleaning

• Sa 2: Extensive Abrasive Cleaning

• Sa 3: Abrasive Cleaning for Visually Clean Steel

SSPC/NACE

In North America, the original surface preparation standard was written in the 1960s by an architectural group for steelworkers in Pittsburgh. The Steel Painting Structures Association was formed within the framework of these standards. SSPC standards consist of textual descriptions and, while not pictorial, include visual guides (VIS) with photographic references. SSPC/NACE numbers them in reverse order according to increasing surface cleanliness.

• SP 5: White Metal

• SP 6: Commercial

• SP 7: Brush-off

The industry was sufficient until a new specification was requested for a level of cleanliness that could reduce costs by substituting for White Metal when near-white was considered good enough. ISO adapted this degree into its existing hierarchy as Sa 2 1/2, Very Thorough Abrasive Cleaning; SSPC deviated from the order and added SP 10 Near White. The two versions were not equivalent: Sa 2.5 allows stains, marks, and darkening from rust, mill scale, and coatings to remain on up to 15% of the surface, whereas SP 10 allows only up to 5%.

In 2000, SSPC and NACE published joint standards, anticipating a merger between the organizations, whose memberships largely overlapped. The merger did not succeed, but the new numbering system from NACE, which aligned NACE No. 1, 2, 3, 4 with SSPC SP 5, 6, 10, 7, continued to be used.

Despite the differences, the cleanliness degrees are generally considered compatible.

*Estimated surface area. ISO 8501-1 is a pictorial reference and does not explicitly state percentages.

Why Surface Preparation Standards?

Surface preparation standards exist to maximize coating life and minimize costs.

Surface preparation accounts for 40% of the painting project cost; facility owners want to limit the materials and hours spent on blasting. Achieving White Metal is costly, especially in maintenance work, and is typically reserved for critical applications where the cost of failure is devastating.

During the surface preparation stage, when selecting a coating, the costs of blasting and painting should be assessed and compared against the risk of early degradation of the coating.

Surface Preparation Standards

Solvent Cleaning

SP 1 / ISO 8504

• Loose adhering material: 100%

• Firmly adhering material: 100%

• Stains, lines, darkening: 100%

Abrasive blasting will not remove oils and greases; it leaves their stains on the surface, causing early degradation of coatings. Visible oil, grease, and dirt deposits must be cleaned before abrasive blasting. SP 1 is a prerequisite for other SSPC abrasive blasting specifications.

Numerous methods are specified for solvent cleaning. The most common and least effective method is washing with soap, water, and a cloth. A dirty cloth can also transfer grease and oil: care should be taken to frequently wipe, fold, repeat, and replace. Although soap residue can prevent adhesion of the coating on large surfaces and rinsing may be necessary, pressure washing with soapy water is recommended.

All specifications can be found at sspc.org.

Brush-off

SP 7 / NACE #4 / Sa 1

Light Abrasive Cleaning, or sweeping the sand.

• Loose adhering material: 0%

• Firmly adhering material: 100%

• Stains, lines, darkening: 100%

Brush-off is recommended for corrosive environments with a short expected coating life, such as ship hulls with anti-corrosive paint or non-corrosive service environments like the outside of a tank.

All specifications can be found at sspc.org.

Industrial Abrasive Cleaning

SP 14 / NACE #8

• Loose adhering material: 0%

• Firmly adhering material: 10%

• Stains, lines, darkening: 100%

Industrial Abrasive Cleaning specifies that 90% of firmly adhering material must be removed. Stains, lines, and darkening from rust, mill scale, and old coatings should be present on 100% of the surface.

This is defined for conditions where the existing coating is thin, well-adhered, and compatible with the new coating. It is the latest abrasive blasting standard and is not commonly specified. ISO does not have a corresponding specification for this.

All specifications can be found at sspc.org.

Commercial Abrasive Cleaning

SP 6 / NACE #3 / Sa 2

Extensive Abrasive Cleaning

• Loose adhering material: 0%

• Firmly adhering material: 0%

• Stains, lines, darkening: 33%

Commercial Abrasive Cleaning states that all firmly adhering materials must be removed. Stains, lines, and darkening can remain on up to 33% of the surface.

All specifications can be found at sspc.org.

Near White Abrasive Cleaning

SP 10 / NACE #2 / Sa 2.5

Very Thorough Abrasive Cleaning

• Loose adhering material: 0%

• Firmly adhering material: 0%

• Stains, marks, darkening: SP 10 5%, Sa 2 ½ 15%

Near White Abrasive Cleaning specifies that darkening, marks, and stains should be limited to 5% of the surface area. The advantage of blasting to White Metal is determined when the added cost is not justified.

High-performance coatings exposed to harsh environmental conditions, such as high humidity and proximity to saltwater, are specified for steel on oil drilling platforms, shipyards, and other marine environments.

All specifications can be found at sspc.org.

White Metal Abrasive Cleaning

SP 5 / NACE #1 / Sa 3

Abrasive Cleaning for Visually Clean Steel

• Loose adhering material: 0%

• Firmly adhering material: 0%

• Stains, lines, darkening: 0%

White Metal is the highest class of abrasive blasting cleanliness. No darkening, lines, or stains are permitted. When viewed without magnification, the surface should be free of visible oil, grease, dust, dirt, mill scale, rust, coating, oxides, corrosion products, and other foreign matter.

White metal is specified for steels where the destructive effects of coating degradation can justify the extra cost, such as steels working in high temperatures, high pressure, and corrosive environments, as well as in nuclear reactors, turbines, chemical tank linings, submarines, etc.

All specifications can be found at sspc.org.

Using the Standards

ISO 8501 includes pictorial references that show how each blasting specification appears at various rust grades and initial conditions. It is available in a hard cover A5 format for direct comparison to surfaces. SSPC/NACE also sells visual guides for direct comparison with surfaces but provides written descriptions as standards.

Surface preparation standards provide a basis for the service level agreement between blasting operators, contractors, inspectors, and project owners. Familiarity with the standards is important for any abrasive blasting operator and a requirement for certified blasting operators.

The specifications include detailed methods and practices for surface preparation. The specifications are frequently updated, so project standards should be referenced with their names, dates, version numbers, and appendices when discussed.