abrasive blast cleaning

Abrasive Blast Cleaning of Non-Ferrous Metals

Most of the SSPC surface preparation standards focus on the preparation of steel, either by abrasive blast cleaning (dry and wet), power tool cleaning, or water jetting.  Recognizing that galvanizing, non-ferrous metals and various types of stainless steel also need to be prepared for painting, SSPC published a standard for brush-off blast cleaning these metals in 2010: SSPC-SP 16, Brush-Off Blast Cleaning of Coated and Uncoated Galvanized Steel, Stainless Steels, and Non-Ferrous Metals.  In 2017, SSPC initiated the development of a standard for the more thorough blast cleaning of the same metals.   The new standard is preliminarily titled “Thorough Abrasive Blast Cleaning of Non-Ferrous Metals or Stainless Steels.”

This article summarizes the requirements of SSPC-SP 16, the likely requirements of the new standard, and the abrasives used for blast cleaning of non-ferrous metals.

SSPC-SP 16, Brush-Off Blast Cleaning of Coated and Uncoated Galvanized Steel, Stainless Steels, and Non-Ferrous Metals

abrasive blast cleaning
Figure 1 – SSPC-SP16

The definition of SSPC-SP 16 is as follows:

2.1 A brush-off blast cleaned non-ferrous metal surface, when viewed without magnification, shall be free of all visible oil, grease, dirt, dust, metal oxides (corrosion products), and other foreign matter. Intact, tightly adherent coating is permitted to remain. A coating is considered tightly adherent if it cannot be removed by lifting with a dull putty knife. Bare metal substrates shall have a minimum profile of 19 micrometers (0.75 mil).

2.1.1 The entire surface shall be subjected to the abrasive blast to achieve the specified degree of cleaning and to produce a dense and uniform surface profile on the bare metal substrate. The peaks and valleys on the surface shall form a continuous pattern, leaving no smooth, unprofiled areas. Tightly adherent coating is permitted to remain. A coating is considered tightly adherent if it cannot be removed by lifting with a dull putty knife.

2.1.2 Intact coatings that are present shall be roughened and cleaned as specified in the procurement documents. If the surface profile is not specified in the procurement documents, the abrasive selected shall roughen the cleaned surface to the degree required by the product data sheet for the coating to be applied.

To comply with SSPC-SP 16, areas where the substrate is bare must be densely and uniformly roughened with a minimum surface profile of 19 micrometers (0.75 mil).  Any amount of coating can remain if it is intact and tightly adherent and the surface is roughened.  The same dull putty knife test that is used in the other SSPC surface preparation standards to determine whether the remaining coating is tight is also used in SSPC-SP 16.

There are no industry guides that depict the extent of roughening required by SSPC-SP 16, but Figure 2 is an example of an approach taken for a failure investigation.  Before visiting the site, a stainless steel coupon was blast cleaned according to the requirements of SSPC-SP 16 to show the expected degree of roughening. The coupon was compared with the substrate beneath the failing coating in multiple locations to determine if the roughening complied with the requirements of SSPC-SP 16.

abrasive blast cleaning
Figure 2 – The specification required that the stainless steel be blast cleaned to SSPC-SP 16. Because visual guides illustrating SSPC-SP 16 are not available, a stainless steel coupon was blast cleaned to SSPC-SP 16 to demonstrate the expected degree of roughening for comparison. The surface did not comply.

Draft SSPC Standard, “Thorough Abrasive Blast Cleaning of Non-Ferrous Metals or Stainless Steels”

Since the standard is still being developed by the committee, it is not appropriate to provide the actual text, but the cleaning requirements and commentary on abrasive selection can be summarized as follows, per Draft 1b, dated March 17, 2019.The standard requires that the entire surface be abrasive blast cleaned to remove all visible oil, grease, dust, dirt, oxides (corrosion products), coating, and other foreign matter with random color variations on the surface allowed.  The color variations are confined to no more than 5 percent of each 5,800 mm2 (9 in2) unit area of surface.  The entire surface must exhibit a dense, uniform surface profile, of a depth appropriate for the coating being applied.

The draft also provides guidance on abrasive selection for aluminum, copper alloys, and stainless steel: aluminum oxide, garnet, stainless steel, encapsulated (sponge), plastic pellets, agricultural abrasives (corn, walnut shells, peach pits), glass beads, sodium bicarbonate (backing soda), and carbon dioxide (dry ice). Steel abrasives are to be avoided. 

An abrasive that creates a given profile depth on steel, will typically provide a deeper surface profile on softer alloys such as aluminum and copper. To prevent or minimize damage to softer alloys or warping of thin gage metals, finer, softer, and less dense abrasives should be used, provided they achieve the desired surface profile (e.g., aluminum/magnesium silicate, soft mineral sands, soft crushed glass and glass bead media, and organic media such as corncobs or walnut shells).  Another option to minimize damage and warping is to reduce the blast pressure (e.g., 0.45 MPa [65 psi]), and use greater stand-off distances.

Comparison of the Two Standards

SSPC-SP 16 requires the removal of all loose coating.  The entire surface, both the bare metal and the remaining coating, is roughened.  The new draft standard removes all coating and surface interference material, and completely roughens the substrate.  The differences between the two are similar to SSPC-SP7, Brush-Off Blast Cleaning and SSPC-SP10, Near-White Blast Cleaning for carbon steel.

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