The wet abrasive blasting standards explained

Standards to determine the level of cleanliness and flash rust on steel surfaces after wet abrasive blasting

When is a steel surface ready to guarantee a good coating adhesion?

The wet abrasive blasting standards help to determine the cleanliness, flash rust, and surface profile of blasted steel surfaces. They need to be separately specified when preparing a blast-cleaned steel surface.

The main standard for steel surface preparation is ISO 8504-2. This international standard describes different abrasive blast cleaning methods for the preparation of steel surfaces. Since vapour blasting is the latest blast cleaning method, it is not described. However, we can easily extrapolate vapour blasting to the ISO 8504-2, ISO 8501-4, and ISO 8501-1
 

There are no dedicated ISO standards for determining the visible cleanliness of blasted surfaces. The best method is the IP method, which introduces internal standards that combine ISO 8501-1 – visual assessments of blasted surfaces – and ISO 8501-4, which describes flash rust grades.

The best standards describing the cleanliness of WAB (wet abrasive blasting) are joint NACE and SSPC standards. They cooperate with the previously issued SSPCVIS5/NACE VIS9, which describe the flash rust grades.

These standards combine elements of the existing standards for dry abrasive blast cleaning with elements of the 2012 SSPC/NACE water jet cleaning standards. The definitions of cleanliness for the steel surface immediately following wet abrasive blast cleaning are identical to the definitions in the five dry abrasive blast cleaning standards:

  •  SSPC-SP 5 (WAB)/NACE WAB-1: White metal wet abrasive blast cleaning
  • SSPC-SP 10 (WAB)/NACE WAB-2: Near-white metal wet abrasive blast cleaning
  • SSPC-SP 6 (WAB)/NACE WAB-3: Commercial wet abrasive blast cleaning
  • SSPC-SP 7 (WAB)/NACE WAB-4: Brush-off wet abrasive blast cleaning
  • SSPC-SP 14 (WAB)/NACE WAB-8: Industrial wet abrasive blast cleaning

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The five wet abrasive blasting standards define cleanliness for steel surface
Original Brush-off (1) Industrial blast cleaning (2) Commercial blast cleaning (3) Near-white blast cleaning (4) White metal blast cleaning (5)
Standard SSPC SP7
NACE No. 4
ISO SA 1
SSPC SP14
NACE No. 8
ISO -
SSPC SP6
Nace No. 3
ISO SA 2
SSPC SP10
NACE No. 2
ISO -
SSPC SP5
NACE No. 1
ISO SA 3
Loose material 0% 0% 0% 0% 0%
Tight material 100% Up to 10% 0% 0% 0%
Stains & shadows 100% 100% Up to 33% Up to 5%* 0%
Why assessing the flash rust level is important

The cleanliness levels for wet abrasive blasting are identical to those of dry abrasive blasting. However, there is one important difference: the formation of flash rust as a result of wet abrasive blasting. Flash rust forms when the water used for cleaning evaporates and forms an oxidation layer on the cleaned surface. Flash rust occurs typically within 30 minutes and the colour depends on the characteristics of the steel and the level of wetness.

Not all coatings have the same tolerance for the presence of flash rust. Before applying a protective coating, the contractor must know the maximum permissible level of flash rust on the steel surface. Flash rust can be reduced by drying the surface after blasting or using an inhibitor.
 

4 degrees of flash rust

The wet blasting standards define four degrees of flash rust that help to assess how much flash rust has developed.

These definitions are based on the extent to which the flash rust obscures the underlying steel substrate, the ease with which it can be removed by wiping with a cloth, and the amount of material that appears on the cloth after the surface is wiped.

·       No flash rust
No rusting is visible with the naked eye.

·       Light flash rust
Small portions of rust, whether evenly distributed or in certain areas, are visible with the naked eye. It is strongly adherent and hard to remove by lightly wiping with a cloth.

·       Moderate flash rust
A rust layer that covers significant parts of the carbon surface. It is reasonably adherent and leaves light marks on a cloth when lightly wiped.

·       Heavy flash rust
A heavy rust layer that hides the steel surface entirely. It is only lightly adherent and easy to remove, leaving substantial marks on a cloth when lightly wiped.

Flush rust can be classified in three levels
Light flash rust (1) Moderate flash rust (2) Heavy flash rust (3)
Surface is visible
Small quantities of rust are observed
Surface is obscured
Rust is well-adhered
Surface is obscured
Rust is loosely adhere
Surface profile

In the blast cleaning process, grains of abrasive are propelled with great force and energy at the work surface. Upon impact, the grains ‘dig’ into and then rebound out of the surface leaving a rugged, miniature ‘mountain and valley’ finish. This surface texture is expressed by the surface profile, which measures the size of the ‘shape’ created.

Flash rust removal can be done with the propelled grains of abrasive

The surface profile is critical to coating performance as it:
·       Increases the surface area
·       Provides a ‘key/tooth/anchor’ pattern for the coating to adhere to

Conclusion

The class of blast expresses the ‘cleanliness’ of the surface finish. It determines to what degree the rust, paint, and other contaminants have been removed. Both the surface profile and the class of blast are important features of the surface finish and need to be separately specified when preparing a blast-cleaned steel surface. 

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