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Surface finish from abrasive blasting depends on the initial surface condition as well as the operation of abrasive blasting and the abrasive size, hardness and shape.

Initial surface condition of steel can be categorised as:

Rust Grade A

Steel which is completely covered by mill scale.

Rust Grade B

Steel where mill scale has started to flake and light rusting occurs.

Rust Grade C

Steel where all mill scale has flaked off and complete rusting has taken place.

Rust Grade D

Steel where pitting and complete rusting has occurred.

Key

Rust Grade A

Key 1
Steel which is completely covered by mill scale.

Rust Grade B

Key 2
Steel which has started to flake and light rusting occurs.

Rust Grade C

Key 3
Steel where all the mill scale has flaked off and complete rusting has taken place.

Rust Grade D

key 4
Steel where pitting and complete rusting has occurred.


Surface cleanliness is categorised into four grades:

  1. Brush Off
  2. Commercial
  3. Near White Metal
  4. White Metal

Brush Off

Brush-Off blast cleaning is a method of preparing steel surface which, when viewed without magnification, shall be free of all visible oil, grease, dirt, mill scale and loose rust. Tightly adhered mill scale and rust may remain on the surface. Mill scale and rust are considered adherent if they cannot be removed by lifting with a dull putty knife.

SA-1 Swedish Standards Institute
SSPC-SP7 SSPC – The Society for Protective Coatings (USA)
NACE No.4 National Association of Corrosion Engineers (USA)

 

brushed off 1
Brushed off 2
Brushed off 3

Commercial

Commercial blast cleaning is a method of preparing steel surfaces which, when viewed without magnification, shall be free of all visible oil, grease, dirt, dust, mill scale, rust and paint/coatings. Generally, evenly dispersed very light shadows, streaks and discolourations caused by stains of rust or mill scale may remain on no more that 33% of the surface.

3rd Quality UK Standards (BS 4232)
SA-2 Swedish Standards Institute
SSPC-SP6 SSPC – The Society for Protective Coatings (USA)
NACE No.3 National Association of Corrosion Engineers (USA)

 

Commercial 1
Commercial 3
Commercial 5

Near-White Metal

Near-White blast cleaning is a method of preparing steel surfaces which, when viewed without magnification, shall be free of all visible oil, grease, dirt, dust, mill scale, rust and paint/coatings. Generally, evenly dispersed very light shadows, streaks and discolourations caused by stains of rust or mill scale may remain on no more that 5% of the surface.

2nd Quality UK Standards (BS 4232)
SA-21/2 Swedish Standards Institute
SSPC-SP10 SSPC – The Society for Protective Coatings (USA)
NACE No.2 National Association of Corrosion Engineers (USA)

 

Near White Metal 1
Near White Metal 2
Near White Metal 3

White Metal

White Metal blast cleaning is a method of preparing steel surfaces which, when viewed without magnification, shall be free of all visible oil, grease, dirt, dust, mill scale and rust.

1st Quality UK Standards (BS 4232)
SA-3 Swedish Standards Institute
SSPC-SP5 SSPC – The Society for Protective Coatings (USA)
NACE No.1 National Association of Corrosion Engineers (USA)

 

White Metal 1
White Metal 2
White Metal 3

Another surface quality is roughness or profile. This is the etched surface or “tooth” obtained by abrasive striking the metal.Surface profile is controlled by:

Type, grade and shape of abrasive
Blasting process and speed of abrasive impact
Condition of steel before blasting
Surface profile affects the ability of the surface to allow protective coatings to adhere. It is important that the surface roughness be sufficient for good paint adhesion.

Surface profile can be measured by several field methods. The best method of ensuring specific profileIs to specify the type and size of abrasive to be used for blasting.

Degree of surface cleanliness and specific profile on any job are established by coatings manufacturer.

Back to Degrees of Cleanliness

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