Concept · Surface preparation

Surface cleanliness grades

Rust grades A–D, the Sa and St preparation grades and their SSPC/NACE counterparts, and what staining each grade allows.

Cleanliness grades say how much rust, mill scale, old coating and staining may remain after preparation. The inspector judges them by eye, against a written definition and reference photographs. For the SSPC/NACE and ISO grades matched one to one, read the surface preparation grades guide.

The why

A coating is only as strong as the layer under it. If the paint sits on loose rust or flaking mill scale, the whole film lifts when that layer lets go. No grade lets loose material stay. A grade sets how much firmly adhering residue or staining may stay.

ISO 8501-1 tests this with a blunt putty knife. Material is poorly adhering if the knife lifts it. SSPC standards use a dull putty knife in a similar way. Firmly adhering residue can stay at some grades. The knife separates it from loose material.

A clean look is not a clean surface. Dry blast cleaning cannot fully remove water-soluble salts from pits (ISO 8501-1). A high visual grade does not prove that salts are gone. See soluble salts and the Bresle test.

Read the label

A label such as B Sa 2½ has two parts. The letter is the starting rust grade of the steel. The Sa, St or Fl part is the preparation grade. The reference photographs combine both, so pick the photograph with the same rust grade as the steel.

• A: mill scale covers most of the steel, with little rust.
• B: rust has begun and mill scale is flaking.
• C: mill scale has rusted away or can be scraped off, with slight pitting.
• D: mill scale has rusted away and general pitting is visible.

Grades and counterparts

The pairs are close equivalents, not identical definitions. Use the standard that the specification names. The cleanliness grade guide adds the power tool grades and the full cross-reference.

The two systems judge differently. ISO 8501-1 uses words and photographs, without magnification and without percentages. SSPC/NACE standards add a limit for each unit area, which is a square of about 76 × 76 mm (3 × 3 in). For SSPC/NACE grades, the written definition prevails over the SSPC-VIS photographs.

In the field

1. Read the specification for the standard, the grade and the starting rust grade.
2. Make sure that oil and grease were removed first by solvent cleaning (SSPC-SP 1). Blasting spreads them.
3. Check the blast air with the ASTM D4285 blotter test.
4. Look at the steel without magnification. Light it well and use a hand lamp in dark corners, because poor light hides stains.
5. Compare the steel with the written definition first, then with the photograph of the same rust grade. Use the printed photographs, because a copy on a screen does not give a true reference.
6. For an SSPC/NACE grade, mark off a unit area at the worst-looking spot. Judge the share of staining in that unit area, not an average of the structure.
7. Lift doubtful material with a dull putty knife.
8. Look into pits, at welds and at corners. The grade applies to the whole surface, pits included.
9. Judge again just before the first coat. Remove rust that formed after blasting.

During the final dry blast, the steel should be at least 3 °C above the dew point (5 °F in SSPC-SP 10/NACE No. 2). After the visual check, test the dust (ISO 8502-3) and the soluble salts (ISO 8502-6 and -9).

Worked example

The specification asks for SP 10. In one unit area of 5,800 mm², the inspector finds a stain of about 25 × 24 mm, which is 600 mm². The limit is 5 % of the unit area.

Check: 5 % of 5,800 mm² is 290 mm². The stain share is 600 ÷ 5,800 = 10.3 %. The area fails SP 10 and needs more blasting. It would pass SP 6, where the limit is 33 %, or 1,914 mm².

What the inspector records

Record the standard, the grade and the starting rust grade. Record the location, the time of the check and the result for each unit area judged. Add the results of the dust and salt tests. A photograph of each area, with its location, supports the record.

Exam traps

• Trap: Sa 2½ is SP 6. Sa 2½ matches SP 10, and SP 6 matches Sa 2.
• Trap: SP 10 allows 33 % staining. SP 10 allows 5 %, and SP 6 allows 33 %.
• Trap: ISO 8501-1 sets a percentage limit. It uses words and photographs.
• Trap: the staining limit is an average over the structure. It applies to each unit area.
• Trap: Sa 3 may keep light shadows. Sa 3 has a uniform metallic colour.
• Trap: a high grade proves the steel has no salts. Salts are invisible and stay in pits.
• Trap: solvent cleaning comes after blasting. It comes before.
• Trap: D is a preparation grade. D is a starting rust grade.

How the exams treat it

Surface preparation and inspection carries 20 % of the CIP Level 1 domains and 18 % of the CIP Level 2 domains.

Use it: Cleanliness grades: SSPC / NACE vs ISO 8501-1

93 practice items in the members area mention this topic (CIP 1: 42 · CIP 2: 32 · FROSIO written: 19).

Related

• Standards: ISO 8501 · ASTM D4285 · SSPC-SP, NACE No. and WJ · NORSOK M-501 · SSPC-VIS 1 · SSPC-AB 1, AB 2 and AB 3 · NACE SP0178 · ISO 8504 · ASTM D7393 · ASTM D4258 and D4259 · ASTM D4262
• Defects: Undercutting (underfilm corrosion) · Cratering (fish eyes) · Peeling · Crawling and cissing · Flash rusting · Edge corrosion (thin film at edges) · Contamination and inclusions · Steel surface imperfections
• Surface preparation: Surface profile (anchor pattern) · Soluble salts and the Bresle test · Waterjetting and flash rust