NACE - National Association of Corrosion Engineers Surface Preparation and Contaminants Questions and Answers — Questions and Answers
Question 1: A steel substrate was prepared for a critical immersion service coating. After abrasive blasting, the inspector notices dark, tightly adherent, bluish-black patches on the surface. What is this contaminant, and what is the primary risk if it is not removed?
- Weld slag; it will create a brittle layer under the coating.
- Soluble salts; they will cause osmotic blistering.
- Mill scale; it can detach, leading to premature coating failure. (Correct answer)
- Oil and grease; they will prevent proper wetting of the surface.
Correct answer: Mill scale; it can detach, leading to premature coating failure.
Mill scale is a form of iron oxide that forms on steel during hot rolling. It is cathodic to the steel substrate, meaning if any breaks exist in the coating, the steel will corrode preferentially. More importantly, mill scale has a different coefficient of thermal expansion than steel and is brittle. Changes in temperature or mechanical stress can cause it to disbond from the substrate, taking the coating with it and leading to catastrophic failure.
Question 2: According to the joint NACE/SSPC standards, which of the following surface preparation methods does NOT inherently create a surface profile?
- SSPC-SP 10 / NACE No. 2, Near-White Metal Blast Cleaning
- SSPC-SP 11, Power Tool Cleaning to Bare Metal
- SSPC-SP 5 / NACE No. 1, White Metal Blast Cleaning
- NACE No. 5 / SSPC-SP 12, High-Pressure Waterjetting (Correct answer)
Correct answer: NACE No. 5 / SSPC-SP 12, High-Pressure Waterjetting
NACE No. 5 / SSPC-SP 12 describes cleaning using high- and ultra-high pressure waterjetting. This method is excellent for removing contaminants, soluble salts, and old coatings but does not abrade the steel to create a new surface profile. Instead, it exposes the original profile that was created by a previous process like abrasive blasting.
Question 3: An inspector is qualifying a surface prepared to SSPC-SP 6 / NACE No. 3, "Commercial Blast Cleaning." According to the standard, what percentage of each unit area (e.g., 9 sq. in.) is permitted to have tightly adherent, residual staining?
- 0%
- Up to 5%
- Up to 33% (Correct answer)
- Up to 10%
Correct answer: Up to 33%
The SSPC-SP 6 / NACE No. 3 standard for Commercial Blast Cleaning requires the removal of all visible oil, grease, dust, mill scale, rust, and coating. However, it permits staining from rust, mill scale, or previous coatings on up to 33% (one-third) of each unit area.
Question 4: A coating project in a marine environment is experiencing premature failure in the form of blistering. The surface was abrasive blast cleaned to a near-white metal standard (SSPC-SP 10). Testing of the substrate underneath the blisters reveals a high concentration of chlorides. What is the most likely cause of this failure?
- The abrasive media was too angular, creating an excessive profile.
- Flash rust occurred between blasting and coating application.
- The coating was applied at too low a temperature.
- Soluble salt contamination was not removed prior to or during blasting. (Correct answer)
Correct answer: Soluble salt contamination was not removed prior to or during blasting.
Soluble salts like chlorides are hygroscopic and draw moisture through the coating film via osmosis, leading to blistering and under-film corrosion. Dry abrasive blasting alone may not effectively remove these salts, especially from pitted surfaces. Wet abrasive blasting or a separate washing step is often required to remove soluble salts before coating.
Question 5: Which of the following is the primary purpose of SSPC-SP 1, Solvent Cleaning?
- To create a specific surface anchor profile for coating adhesion.
- To remove all rust and mill scale from the substrate.
- To remove non-visible contaminants like soluble salts.
- To remove visible oil, grease, soil, and other soluble contaminants. (Correct answer)
Correct answer: To remove visible oil, grease, soil, and other soluble contaminants.
SSPC-SP 1 is a prerequisite for most other surface preparation methods. Its specific purpose is to remove soluble contaminants like oil, grease, and cutting fluids. Failure to perform solvent cleaning first can result in abrasive blasting pressing these contaminants into the steel profile, leading to poor adhesion.
Question 6: During surface preparation with power tools, an inspector is assessing if the remaining material is 'tightly adherent' as per SSPC-SP 7 / NACE No. 4, Brush-Off Blast Cleaning. What is the standard method for making this determination?
- Using a portable adhesion tester to measure the pull-off strength.
- Observing if the material can be lifted with a dull putty knife. (Correct answer)
- Striking the surface with a hammer to check for flaking.
- Wiping the surface with a solvent-dampened cloth to check for residue.
Correct answer: Observing if the material can be lifted with a dull putty knife.
The definition of 'tightly adherent' in standards like SSPC-SP 7 and SSPC-SP 3 is practical and field-oriented. Material (mill scale, rust, or coating) is considered tightly adherent if it cannot be removed by lifting it with a dull putty knife.
NACE - National Association of Corrosion Engineers Surface Preparation and Contaminants Questions and Answers
A steel substrate was prepared for a critical immersion service coating.
After abrasive blasting, the inspector notices dark, tightly adherent, bluish-black patches on the surface.
What is this contaminant, and what is the primary risk if it is not removed?