KTA’s Certified Coating Inspector Forum Issue No. 6 – October 2022
KTA’s Certified Coating Inspector Forum is designed to provide professional development/continuing education on standards, inspection practices, new instruments, and other topics to help keep certified AMPP and FROSIO coating inspectors current. It represents the views of the author and KTA-Tator, Inc. It may or may not represent the views of AMPP: The Association for Materials Protection & Performance, even though SSPC, NACE, and AMPP standards are frequently referenced in the content.
Introduction
When considering surface preparation by abrasive blast cleaning our “inspector minds” often focus on “post-blast” inspections such as surface cleanliness using the SSPC VIS- 1 Guide, surface profile measurements using a depth micrometer or replica tape, and visual verification of dust/abrasive removal (or using the ISO 8502-3 tape test). But one of the more critical inspections needs to occur before abrasive blast cleaning begins, and that is abrasive cleanliness.
Verifying the cleanliness of the abrasive (whether new or recycled) is often overlooked despite its importance in the surface preparation process. Oil and/or salt contamination on an abrasive may be transferred to the surface during blast cleaning operations and adversely affect long-term coating performance.
There are many factors that impact quality during abrasive blast cleaning operations and abrasive cleanliness is one of them. Most abrasives embed into the steel. The degree of embedment from abrasive blast cleaning operations can vary widely even within a generic category of abrasive. Abrasive embedment is inevitable and is not normally detrimental – unless the abrasive is contaminated.
Oil transferred to the substrate can cause film defects (e.g., fisheyes) and potentially affect adhesion of the primer. Water soluble salts transferred to the surface can cause accelerated rust back, osmotic blistering, under-film corrosion, and premature coating failure. Without routine monitoring of abrasive cleanliness, especially when the abrasives are recycled, these contaminants can go unnoticed, only to be revealed when coating performance is impacted.
Unfortunately, abrasive testing is often not specifically addressed by many specifications, and as inspectors, we are taught that we can’t invoke requirements that aren’t included in the project specification. This is where a deep comprehension of the SSPC/NACE Surface Cleanliness standards is of great value. While abrasive cleanliness may not be specifically required in a project specification, the testing is invoked by reference. That is, when a specification references any of the SSPC/NACE joint abrasive blast cleaning standards (e.g., SSPC-SP10/NACE No. 2 Near-White Metal Blast Cleaning), it is invoking the testing of abrasive cleanliness by reference. An excerpt from one of the SSPC/NACE abrasive blast cleaning standards addressing abrasive cleanliness is shown below:
So even if the project specification does not specifically address abrasive cleanliness, it is automatically required when an SSPC abrasive blast cleaning standard is specified. That is, abrasive cleanliness is an indirect or referenced requirement of the cleanliness standard.
Abrasive Testing
While abrasive manufacturers often perform laboratory analysis of their product, their testing does not fully satisfy the requirements of the SSPC abrasive (AB) standards since verification testing for cleanliness is required in the shop or on the job site. Transporting, storing, and recycling of the media can affect the cleanliness of the abrasive. The two tests that are used to verify abrasive cleanliness in the AB standards are oil content and water-soluble salt content. The contractor’s quality control inspector typically performs these tests, while the owner’s quality assurance inspector verifies the testing is done correctly and documented.
According to the AB standards, the presence of oil contamination is determined in accordance with ASTM D7393, Standard Practice for Indicating Oil in Abrasives. This shop/field test, commonly known as the vial test, requires that a sample of abrasive be placed in a clean, sealable container (about half the height of the container). Tap water (68 – 95°F) is added to the container so that it is at least 1 inch above the abrasive. The container is shaken for 1 minute and allowed to stand for up to 5 minutes. The surface of the water is then visually examined for any evidence of oil droplets or sheen. If oil is visually evident, the abrasive fails the test. At least one test for each 50 bags of abrasive, or 3 tests per bulk delivery should be conducted, as well as at least once per shift or every 12 hours (whichever is shorter) for recyclable abrasives. This test also reveals whether the abrasive is excessively dirty. If dust floats to the surface or the water remains cloudy, the overall cleanliness of the abrasive should be questioned as this could impede production and increase the amount of dust on the surface, especially when using metallic abrasives in a centrifugal wheel machine, where static electricity causes it to adhere to the surface.
Water soluble contaminants are the non-visible soluble salts that may be present on an abrasive. The abrasive is tested in accordance with ASTM D4940, Standard Test Method for Conductimetric Analysis of Water-Soluble Ionic Contamination of Blast Cleaning Abrasives. According to the SSPC AB standards, the conductivity of the abrasive cannot exceed 1,000 µS/cm (micro-siemens/cm). This shop/field test requires combining equal parts abrasive and deionized or distilled water (300 mL of each) and stirring twice for 1-minute each (with an 8-minute standing time between stirs). The purpose is to leach any soluble salts from the abrasive. The extract from the water/abrasive slurry is then poured through filter paper to prevent silt from fouling the probe of the conductivity meter. The conductivity of the filtrate is then measured. It is important that the conductivity meter compensates for water temperature and that the probe is verified for accuracy using a control solution (with a known conductivity value) prior to use. An abrasive test kit (pictured) containing all the required equipment for the analysis is available.
SSPC Abrasive Cleanliness Standards
There are four SSPC Abrasive (AB) standards:
- SSPC-AB 1: Mineral and Slag Abrasives
- SSPC-AB 2: Cleanliness of Recycled Ferrous Metallic Abrasives
- SSPC-AB 3: Ferrous Metallic Abrasive
- SSPC-AB 4: Recyclable Encapsulated Abrasive Media
Each standard has different requirements based on the type of media but is identical when it comes to the quality control tests for oil content and water-soluble contaminants. SSPC-AB 2 requires additional field and laboratory tests for cleanliness to help ensure the abrasive recycling equipment is functioning properly and that the abrasive operating mix is still adequately sized. SSPC-AB 4 requires more frequent testing including at least three water-soluble contaminant tests at different times during an 8-hour period.
Summary
Testing of new and recycled abrasive is an important, yet frequently overlooked inspection checkpoint. Abrasive cleanliness is an indirect or referenced requirement for each of the joint SSPC/NACE abrasive blast cleaning surface cleanliness standards. So, if we do not test for abrasive cleanliness, we are not in full compliance with the standards. Testing an abrasive for cleanliness is performed according to ASTM D7393 and ASTM D4940. Test kits are available for both methods.
