Surface Profile and Salt Contamination Before a Zinc-Rich Primer
A fabricator is applying an inorganic zinc-rich primer to blasted steel for an offshore structure. The specification requires a surface profile of 50-75 micrometres and a maximum soluble salt level of 3 micrograms/cm2 chloride equivalent. The blaster has been reusing abrasive and the steel has been sitting outside between shifts. The inspector measures the profile with a comparator disc and finds it visually acceptable, but a Bresle test on one area returns about 8 micrograms/cm2 chloride. The fabricator wants to proceed because "the profile looks fine and the primer is zinc-rich, which tolerates salt." The schedule is tight and reblasting would cost a shift.
1. How should the inspector respond to the fabricator's argument, and what is the technical basis?
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The inspector should reject the argument and hold the work. The technical basis:
- Soluble salts are not neutralized by zinc-rich primers. Chlorides at the steel interface are hygroscopic and promote osmotic blistering and undercutting; a zinc-rich primer does not consume or tolerate them. The specification sets a maximum for a reason, and 8 micrograms/cm2 is well above the 3 micrograms/cm2 limit.
- "Profile looks fine" is not the acceptance criterion for salts. Profile and salt contamination are separate requirements, and passing one does not waive the other.
- Reused abrasive and outdoor exposure are plausible sources. Reused abrasive can carry salt and debris, and steel sitting outside between shifts can pick up chloride from the environment. The inspector should flag both as process issues, not just the one area.
The inspector should require re-cleaning (reblasting or another approved method) to bring chloride below the limit, then re-verify with a Bresle test before priming. The schedule pressure does not override the specification. The inspector documents the failed test, the corrective action, and the retest result.
2. What should the inspector check about the salt test itself before requiring a full reblast?
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Before ordering a full reblast, the inspector should confirm the test is valid, because a single high reading can be a test artifact:
- Bresle patch technique. The patch must be applied to a clean, defined area, the specified volume of deionized water injected and recovered, and the extraction time per the method. Poor technique or contaminated water gives false highs.
- Test location. The high reading should be confirmed at additional locations to map the extent; if only one area is high, the problem may be localized.
- Blank/control. A blank patch (no steel contact) run through the same procedure checks for contamination in the water or patch.
- Calibration and method. The conductivity/chloride measurement must be per the method and calibrated.
- Profile interaction. Very deep profile can trap salt and make extraction less efficient; the inspector should ensure the extraction is adequate for the profile.
If the retest confirms chloride above the limit, re-cleaning is required. If the retest is below the limit and the original was a technique artifact, the inspector documents the valid result and can proceed, but should still address the reused-abrasive and outdoor-exposure process concerns.