Many of our customers have come to us to help with the problem of blistering under coatings. We sometimes get asked, are blisters really that bad? Sure, it’s not cosmetically appealing when your coating starts to look like a tortilla chip:
But is there actually a structural problem? The short answer is, it depends.
In a recent JPCL/PaintSquare article, Kibler, Cassidy, and Ong discuss the causes of coating blisters and some best practices for determining when they are a problem, and what to do when they are.
Some blisters are formed by improper application of coatings. For instance, when a primer has not been allowed to fully cure and release all of its solvents before another coating is applied. The remaining solvent is released under the next coating layer, causing a blister.
These blisters are not great news, as they weaken the coating by causing distension of the blistered area. However, the substrate is typically not affected by the blistering.
Also, blistering may be caused by cathodic polarization on the substrate (cathodic blistering). This can cause disbondment when the cathodic (or “reduction”) reaction produces Hydroxyl (OH-) ions. If these combine with traces of sodium (Na) on the substrate, it can produce Sodium Hydroxide (NaOH) – the most common active ingredient in household drain cleaners.
While this strongly basic solution can eat away at organic coatings such as epoxies (expanding the blister), it generally doesn’t cause corrosion of steel underneath the blister.
The worst kind of blister is the anodic blister. Such blisters are usually caused by higher than acceptable levels of salts on the surface. Anodic blisters are active corrosion cells, with the metal deteriorating underneath. If you break them open (which is not generally recommended unless you are conducting a test), the fluid underneath will contain dark iron oxide, and be slightly acidic or neutral.
In these kinds of blisters, failure to repair them within an adequate time frame may lead to damaging pitting corrosion.
And the main culprit of both kinds of blisters? Salts on the surface. In the US Navy and other industries, the standard ASTM D714 is used to evaluate blister size and density. Past a certain threshold, the Navy considers high levels of blisters – regardless of which kind they are – to be a coating failure. It stipulates these areas be treated as damaged and openly corroding areas.
So, in short, we might say that wisest and the best don’t want any blisters forming on their coatings.
Check out the above mentioned article for more helpful info. The authors cite several studies which determine that, for immersion service, an acceptable level of surface salts before coating is 3 μg/cm2 – not very much! This corresponds to 30 mS (micro-siemens) on a total salts conductivity test.
If you’re trying to reach – or beat – those low levels of salts on a surface, get yourself some HoldTight 102. It’s the one step, no salt, no rust, no residue solution, which has been the gold standard of surface preparation in the industry for over 30 years. HoldTight 102 has been explicitly approved by nearly all major coating manufacturers.