Yes, you need to verify the presence of salts to prevent osmosis and thus osmotic pressure. This will create blisters or cracks on the coating surface, eventually leading to corrosion of the surface area and possibly, further delamination of the coating.
How to test for salts?
On any project, what you do is determined by the work specifications. Testing for salts on steel should be relatively straightforward, but many specifications are so poorly written that it is often difficult to determine how to test for salts.
Although the terms “chloride” and “salts” are often used interchangeably, they are actually very different. It is important for the inspector to determine the proper tests based on the project specifications and the wording often makes this difficult.
Are you testing for total salts, or do you need to test for chloride, or does it require testing for CSN (chloride, sulfate, and nitrite)? Since there are no US standards to test for salts, many specifiers come up with their own, often confusing, verbiage. Occasionally, specifiers will require testing in accordance with ISO 8502, currently the only internationally recognized salt standard.
Different Salt tests
After reviewing the project specifications, inspectors can start considering different testing options to see which is best for them. Total salt testing is the most common method, used around the world, and involves testing for all salts.
Total Salt testing is the only way to identify the presence of all soluble ions that may be on a surface. The most common testing method for total salts is the Bresle test required by ISO 8502-6 and ISO 8502-9. Though this does not tell you the specific ions present, for coated surfaces, the ion species is not as important as the total number of ions. This is what determines osmotic pressure which can lead to blistering; however, we will leave this topic for a separate blog.
Another testing option for total salts is potassium ferricyanide paper, which changes color if salts are present on an iron surface. When iron interacts with salts, specifically the anion, it forms free iron (a cation). The Free ion (Fe ++ ) reacts with the papers to create a Prussian Blue shade.
While this does not quantify the amount of salts present, the absence of color means no further testing is needed. Because they only indicate whether salts are present, potassium ferricyanide papers are often used in conjunction with other tests to quantify the salts if found.
Specific ion testing helps users look for a single ion. This can be helpful when there is a particular ion you are looking for, but it only finds one half of the salt (the anion). If looking for the chloride ion, this test will tell you how many chloride ions are present but will not distinguish between different types of chloride salts.
A popular test is the “CSN” test. While this test does check for the most common anions, it ignores the cations and any salt that does not have Chloride, Sulfite, or nitrate for the anion in the salt.
Also, a chloride test that shows zero chlorides doesn’t mean that there aren’t any salts present, just that there are no salts containing chloride. If inspectors use this test, they need to make sure they don’t make any false assumptions.
Another variation of this test is the “CSN” test. While this test does check for the most common anions, it ignores the cations and any salt that does not have Chloride, Sulfite, or Nitrate for the anion in the salt.
Regardless of which salt test is used in the early stages, the conductivity test provides the most accurate reading of salts present on a surface and is the only way inspectors can be sure that there is no salt left after cleaning.
How to remove salts from the surface to be coated
Just as there are misconceptions about salt testing, there are also some dangerous myths out there about how to clean salts off a surface. Acid-based cleaners may remove salts containing chloride, nitrate, or sulfate: in fact, since acidic cleaning solutions are salts, they can also add more contamination to the surface than you are removing. This may in turn cause more work since the acid salts have to be cleaned off after application.
A cleaner such as HoldTight® 102 is nonionic and 100 percent volatile. It adds no salts when cleaning off existing salts and evaporates when it is done. Once the surface is dry, you can paint without washing it off.
High-pressure water can help remove soluble salts.
You don’t always need a separate cleaner. To remove water soluble salts only requires Water. If used correctly, you may be able to remove soluble salts with water that doesn’t contain any chemical additives. The key to removing these salts with just water is using high quality water at high pressures, generally 3,000 to 5,000 psi.
MTest has several options available for effective salt contamination testing. We have the Bresle test (TQC-SP- 7310) which is the most used salt test in the world and virtually the only test used in most countries except North America. We now have the new DeFelsko SST probe and Posipatch which is a reusable version of the Bresle test and eliminates the need for syringes with needles.
While I do not believe chloride testing is the best way to test for salts, occasionally chloride testing is required by the specifications. For this reason, we offer the CHLOR*TEST, as well as their CSN Kit. We have just developed our own Chloride Anion test which utilizes the Posipatch and has the added advantages of being less expensive and working as both a Chloride test and Bresle test. We are also developing a CSN test which we hope to have available later this year. For rapid screening, inspectors can purchase a package of several potassium ferricyanide test papers that are the most cost efficient. We also have a SACK test kit and SACK TEST L which will test for salts and/or chloride in abrasives, in the wash water and on the surface of the metal.