Invisible contaminants and how to handle them
Posted by Tom Swan on Mar 29th 2020
|3 invisible contaminants and how to handle them|
The right testing methods will help you uncover these invisible contaminants.
Dealing with invisible contaminants may be tricky - not just because they can't be seen, but because there's so much uncertainty about them. As with other topics in corrosion, it's hard to get reliable information. This makes finding the right testing equipment important, as long as you understand what it is you're looking for and how to test for it effectively.
#1: Soluble Salts
First, some misconceptions: Since the surface you are testing will be coated, the type of salt you're testing for doesn't matter, we only care if the salt is soluble and how many ions are present and not what the ions are. The only way to determine total salts in the field is with a conductivity test. The common specific ion tests only test for the three most common anions and completely ignore the cation which in many instances is more important than the anion.
Regardless of the method you use, another concern is successfully removing salt from the surface of the steel. We recommend high-pressure water and to improve results use a salt remover, HoldTight® 102, to clean the surface. When using HoldTight it is important to have good quality water, We have just released a new test kit to make sure the wash water quality will not hinder the salt removal process. While made specifically for use with HoldTight, it is also a good stand-alone test to check the quality of your was water.
Another source of salt contamination is through abrasives and abrasive recycling. To make sure the abrasive is not contaminated, MTest just released a new test kit, the MT-SACK Test Kitand MTEST-SACKL Test, which can be used to check abrasive contamination in general accordance of ASTM-D4940. In addition, if you want to check for surface contamination, you can add the Bresle Kit to the SACK test.
#2: Chlorides and Total salts
In most cases, measuring the chloride ion only tell you part of the story. That's why you need a test that will give you all the soluble salts.
"Testing for chlorides is not the same thing as testing for salts.”"
A common test for measuring soluble salts that includes chloride salts, use Potassium Ferricyanide test papers shortly after the steel has been blasted. If Prussian Blue forms, it indicates the presence of salts but is not quantitative. If you want a quantitative method to determine total salts, use Bresle Test. When run in accordance with the ISO standard, the extraction rate is as high as 90 percent.
If you need to run a chloride test, MTest has come up with our own version of the test that is less expensive then the competitive test but does requires the DeFelsko Posipatch ring. See our web site for more details.
The Chloride/Suflate/Nitrate test, known as a CSN, is often required by specifications. Keep in mind, though, that even if you find zero of all of these, it does not mean you have a surface free of soluble salts. We do sell a CSN Tet kit and am working on developing our own CSN test which will be ready shortly.
There is no easy test to find oil on the surface. For an inspector, finding the right test can be frustrating. For example, ASTM A-380 states that an ultraviolet or black light may help reveal some oils, but not mineral oils. . At the same time, a drop of water can sometimes show whether oil is present (water break test)—but only if the surface is highly contaminated and this test often does not work well ion surfaces with a high profile. Sometimes something as simple as wiping the surface with a clean rag can help spot oil.
Regardless, the UV/black light test might be the best option sometimes. If so, try using a 365 nm light in a dark area. UV covers 300 to 400 nm so this is in the medium part of the UV spectrum for ultraviolet lights, Black light covers 400 to 450 nm and is not as effective..
Visit our store for more products that might help you detect surface contaminants. You can also contact us or review our Inspectors Handbookfor a detailed guide to different testing procedures.