Surface Cleanliness – “Invisible Contaminants” Tips and Tricks:
NOTE: The following comments are the opinion of the author based on experience using the following equipment in both a laboratory and field environments. Any comments are welcome and can be forwarded to me at tswan@cci-inspection.com
While many factors can lead to coating failure, perhaps the most common reason is Inadequate Surface Preparation. It is important to understand that surface preparation has two components:
Visible
Invisible (Surface Contamination)
What are Visible and Invisible Surface Contaminants?
There is general agreement in the coating industry on the importance of visible surface preparation and that will be dealt with in the Surface Cleanliness – “Visible” Cleanliness. Invisible surface contamination is much more problematic, less understood and less agreed on by many “experts”. Remember, MOST articles on this subject have been written by people with a specific agenda in mind. I am not aware of any “definitive” article on surface contamination written form a pure “research” perspective. Make sure you separate the HYPE from the FACTS
Invisible surface contaminates are generally defined as any substance on the surface (or near the surface) of the substrate that cannot be viewed with the eyes. These are normally Salts. Salts primarily cause two problems:
CORROSION.
OSMOTIC BLISTERING
What are Salts?
In chemistry, salt is a general term used for ionic compounds composed of positively charged cations and negatively charged anions, that combine so that the product is neutral and without a net charge. These ions can be inorganic (Cl-) as well as organic (CH3-COO-) and monoatomic (F-) as well as polyatomic ions (SO42-).
What?
OK. The important part is salt is composed of ions which have a negative or positive charge. When salts are DISSOLVED in water, they form a SOLUTION. The Cations and Anions are necessary for a current to flow in a liquid such as water. The dissolved salts are referred to as TOTAL DISSOLVED SOLIDS (TDS). The higher the SPECIFIC CONDUCTANCE of the ions, the better a current will flow and the easier it is to create a CORROSION CELL. In addition, the total number of ions present in the solution, (TDS) determine the OSMOTIC PRESSURE that will be exerted on a SEMIPERMIABLE MEMBRANE.
What is with all the UPPERCASE WORDS?
In order to understand the importance of Salts, it is important to understand some terms. These definitions may make a chemistry teacher cringe, but they are intended to explain the words in the context of this discussion.
DISSOLVED: The most common salt is sodium chloride (NA+Cl-). When salt is put into water and stirred, it disappears. This happens because the salt breaks apart into ions, the Cation (NA+) and the Anion (Cl-).
SOLUTION: The dissolved salt forms a solution with the water. The ions move into the water matrix and become physically inseparable from the water.
TOTAL DISSOLVED SOLIDS: The total number of ions in a solution usually expressed in ppm.
SPECIFIC CONDUCTANCE: Different ions are more conductive than other ions. That is why sulfuric acid makes a better battery than salt water. The higher the specific conductance of the ions in the solution, the more charge the water can carry and the more efficient corrosion cell it will generate. The conductivity of a solution is often used to estimate the Total Dissolved Solids.
CORROSION CELL: A picture is worth a 1000 words.
For corrosion Cell to occur requires 4 items.
- Anode – Corrosion (metal loss) occurs at the anode.
- Cathode – For this discussion it is sufficient to know you need a cathode.
- Electrolyte – Water that contains IONS (dissolved salts)
- Metallic Pathway – Substrate.
SO, for corrosion to occur requires Salts for the electrolyte. When a surface is painted, it physically separates the electrolyte from the metallic pathway thereby preventing corrosion cells from forming.
OSMOTIC PRESSURE (OSMOTIC CELL): Again this is best illustrated with an illustration:
Nature wants everything to be in balance and will do whatever is necessary to restore that balance. There are Twice as many X’s or Ions on the left side of the membrane as the right side.
To balance things out you can move 15 X’s from the left side to the right side. Unfortunately, the X’s or ions will not go through the membrane so it must balance it out moving the water which will go through the membrane. To balance things out, 1,000 W’s must go through the membrane for every 60 X’s to balance it out
Since there is not enough room for the water, it pushes up on the paint and causes a blister.
The driving force or Osmotic Pressure is determined by the concentration of salts on the surface (substrate) verses the concentration of salts on the liquid side.
For those that want a more technically correct defination read the following.
For those that want a more technically correct definition read the following. Osmosis is actually a vapor phase reaction. Water cannot penetrate the membrane (Coating) in the liquid phase. Water molecules, in the vapor phase will go though the coating. When the water molecules reach the surface of the substrate, they condense. If the surface is free of ions, the pure water is free to re-evaporate and pass back through the membrane in the vapor phase. However, if there are soluble ions on the surface, they will go into solution with the condensate. Since the condensate now has soluble ions present, the vapor pressure of the new solution is increased and the water can no longer evaporate. Osmosis is actually a vapor phase reaction. Since there is little water under the coating, the solution generally has more ions than per ml than the solution on the outside of the coating. Osmotic pressure drives the water molecules through the coating to the surface until the concentration of ions under the coating is equal to the concentration of ions on top of the coating.
NOTE: Solvent Entrapment can also cause blistering.
SEMIPERMEABLE MEMBRANE: Will allow molecules to pass through in the gas phase but will not allow liquids to pass through. In coating applications, Paint can act as a semi-permeable membrane. How permeable the paint is depends on various properties of the paint including type, hardness, porosity, and thickness.
How much Salt is too Much?
My Rule of Thumb: The less salts the better. Some salts on the surface of the substrate to be coated are generally not critical on surfaces that will have atmospheric exposure unless water may pool on the surface or the surface is exposed to consistent condensation. (Remember: Condensation and precipitation are close to pure water) The definition of “some” can be debated. For surfaces that will be in immersion environments, it is important that the “conductivity” of the contaminants on the surface to be coated, be an order of magnitude or less than the “conductivity” of the liquid on the surface of the coating.
Some Specs require testing for chlorides and some for salts, are they the same?
Yes and No. The primary salt present on most surfaces is sodium chloride. Na+Cl-. The chloride ion is one half of the sodium chloride molecule. When running a “chloride” test, the test looks specifically for the chloride ion. Much of the time, this is sufficient to determine invisible contamination on the surface, but it does not give total salts.
The only practical way to measure total salts is by use of conductivity. This is a backdoor approach to measuring salts, because it measures the results of the salts and is not specific to any particular salt. As salts dissolve in a liquid, they break down into their ionic states. The ions increase the conductivity of the liquid so the greater the measure of conductivity, the greater the salt level. This can be used to estimate the Total Dissolved Solids (TDS)
A conductivity sensor measures how much electricity is being conducted through a centimeter of water. Specific conductivity is expressed as mhos per centimeter (M/cm), sometimes called siemens per centimeter (S/cm). Because a mho (or siemen) is a very large unit, the micromho (microsiemen) or millimho (millisiemen) typically is used (mS/cm).
The conversion factor depends on the chemical composition of the TDS and can very between 0.54 – 0.96. A value of 0.67 is commonly used as an approximation if the actual factor is not known
TDS (ppm) = Conductivity (mS/cm) x 0.67
For values in the range sensed by most TDS meters, a rough conversion is that 1 ppm NaCl = 2.2 mS/cm.
Which is more important, Salts or Chlorides?
For most applications, total salts is more important than looking specifically for chlorides. Chlorides have gotten a bad reputation because it is believed they are more aggressive to steel than other ions. For chlorides (or any other ion) to be aggressive, however, there must be a corrosion cell present and if the surface is painted properly, a corrosion cell does not exist. The main purpose for removing salts is to prevent osmotic blistering. Osmotic pressure depends on the number of ions in solution and is independent of the type of ions present. Again, Osmosis will only occur in IMERSION environments and when the CONCENTRATION of IONS on the surface of the steel is Greater than the Concentration of Ions in the liquid on the exterior of the coating.
It is important to note that surfaces that are in atmospheric service that are subject to pooling rain or persistent condensation can have some of the same problems experienced in immersion services. Rain water and condensation are relatively pure which can cause osmotic cells and blistering if surfaces under the coatings are contaminated. IN GENERAL, when properly painted, “some” salt or chloride contamination on a substrate is NOT A PROBLEM for atmospheric service.
Where do Salt Limit Numbers Come From in Specifications?
Every coating is different. The coating manufacturer should be able to tell you how much surface contamination the coating can withstand. Unfortunately, very few manufacturers have a clue so they resort to playing it safe. Most specs, that have salt or chloride limits use one of the following limits.
No Measurable Chlorides (Salts) (SSPC SC-1)
Less than 3 µg / sq cm (Mil Spec) Less than 4 µg / sq cm – Elcometer Recommendation in 130 Manual Less than 5 µg / sq cm Less than 7 µg / sq cm (SSPC SC-2) Less than 20 µg / sq cm Less than 50 µg / sq cm (SSPC SC-3)
REMEMBER: Since most test methods only remove, at best, 50% of the non-visible contamination, the amount of surface contamination is actually ay least twice as high as the amount measured. i.e. If you measure 10 µg / sq cm you probably have at least 20 µg / sq cm on the surface being measured.
When dealing with salts on metal to be painted, less is always better. Generally, if specifications require less than 20 µg / sq cm for non-immersion surfaces, the specifier is being cautious unless extenuating circumstances are present. That doesn’t mean he is wrong in setting the salt limit so low, just that he is being cautious.
When dealing with immersion surfaces, it is important that the ionic strength of the liquid on the outside of the coating be used to determine the amount of salts that can be tolerated on the substrate. When dealing with Demineralized or pure water, less than measurable salts should be present on the substrate. When dealing with potable water, most commonly 7 µg/sq cm is used.
Also the thicker, and less permeable the coating, the more salts can be tolerated on the substrate.
My specs say salt or chloride levels shall be less than X ppm. What does this mean?
It means the person who wrote the specs doesn’t know what they are doing. Contact the spec writer, thoroughly chastise them for their stupidity, and ask them to rewrite the spec. You may also want to suggest they contact CCI Inspection Services’s Consulting group for specification review.
To get from ppm to ug/sq cm you need to know:
1. The amount of surface area used to collect the sample.
2. The amount of water used to collect the samples.
Equation:
What is the best way to test for chlorides or salts?
There are various methods of testing for salts. The first thing that needs to be determined is do you need to test for total salt or chlorides. If the test method uses conductivity, you are testing for Salts, not chlorides. If the test uses chemistry, (Quantabs, drops or Kitagawa tubes), you are testing for chlorides.
Salt tests include the:
Elcometer 130 Salt Meter Bressel Test with the Conductivity Meter Surface Contamination Test (SCAT) with a Conductivity Meter Chloride Tests include
Elcometer 134 Chlor*Test Bressel Test with Quantabs or Kitagawa Tubes Scat Test with Quantabs. or Kitagawa Tubes
NOTE: The Chlor-Condoms can be used with a conductivity meter but do NOT use the solution that comes with the test. The solution has conductivity and will not give an accurate reading for salts.
What is the lest expensive way to run salt or chloride tests?
The least expensive test is the Scat Test. It involves drawing a 6 in x 6 in (10 cm x 10 cm) square and swabbing it with DI water and a cotton ball. This is also the least accurate.
For the others, lets do some math:
Elcometer Salt meter: Initial cost $5,625.00 plus $2.75 per test Bresle Test: Initial Cost (by conductivity) $380.00 plus $6.00 per test Chlor*Test: Initial Cost: $0.00 plus $17.90 per test
Equation:
$5,625.00 + X*$275 = $380.00 + Y*$6.00 = Z*$17.00
NOTE Salt meter is $5,900 - $275 for papers that come with meter = $5,625
Bressel Kit is $530 - $150 for patches that come in the kit.
It takes 36 Bresle tests to be less than the Chlor*Test It takes 396 Salt Meter tests to be less than the Chlor*test It takes 1,615 Salt Meter Tests to be less than the Bresle Test
How long do the tests take?
The Scat Test, Chlor*Test and Bresel Test take about 10 minutes per test. The Elcometer Salt Test takes about 3 minutes. For large numbers of tests, the time savings with the salt meter can be substantial.
Which test is most Accurate?
Accuracy comes into play in the effectiveness of removing the salts to be measured from the surface as well as the method of measurement. It is generally accepted that the SCAT Test removes about 25% of the salts from the surface and at best the other methods remove 40% to 50% of the salts from the surface so the number you get is probably half of the amount of actual salts on the surface being tested. The Elcometer SCM 130 manual says that tests show the effectiveness of the salt meter on a blasted surface is 70% to 80%.
How do I get rid of Salts/Chlorides?
The best way to remove salts is plenty of clean water. Since water by itself has a lot of surface tension, using high presure helps the water better wet the surface to help dissolve the salts. The quality of the water is important because as the water evaporates off the surface, any salts that were in the water will remain on the surface. Usually potable water is sufficient, but the better the quality of the wash water the more efficent the surface will be cleaned.
Sometimes it helps to add a surfactant such as HoldTight 102 to the water. This provides the same benifits that using soap does to help clean when washing. HoldTight is formulated to be 100% volitle and contains no ions. It evaporates from the surface leaving no film. There are other salt removers on the market, buit these are all ionic, and while they may remove chlorides, they will deposit other salts on the surface. The product used to rebove the chlorides must then be washed off the surface. You can get more information on HoldTight at www.holdtight.com.
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