Six common myths are presented and analyzed for factual truth and understanding. The reader is cautioned to review articles and other presentations carefully to separate myth from fact.
Many specifiers intermix the term “salt” and “chloride.” While most of the salts on metal or concrete
surfaces are generally chloride salts, not all salts contain chloride. The only way to determine “salt” in the field is to
measure conductivity and make some assumptions to calculate total salts or total dissolved solids (TDS). The only
way to measure the chloride anion is to use a test specific for chlorides. To determine which test is best for a specific project, we need to understand the specific concern of “invisible surface contaminants.”
Salt is just a Molecule
The first step is to understand what constitutes a salt. Often, salts are referred to as molecules. The fact is, salts are
not molecules but should be more properly referred to as “formula units.” This is easy to explain if we look at the
lattice structure of a salt crystal. Let’s examine a sodium chloride (NaCl) crystal (Figure 1). Figure 1 shows that each
chloride ion is bound to four (six for face-centered cube lattice) sodium ions by strong electrostatic charges called
ionic bonds, and each sodium ion is in turn bound to four other chloride ions by ionic bonds. Since no single ion is
attached to any other ion, this is not a molecule but a lattice of 1 chloride anion (Cl –) to 1 sodium cation (Na+) or
NaCl. Salts are typically composed of a metal atom (cation) and a non-metal atom (anion) joined by ionic bonds.
Other salts contain two nonmetal atoms.
(ammonium chloride [Na4 Cl])
or two metal atoms
(calcium plumbate [Ca(PbO )]).
An accepted definition is that a salt is an ionic compound formed by the reaction of an acid with a base.
What are bad salts?
The industry has been deluged with articles on chloride and salts that imply or state that some salts are worse than others (e.g., chloride, sulfate, and nitrate are bad). The following terms needs to be understood:
- Solution—A solution is composed of a solvent and a solute. For this article,the solvent is water and the solute is the substance on the surface under the paint that is water soluble.
- Osmosis—This is a colligative property of solutions that depends on the number of molecules in a given volume of solvent and not on the mass or type of the molecules. Osmosis is independent of the chemical nature of the solute, but it very definitely depends on the solvent. What is dissolved can be ions, solvents, or organics such as sugar.
- Osmotic pressure—Osmotic pressure depends on the number of dissolved molecules, not the chemical nature of the solute. So, what the solute is doesn’t matter. But what the solvent is does matter. This will be described in greater detail below.
Chlorides Pull Water Through Membranes
Many articles state some salts containing chloride anions are hygroscopic, making these salts worse than other salts.
This myth further states that the hygroscopic chloride anion, generally in the form of NaCl, pulls the water through
the coating and causes an osmotic cell to grow.
Chloride salts don’t pull water through the coating. The air or solution on one side of the coating cannot directly
detect what is on the other side unless there is a hole in the coating. In addition, the salt normally cited as the
problem, NaCl, is not particularly hygroscopic when compared to many other salts.
A coating is semipermiable, which means it will let small molecules pass through (water) but not larger molecules
(anions and cations). When water is on the surface of the coating, small amounts of water migrate through the
coating as individual molecules. When the water molecules reachs the substrate under the coating, the water
dissolves any soluble salts or other water-soluble materials to form a solution. The dissolved particles lower the
vapor pressure of the solution, which prevents the liquid from leaving the solution and creates a driving force
(osmotic pressure) from the higher vapor pressure side (the liquid outside the coating) to the lower vapor pressure
side (the solution beneath the coating).
Osmotic pressure is determined by the relationship of two solutions separated by a semipermiable membrane. The number of molecules (molar concentration) in the solution, not the composition of the molecules, determines what the osmotic pressure is. Molecules can be soluble salt ions such as Na+ and Cl- or soluble organics such as sugars or solvents.1-3 The important thing to note is this: the solute that is dissolved has no bearing on osmosis. It does not even have to be a salt. What matters is the number of molecules that are dissolved.
The theory that treats the concentration of solute molecules in accordance with the “Gas to all solutes and is true only at extreme dilution, but the theory is valid for this discussion.While the nature of the solute has no bearing on osmosis, it does have a bearing on the creation of a corrosion cell. If the solute is ionic, the liquid under the coating is now an electrolyte and will form a corrosion cell. The solute that is dissolved will affect the strength of the corrosion cell. If salts present in the solution render the pH acidic, the corrosion cell will be more corrosive than if the solution is basic.
Chemical Additives are Necessary to Remove Water-Soluble Salts
This leads into the discussion of using chemicals to reduce water-soluble salts or chloride contamination. In most
situations, the use of high-pressure water is sufficient to remove salts to a level low enough to meet the extraction
criterion. It is important to use good-quality water to prevent further contamination of the surface. The objective is
to have the surface meet the extraction criterion.
Pressurized water cleans a surface better than just pouring a bucket of water on it. The energy imparted to
pressurized water helps break the surface tension of the water to make it more effectively wet the surface.
Typically, higher purity water will dissolve more salts. As the wash water quality decreases, or the surface is
corroded or pitted, surface cleaners can help remove salts. If the wash water is too contaminated, not even cleaners will help clean the surface. They may even increase the contamination.
After washing, the surface should be tested to measure the amounts of salts on it. All cleaners work in generally the same way. To remove the salts, the solvent (water) needs to reach the solute (salts). It is necessary for the cleaner to reduce the surface tension of the water to better access the salts. The surfactant in the cleaners, along with the high pressure of the water, pull the salts into solution, which also helps to remove insoluble particles and dissolve any soluble salts. This is why most salt removal products require the use of high-pressure water. Generally, the higher the pressure, the more efficient the wash.
Only Acid-Based Cleaners will Remove Chloride, Sulfate, or Nitrate
Many articles state that the only effective way to remove chloride-, sulfate-, or nitrate-containing salts is to use
acid-based cleaners. This is a myth. There are several products on the market to remove salts from steel. Acidic,
neutral, and basic cleaners can all be used effectively.
The only way to be certain the surface is salt free, even after
cleaning, is to run a conductivity test.
Thomas Swan
A two-year study conducted at Kennedy Space Center compared four cleaners formulated to remove chloride
ions.4 The cleaners, all off-the-shelf products generically called chemical rinse aids (CRAs), were already in use for
washing down aircraft. These CRAs included acidic (initial pH = 3.5 ), neutral, and basic (initial pH = 9.0) cleaners.
The researchers rinsed various metal coupons and let them dry in a beach exposure. Comparisons were made for
coupons rinsed with demineralized water, seawater, and unrinsed. They found that the pH of the cleaner had no
effect on how well it removed salts on steel to stop atmospheric corrosion.
Acid-based cleaners are ionic. Once they are used to remove the salts, the cleaners themselves must be removed
since they are salts. If the cleaner is still present when the surface is painted, it will form a corrosive solution.
Basic cleaners are generally based on amines; they are nonionic and contribute little or no conductivity to wash
water. Some are 100% volatile, meaning they will evaporate from the surface with the water. Should any residue remain, a corrosion cell will form, but as the solution becomes more basic, and thus inhibits or reduces corrosion.
Testing for Chloride, Sulfate, and Nitrate Ions Ensures a Salt-Free Surface
Since chloride, sulfate, and nitrate ions are in the majority of salts present, they are usually a good indicator of salts
left on the surface, and many specifications require testing for one or more of these ions. If the prime concern is
chloride-containing road salts, testing for chloride will probably find the majority of these ions still present. It also
may not.
For example, acid-based cleaners contain other salts, which may contaminate the surface during cleaning; however,
they do not contain Cl+ , SO- , or NO ions.
With nonionic cleaners, this is not a concern. The only way to be certain the surface is salt free, even after cleaning, is to run a conductivity test. If there is no conductivity, there are no chloride, sulfate, nitrate, or any other ions present in solution. European standards and the new International Marine Organization (IMO) standards require conductivity testing for this reason.
Amine Based Cleaners Do Not Remove Salts, They Mask Salts
It is often stated that some cleaners may mask salts, so they are not detected during testing. This could be either a
myth or fact, depending on the chemistry of the cleaner. A few inhibitors work by forming a barrier layer on the
surface so that oxygen and water cannot easily migrate through the barrier. A common example is the use of
phosphoric acid (H PO ) or phosphates to form iron phosphate on the surface. Phosphatizing steel is a very accepted
practice. In the most general terms, an acid is used to react with or etch the steel to create a water-insoluble salt
coating. Other very basic inhibitors could form barriers of iron oxide or iron hydroxide [Fe(OH) ] by reacting with a
water-soluble iron salt to form a water-insoluble salt coating. The barrier would mask salts if they were present on
the steel under the barrier.
Amines, which are derivatives of ammonia (NH ), are weak bases and often cited as a film-forming product that can
mask salts. Stating it this way is misleading. Some amines are formulated to create a stable hydrophobic film on the
metal surface to repel water. These types of amines are often used in the water treatment industry to inhibit
corrosion. Again, the amine barrier would mask salts under the barrier if they were present on the steel.
Amines can be formulated to have many different properties, however, including being aromatic. Because they are
non-ionic, aromatic amines used in cleaners will not leave a film that masks salts on surfaces, add any salts to the
cleaned surface, or form any impenetrable hydroxyl layer on the steel.
Conclusion
But knowledge is power. Dust off that old chemistry book, when references are given, take the time to check out the reference to see if it was properly cited, and do not believe something just because someone said it in a lecture or printed it in an article.
References
1 C. Hare, JPCL 10 (2007): p. 77.
2 C. Hare, JPCL 2 (1998): pp. 45-63.
3 C. Hare, JPCL 3 (1998): pp. 17-34.
4 J.J. Curran, J.P. Curran, L. MacDowell,
Tom Swan
5750 N Sam Houston Prky E
Suite 1016
Houston, TX 77032
Office: (281) 359-2215
Cell: (281) 300-9435
tswan@m-testco.com
