Residual water-soluble salts on steel surfaces are an important, yet widely debated topic. Basically, residual soluble salts are contaminants on a substrate surface that will affect the performance of the coating material, says NACE International member Mario Moreno, engineer with StonCorCanada Group (Whitby, Ontario, Canada), a NACE-certified Coating Inspector Program (CIP) Level 3 Inspector, and a NACE CIP instructor. Because these residual soluble salts are colorless and small—often only a few microns in size—they are not readily visible on a surface.
Atmospheric contaminants are the most dominant soluble salts. The primary soluble salts that the coatings industry must manage are in the form of chlorides, sulfates, and nitrates, comments NACE International member Gil Rogers, a consultant with Rogers & Associates (Sherwood Park, Alberta, Canada), a NACE-certified CIP Level 3 Inspector, a NACE and SSPC Protective Coating Specialist, and a NACE instructor. Chlorides are generally present in a marine environment and also are prevalent in deicing salts for roads and bridges. Sulfates, a result of coal and liquid hydrocarbon fuel combustion, can be found in industrial environments where there is wide use of sulfuric acid (H2SO4). Nitrates are commonly found in the fertilizer industry and also result from nitrogen oxide (NOx) emissions from vehicles and other combustion engines. Abrasives used for surface preparation sometimes contain salts that can contaminate the substrate surface.
Residual soluble salt contaminants can contribute to corrosion of metal substrates as well as reduce the service life of coatings. Although protective coatings provide a barrier to restrict environmental water, corrodents, and oxygen from reaching the metal surface, these substances can eventually permeate the coating and reach the substrate. There are two primary ways that residual soluble surface salts affect the substrate or the coating, Moreno explains. First, residual soluble salts on metal surfaces accelerate the corrosion process under the coating film when moisture is present by dissolving and increasing the conductivity of the electrolyte solution. This speeds up the crevice, pitting, and general corrosion that can occur even when salt contamination is not present.
Second, residual surface salts can cause osmotic blistering, which occurs when moisture diffuses through the coating film via osmosis and dissolves water-soluble salt trapped underneath the coating. Because coatings are semipermeable membranes, Rogers says, the entrapped salt, being hygroscopic, draws moisture from the air. The pressure of the concentrated moisture beneath the coating may exceed the bond strength of the coating and cause local areas of coating disbondment, which appear as blisters. Corrosion can then develop underneath the blisters, depending on whether or not oxygen is present. If a blister breaks, Moreno adds, the metal substrate is exposed and corrosion is further accelerated. Depending on the type of coating and coating thickness, osmotic blistering can occur if the residual salt levels are high enough and there is sufficient time of wetness.
According to NACE Publication 6G186,1 a report prepared by NACE International Task Group (TG) 142—Surface Preparation of Contaminated Steel Surfaces, the best lifecycle performance for a coating is usually achieved when the coating system is applied over an uncontaminated or minimally contaminated surface. A higher level of nonvisible residual soluble salt surface contamination, however, may not significantly compromise the performance of the coating system. The level of soluble salts that will cause a detrimental effect on coating performance varies widely, the report says, depending on factors such as the type of service, coating thickness, generic coating type, and the presence of moisture. Additionally, some soluble salt contaminants are more corrosive to steel than others. The corrosiveness of a salt is directly proportional to the conductivity of the electrolyte formed when it dissolves.
Very little definitive information is available regarding the amount of surface salt contamination and its relationship to coating performance, the report notes. Objectively evaluating the detrimental effects of soluble salts on coatings can be difficult due to diverse and varied resins, pigments, and other components used to formulate single- or multicoat coating systems. Other variables include the thickness of a given coating or coating system and the nature and severity of exposure environments. These combined variables make it challenging to prepare a simple, convenient table or chart that establishes acceptable tolerance levels of residual soluble salts beneath a coating, although the industry is moving toward a consensus of what would be considered acceptable levels of salt contamination, note Rogers and Moreno. Information is available in ISO/TR 15235:20012 on the levels of water-soluble chloride and sulfate salts allowed without increasing the risk of coating failure, which is based on an evaluation of published data from technical literature as well as unpublished data from coating system users and manufacturers.
Typically, coating specifiers refer to the coating manufacturers’ product data sheets or contact the coating manufacturers to determine allowable levels of residual soluble salts. In general, some types of coatings are more tolerant of water-soluble surface salts than others. For example, the TG 142 report notes that inorganic zinc-rich coatings and metalized coatings are generally considered to be more tolerant of soluble surface salts than organic coatings such as fusion-bonded epoxies and epoxy-phenolics. Also, the total thickness of a coating system can impact its ability to tolerate the effects of salts on a surface. For any given coating system, thicker systems are typically more impervious to water and have a greater salt tolerance than thinner systems.
Frequently the specified allowable level (measured in µg/cm2) of residual soluble salts on a substrate surface—the maximum allowed to avoid a coating failure—is relative to the expected service life for a specific coating system in a particular environment. For example, Rogers says, an application in immersion service generally calls for very low levels of residual soluble surface salts: <3 µg/cm2. He says that coating applications in service environments such as atmospheric exposure can tolerate a higher level of residual soluble salts (around 30 µg/cm2). He adds, however, that a residual soluble salt level >50 µg/cm2 will more than likely result in a rapid failure in any coating system.
As stated in the TG 142 report, some owners and specifiers want to keep any salt contamination to a minimum prior to coating to reduce the risk of coating failure. Although it is desirable to have zero soluble salts present on the surface to be coated, there are costs associated with their detection, removal, and testing. Moreno says that a completely clean surface with no level of contamination may require lengthy, resource-consuming surface preparation, as well as multiple rounds of surface testing to verify residual salt levels. Attaining this level of surface cleanliness can be very costly to achieve, he adds, and may not be necessary to avoid coating failure.
When weighing the costs of removing soluble salts to very low levels vs. the risk of reduced coating performance, conducting a risk assessment for individual coating projects can help. When the specifier understands the risk of coating failure, Rogers says, he/she is better able to make a decision on the acceptable level of surface salt contamination and determine if residual soluble salt removal is necessary.
“Risk is understood as the possibility that a certain event will happen, multiplied by the impact this event will have if it does happen,” says Moreno. Assessing the risk of coating failure due to residual soluble surface salts involves identifying the potential hazard (the presence of soluble salts on the surface), estimating the probability that the hazard will create a risk (which is related to the quantity of soluble salts on the surface and other factors), and defining the consequence if the hazard causes an event (corrosion of the substrate or a premature coating failure). Such a risk assessment can be used to quantify the risk associated with the presence of soluble salt contamination on the surface and the risk reduction associated with the removal of the soluble salt contamination. Both Rogers and Moreno comment that the higher the consequence of a coating failure, the more stringent the soluble salt level threshold should be.
Risk assessment draws on a number of factors, including the service environment to which the substrate will be exposed, the type of coating system, the thickness of the coating system, the method of surface preparation, and the degree of surface cleanliness prior to coating. In some service environments, there may not be any evidence of salt contamination or salt-induced corrosion, which indicates that residual soluble salts may not be a problem. In environments where salt-induced corrosion is seen or where salt contamination is suspected, surface salts may be present. In these environments, testing is usually performed before any coating operations are conducted to ensure that unacceptable levels of soluble salt contamination are not present.
Several recognized tests are available to detect the presence of salt contaminants. Moreno and Rogers note that two quantitative tests commonly used are the sleeve test and the Bresle method. Field test methods for retrieving and analyzing soluble salts are described and explained in detail in SSPC-Guide 15.3 One of the fundamental requirements for any test, they comment, is the ability to recover all salts on the surface prior to testing. Although measurements of salts extracted from the surface are accurate, the degree of salt extraction from a surface may vary considerably. For example, Moreno and Rogers point out that salt concentration levels in crevices and localized areas where salt can accumulate may be three to five times higher than salt concentration levels on flat surfaces.
Salt removal methods include wet and dry abrasive blast cleaning, waterjetting, chemical cleaning, wet-heat/steam cleaning, and hand or power tool cleaning. Industry experience indicates that these methods do not always remove salt contaminants to the specified level of cleanliness after a single cleaning. Repeated use of a single method or a combination of cleaning methods is necessary in some instances to remove surface salts down to the level desired. To determine final surface cleanliness prior to the coating application, one or more of the salt contaminant tests is performed.
Areas tested for the presence of salts typically include locations where corrosion has previously taken place, including pitted areas, moisture drain or drip points where higher salt concentrations potentially exist because of evaporation concentration, surface areas known to be exposed to salts, areas on floors or horizontal regions where salts may concentrate, and areas where rust-back occurs quickly. NACE Standard SP07164 designates where and how often to test for the presence of surface soluble salts on previously coated surfaces before applying a coating system.
More information on residual soluble salts and their effect on coatings performance can be found in Rogers’s and Moreno’s CORROSION 2016 paper no. 7539, “Residual Soluble Salts and Coating Performance—Separating Myth from Reality.”
References
1 NACE Publication 6G186, “Surface Preparation of Soluble Salt Contaminated Steel Substrates Prior to Coating” (Houston, TX: NACE International, 2010).
2 ISO/TR 15235:2001, “Preparation of steel substrates before application of paints and related products—Collected information on the effect of levels of water-soluble salt contamination” (Geneva, Switzerland: ISO, 2001).
3 SSPC-Guide 15, “Field Methods, Retrieval, Analysis, Soluble Salts, Steel, Nonporous, Substrates” (Pittsburgh, PA: SSPC).
4 NACE Standard SP0716-2016, “Soluble Salt Testing Frequency and Locations on Previously Coated Surfaces” (Houston, TX: NACE, 2016).