Removal of soluble salts

By: Pedro Sánchez*

The costs associated with corrosion at the global level represent a considerable amount that cannot be overlooked, it is estimated that it represents 3% of GDP (Gross Domestic Product) of developed countries. Therefore, it is important in the early stages of the projects and in the successive maintenance throughout the useful life of this immense variety of elements, all the aspects that can influence the premature failure of a coating such as the presence of salts are taken into account.

This problem is often overlooked in specifications and is not given the attention it deserves. The intention of this article is to expose certain important considerations on the subject.

How to remove salts?
The removal of soluble salts can cover a wide range of possible solutions, from the simplest to the most complex, depending on the conditions, which can vary greatly. Consider a very common salt like Sodium Chloride. The chloride ion is never found alone.  It is common to find in the coatings industry that the chloride ion is always accompanied by something else. For chemical stability, the chloride ion (this also applies to sulfate and nitrate ions) seeks to be together with something else, forming compounds such as sodium chloride, zinc chloride, lead chloride, iron chloride or any other of many combinations.

When a coated surface is contaminated with sodium chloride, salts are found on the surface. Because chloride ion has a strong affinity for metals and has an extremely small size it can pass through truly small imperfections (notches, cracks or micro cracks) in the coating, to the surface of the metal. Once on the surface of the metal, having the chloride ion affinity for the metals, it leaves the sodium and adheres to the metal surface that allows greater stability. The chloride ion forms an electrochemical bond with the metal, making it an extremely strong bond. This is why it cannot be easily removed.

Where before sodium chloride could be relatively easy to remove from the surface of the coating, the chloride ion has now adhered to the surface of the metal and is truly difficult to remove. For example, most salts on the surface of a coating can be removed using low pressure water shot blasting, but salts that have been glued and reacted with the metal substrate sometimes cannot be removed using Ultra High Pressure WaterJet (UHP WJ) - 40,000 psi.

Once the chloride ion has adhered to the metal substrate, a very high level of energy is required to overcome the electrochemical bond. This energy can be in the form of mechanical energy, chemical energy, or both. The properties of mechanical energy are widely known, but many coating professionals are not aware of the chemical requirements of a salt remover. Since a series of reactions took place and resulted in the binding of the salt anion to the metal substrate, chemical detachment is the most effective method and has the highest probability of success in removing it, if the right product is used.

For maximum effectiveness, the salt remover should have several distinctive properties:
1) The pH of the remover must be less than 7 to facilitate detachment, this has been supported by both chemistry books and history. Products with pH above 7 accelerate the formation of a layer (barrier) which can hide the remaining salt on the surface.
2) The remover must be able to be used with water containing certain salt content, such as 600 ppm chlorides, as is often found in the field.
3)It must be able not to leave any residue or film that may interfere with the adhesion of the coating.

As has been established, the single mechanical force of pressurized water sometimes cannot do the job, but when combined with the chemical energy of the salt remover, the work can be successfully accomplished. One form of energy assists the other, thus providing the energy required to accomplish the work of removal.

In this way, on a clean surface by means of shot blasting, chlorides can be properly removed by the mechanical energy of 3000 psi of pressurized water treated with salt remover. The water pressure is important to penetrate all the small cracks of the surface that exist in those metal surfaces that have been subjected to cleaning by shot blasting. The chemically balanced action of the salt remover must take on the challenge of peeling off chloride ions from the metal while the water tries to wash them away.

When we place water on the bare surface of the metal, the theme of sudden oxidation or "Flash Rust" invariably appears. Contaminants including minerals, cause immediate corrosion so when contaminants are removed, the cause of immediate corrosion is also removed.

Considering that it is not common to completely remove all contaminants from the surface, there is usually some very low level of contaminants (salts) that remain after cleaning. This is due to the irregularity of the structure of the metal surface after shot blasting. When a shot surface is observed through a magnifier, it can be observed as the abrasive has affected the surface, forming valleys and peaks in its profile.

At the same time, rough edges are also visible in the peaks created by the collision of the particles that form cavities. Removing 100% of these incredibly small ions becomes almost impossible due to their location in these spaces. Even this small amount of contamination can cause some immediate corrosion, but it is typically very mild and in many cases no blasting is required. Some coating manufacturers require a surface prepared to "White Metal" without immediate corrosion residue "Flash Rust". Removing any small amount of "Flash Rust" usually requires light blasting or the application of an inhibitor to prevent oxidation.

Typically a surface has to be shot blasted, allow the rust to appear again and then re-shot. It is not unusual to blast a contaminated surface several times to reach an acceptable level of cleanliness, particularly on highly contaminated surfaces that have been in marine environments or bomb casings. The oxide that appears later is activated by the humidity of the environment or is accelerated by washing with water. Simply the presence of moisture is enough for the salts to do the rest. The cost associated with this process makes it an economically questionable process.

Of the methods that use water, there are many options, such as UHP WJ (Ultra High Pressure Water Jetting), waterjet shot blasting and dry shot blasting in conjunction with a water wash, with the addition of a salt remover, which sometimes requires repeating the shot blasting as mentioned above. There are other less common methods, such as the use of special abrasives, but a particular type of abrasive alone will surely not remove all salts, although some abrasives will "scrub" the surface better than others.

The standard that regulates the use of Waterjeting according to NACE and SSPC (Society for Protective Coatings) is the NACE 5 / SSPC SP 12 standard (Surface Preparation and Cleaning of Metals with Pre-Coating Water Jet) and the NACE VIS 7 / SSPC-VIS 4 visual standard (Guide and Photographic Reference for the Preparation of Steel Surface by Water Jet).

By reviewing the various options available, you can determine which method may be most suitable for a particular job.

UHP WJ may, but not always, result in a desired level of cleanliness. If you want to make sure you get the desired level of cleanliness in one go, you should add a convenient salt remover to the water. This combines both energies, chemistry and mechanics, one assists the other. This allows the operator to proceed quickly while visually inspecting ensuring that non-visible contaminants will be removed. Maximum productivity can be achieved in this way.

A shot blast with water or using a shot blast with mixture, can reduce chlorides, but may not get the desired result in a simple pass.

Blasting using water is not widely used because many contractors and operators are used to dry shot blasting. Many contractors do not want to make significant investments in equipment, unaware that they can adapt some very simple accessories to existing shot blasting equipment.

Since blasting with water is dust-free, environmental pollution is not a problem. It offers the operator better visibility, a variety of nozzles can be used to help complete the work more quickly and help save time, in addition to the cleanliness of the operation, the adhesion of the coating to the substrate is better.

The most frequently used technique is the blasting with dry abrasive for areas covered with oxide, this allows to remove the oxide and expose the bare substrate of the metal, based on the standard of industrial grade shot blasting, depending on the extent of the corrosion, since they can be isolated areas such as the entire surface. This shot blast can be simply to remove any material that can serve as a barrier such as, previous damaged coatings or rust that can accommodate the salts, therefore they must be removed. A wash with pressurized water 3000 psi that includes the salt remover, must be carried out, so that the combined action of mechanical and chemical action helps the removal of salts. A final shot blast is made following the speciations. Another technique can also be carried out by performing as a first step a dry shot blasting, then a washing with water (and salt remover) under pressure and finally rinsing using water incorporating an inhibitor or a light shot is performed. Either method works well and helps to achieve the desired result if applied correctly.

Of the two cases presented above, the only difference is that each work is particular and the method that best suits our situation must be chosen. These methods are very convenient and do not cause major problems to workers, the environment and do not generate waste.

Attention should be drawn when determining what kind of salts we are trying to remove. For this we must perform the appropriate test that allows us to determine the type of salt present in the substrate, since there are some methods to determine contamination by chlorides that are not able to determine other kinds of salts such as nitrates.

What are the permitted levels?
The coatings industry has sought guidelines to follow regarding the levels of contamination with permitted salts. For general information concerning coatings, the manufacturer is usually called upon. Manufacturers provide the required profile information, layer thicknesses to be applied, waiting times between layers, etc. Since they are the ones who know your product best, they can give a guide to what the allowed levels of salts are. For them to be able to offer this truthful information, they must know what is the life expectancy in service that their product will have. In addition, the manufacturer should ask you about other variables that will be present in the service conditions of the coating. By proceeding in this way we will be able to provide you with the best answer to your questions. There are salts such as Chlorides, Sulfates and Nitrates and in each case there are minimum permitted levels stipulated by the standards and that may vary according to each particular job. In any case to avoid premature failures in the coatings, excess contaminant salts must be removed.

* Belzona ®