Let’s Talk About Surface Soluble Salt Remediation Practices

KTA’s Certified Coating Inspector Forum Volume 3, Issue No. 4 – April 2024

William Corbett, COO

AMPP Senior Certified Coating Inspector & Certified Protective Coating Specialist

KTA’s Certified Coating Inspector Forum is designed to provide professional development/continuing education on standards, inspection practices, new instruments, and other topics to help keep certified AMPP and FROSIO coating inspectors current. It represents the views of the author and KTA-Tator, Inc. It may or may not represent the views of AMPP: The Association for Materials Protection & Performance, even though SSPC, NACE, and AMPP standards are frequently referenced in the content.

Introduction

The December 2022 edition of the Certified Coating Inspector Forum[1] included information on the consequences of coating over surface soluble salt contamination and the importance of detection (prior to coating) using any of the extraction and analysis methods described in SSPC Guide 15[2]. This article describes salt threshold levels, presents options for soluble salt remediation, describes products that may be used to enhance the effectiveness of salt removal when pressure washing/high pressure water cleaning, and steps to follow to verify the products are being used properly.

What Industry Standards Address Salt Threshold Levels?

While there are guides[3] and standard practices[4] published by AMPP for testing frequency and test locations on new and previously coated steel surfaces, currently there isn’t definitive industry guidance on threshold levels of surface salt contamination.  But as of this writing, AMPP is developing a guide for specifying soluble salt limits (tentatively Guide 21561) based on the prevailing service environment. Although an industry guide on salt thresholds has not yet been published, SSPC-SP COM[5] identifies three levels that have been commonly specified in the past (shown in Table 1).

Table 1: Commonly specified levels according to SSPC-SP COM

While nitrates may also be of concern, they are not addressed in SSPC-SP COM.

SSPC-SP COM also states that the US Navy has established chloride thresholds based on surface concentration and conductivity for both immersion and non-immersion (topside) environments using the adhesive patch/conductivity meter method, as shown in Table 2.

Table 2: US Navy Maximum Allowable Chloride Levels

Similar to the Navy, when a project specification invokes surface soluble salt testing, it should indicate the maximum allowable concentrations (either total dissolved salts via conductivity or salt-specific, such as chloride). Frequently, specified levels are based on the generic coating type and service environment. Immersion environments typically have a lower threshold than atmospheric environments since immersion poses a greater opportunity for osmotic blistering and accelerated corrosion.

Options for Surface Soluble Salt Remediation

The industry has not yet developed guides for the optimum methods to reduce the level of salts on a surface. In some cases, specifications will establish the process to be followed while in others, the means and methods are left to the contractor. Selection of the method(s) may be based on the concentration of any salts present. Some procedures that have been used in the past include:

1. Normal dry or wet methods of surface preparation.
2. Use of a blend of abrasive that includes smaller particles to scour the surfaces during abrasive blast cleaning.
3. Steam cleaning.
4. Dry abrasive blast cleaning, but allowing the surface to remain uncoated overnight. Residual soluble salts will draw moisture from the air developing dark rust spots where soluble salts remain. These locations can then be further treated (e.g., blast cleaned again). Note that the formation of distinct rust spots on a bare blast cleaned surface overnight is an excellent indication that salts are present.
5. Pressure washing/high pressure water cleaning/water jetting either with or without the addition of a liquid surface soluble salt remover.
6. Application of a Direct-to-Surface soluble salt remover.

Verifying Proper Use of Liquid Soluble Salt Removers

As certified coating inspectors, one of our roles is to verify specified or selected products are used properly, and to verify their effectiveness. Surface soluble salt removers are no exception. Here are 10 key inspection checkpoints/steps to consider when surface soluble salt remediation is required by project specifications or is determined to be necessary based on test results. While there are other salt remediation products available, much of the discussion below is based on products of Chlor*Rid.

1. While a project specification may not require the use of a product that improves the effectiveness of surface salt remediation processes, the contractor may elect to use them to reduce the chance that multiple steps will be required, and to minimize project delays. When this happens, verify that the contractor has received the necessary approvals (in writing) from the owner/specifier prior to use.

2. Obtain a copy of the product application and safety data sheet (SDS) and read them carefully. Verify the contractor has done the same. An example regarding safety – the pH of Chlor*Rid Liquid Soluble Salt Remover is acidic (~3.3) when diluted. The pH of Chlor*Rid DTS (Direct-to-Surface) is similar in acidity (~3.4).

3. Prior to application, verify all visible grease, oil, and other contaminants have been removed from the surfaces per SSPC-SP 1, Solvent Cleaning.

4. The Chlor*Rid Liquid Soluble Salt Remover and the Chlor*Rid DTS are designed to work on surfaces that are free of barrier materials such as rust or mill scale, and coatings. Therefore, if dry abrasive blast cleaning is used to remove existing materials, and the soluble salt remover is subsequently applied, the prepared surfaces will need to be re-blast cleaned or, if approved by the specifier and coating manufacturer, Chlor*Rid SP8 Gel/Rinse used to remove the flash rust. Alternatively, again if approved, the surfaces can be immediately treated with a surface passivator such as Hold*Blast to prevent rust-back from occurring.  

5. While Chlor*Rid Liquid Soluble Salt Remover is biodegradable and non-hazardous, it is a good idea to confirm that the contractor is aware of any local environmental regulations that may be impacted by its use.

6. Verify conformance to storage temperature and application temperature ranges, which may be product specific. These ranges are typically noted on the product data sheet or application instructions.

7. Verify the concentrated product is diluted with potable water and that the correct ratio of product-to-water is used. For example, Chlor*Rid is recommended to be used at a 1:100 dilution (e.g., 1 gallon of Chlor*Rid to 100 gallons of potable water) when spraying it onto prepared steel surfaces[6]. If dilution is not desirable or feasible, Chlor*Rid DTS (Direct-to-Surface) may be used instead. If the soluble salt remover will be incorporated into the water for wet abrasive blast cleaning, slurry blasting, vapor blasting, or water jetting, verify that the contractor follows the methods for dilution, etc. as directed by the manufacturer of the salt remover.

8. Performing a surface soluble salt test before and after washing a small “test area” (~20-30 sf) will help determine whether the traverse rate (speed of moving the nozzle), water pressure, wand-to-surface distance, and dilution ratio are effective in reducing surface salt concentrations to acceptable levels. Performing this step prior to production washing can save labor and material costs associated with repeated washing.

9.  Verify the application equipment and any minimum pressures that are required. For example, the Chlor*Rid product should be applied to prepared surfaces using a minimum 3,000 psi pressure washer (equipped with a zero-degree rotating nozzle or narrow fan spray nozzle), where the diluted concentrate is drawn from a reservoir tank[7]. Conversely, the Chlor*Rid DTS product can be applied by brush, roller, airless spray, or a pump sprayer. After application, no additional washing or removal steps are required for either product.

10. Lastly (and most importantly), verify the effectiveness of any surface salt remediation process employed by testing the surfaces using one of the methods of extraction and analysis described in SSPC Guide 15. Note that the method to use may be specified. Unless the project specification invokes a testing frequency, SSPC Guide 24 (new steel) and NACE SP0176-2016 (existing steel) may be useful in determining the number of locations to test. If surface soluble salt concentrations remain higher than the specified threshold, then additional washing/remediation procedures will likely be required.

Summary

This article addressed soluble salt thresholds, offered options for remediating surface soluble salt concentrations, and described how salt contamination may be removed from prepared steel surfaces by employing surface soluble salt removers. The article also listed 10 key checkpoints/steps that an inspector needs to consider when inspecting a project that specifies the use of surface soluble salt remover removers, or when a contractor employs them.

[1] Let’s Talk Surface Soluble Salt Testing; KTA’s Certified Coating Inspector Forum, Issue No. 8 – December 2022

[2] SSPC Guide 15, Field Methods for Retrieval and Analysis of Soluble Salts on Steel and Other Nonporous Surfaces

[3] SSPC Guide 24, Soluble Salt Testing Frequency and Locations on New Steel Surfaces

[4] NACE SP0176-2016, Soluble Salt Testing Frequency and Locations on Previously Coated Surfaces

5 SSPC-SP COM, Surface Preparation Commentary for Metal Substrates

[6] Dilution ratios can be increased but should never exceed 1:25.

[7] Other application methods may be approved by the manufacturer. Consult the application Guides for options.