This article offers a proven way to remove soluble salts, which are a leading cause of premature coating failures when residual surface salts are not removed during surface preparation. Case studies document cost-effective and environmentally sound methods to clean surfaces of soluble salt contaminants. These methods provide short-term and long-term cost-savings benefits due to the potential of extending maintenance coating cycles.
History of Surface Preparation Practices to Remove Soluble Salts
For some historical perspective, the process of surface preparation and treatment for steel has been in practice in various forms for about 90 years. (For an introduction to this topic, see Five Key Factors in Understanding the Role of Soluble Salts in Coatings Failures.) We can and have borrowed from the experiences of the metal fabrication segment of the auto industry and, more recently, the experiences of the pipe coating industry.
Steps for Effective Soluble Salt Removal
Free machined parts are cleaned in a disciplined A-A-A three-step process. An alkaline cleaner is used to remove machining oils and dirt. This step is followed by an acidic solution to decontaminate the surfaces of any residual salts.
Lastly, an alkaline dip is used to passivate parts, keeping visually unpleasant or unacceptable rust from forming. The three steps are distinct and cannot be combined because each has a separate chemical functionality. The pipe coating industry tests pipes prior to processing to determine the presence of salts.
Should salt contamination exist after using automated abrasive blasting equipment to remove mill scale and rust, a phosphoric acid solution is sprayed on the surface to displace the soluble salts. Excess phosphoric acid is removed with a deionized water rinse, and the pipe is then dried and coated. For projects in the field or in other non-protected conditions, the benefit of enclosed and controlled environments does not exist as outlined in the above examples where systems are in place to handle waste chemicals, with the treatment and disposal of these meeting local and federal requirements. Yet, similar chemically functional products can be used to cost-effectively achieve the objective of salt decontamination. (Learn more about surface preparation in the article Substrate Surface Preparation for Corrosion Prevention.)
The first step is to accurately identify and quantify the presence of the anionic salt species of concern or that are specified because of their deleterious effect on coating performance. Should the levels be above specified limits after abrasive blast deoxidation (removal of mill scale and oxide layers), a proven and approved acidic soluble salt remover can be applied in a pressure washer solution with fresh potable water to remove surface-bound salts. The rinse from the pressure-washing step can be channeled as an industrial waste without concerns about the costs associated with hazardous waste disposal. The one-step process can and should be confirmed with the retesting of affected surfaces to ensure that the application of the acidic salt remover has been completed as instructed in the manufacturer’s directions.
It is important to dwell on the cost benefits of incorporating testing and the removal of salts to improve life cycle coating performance because it is these parameters that provide justification for the consideration of changes in procedures and the enforcement of revised project specifications. Early adopters have generally not experienced additional or incremental costs for incorporating testing and removal, in large part because compliance to surface cleanliness standards is representative of contractors' compliance to all the requirements, thus reducing the risk of failure and rework. More importantly, if there are any incremental project costs, these become meaningless because the potential for life cycle coating performance is improved, and this has been proven repeatedly by those who have incorporated the outlined steps.
To summarize, once the surface-bound soluble salts have been identified during surface preparation for a protective coatings project, an environmentally friendly acidic soluble salt remover should be applied to meet the growing stringent surface cleanliness requirements that will allow the coating to achieve full life cycle.