What Does Surface Contaminants Mean?
Surface contaminants are the impurities that bond or settle on metal surfaces and affect the physical and sometimes chemical properties of the metal, as well as the coatings. Surface contaminants may be a result of a manufacturing process, the environment, or part handling. Surface contaminants affect the adhesion of coatings and paints, and may interfere with the protective features of the coating.
Some contaminants may react with metal to create initiation points for corrosion, or cause degradation of the coating protection, hence leading to a shorter lifespan of the protection method.
Corrosionpedia Explains Surface Contaminants
Surface contaminants originate from various places and differ depending on the source. Typical surface contaminants include:
- All soluble salts, oil, grease compounds; dust and grime arising from cutting and drilling fabrication processes.
- Residual adhesives from tape and protective plastic.
- Mineral contamination in plant rinse water.
- During electrolytic processes where stray currents introduce contaminants into process solutions.
- Improper part handling with unclean hands and placing parts of wet parts on brown Kraft paper or dirty surfaces.
The atmospheric and industrial contaminants contain soluble salts from acid rain, industrial pollution, marine spray, splash, immersion, spillage, and chemical processes. Contaminants containing soluble salts (such as sulfates and chlorides) react with the steel to form corrosion cells, which end up accelerating the corrosion degradation of the metal. Only a small amount of these contaminants is required to induce deterioration in coatings.
Additionally, the soluble salts cause the breakdown of the protective coating by osmotic blistering and disbondment if they are deposited between the coatings. An osmotic action draws moisture through the coating and forms blisters beneath the coating, which leads to internal pressure build-up. In addition, the moisture reacts with chlorides to form corrosion cells due to the reduced pH.
Testing kits are used to analyze surfaces and determine the presence of contaminants and if acidic, alkaline, or another type. These are either portable kits or based in the laboratories.
Surface contamination is preventable; yet if not, there are various removal methods determined by the nature of impurities. Some of these methods are:
- Mechanical – wire brushing, blasting
- Electrochemical – electropolishing
- Chemical – pickling, chelating agents, or passivation
- Solvent washing followed by wipe drying
- Use of proprietary emulsions, degreasing compounds
All contaminants—particularly the salts, hydrocarbons, and other chemicals—need to be removed from surfaces before and after abrasive cleaning, as well as during surface preparation before applying a coating.