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Corrosion Control, Protective Coatings and Testing

Corrosion, corrosion control and testing are main concerns for industries who produce products that will be affected by corrosive elements such as chemicals, oxygen, water, electrolytes and UV light. This article will detail what corrosion is, how to prevent it, protective coatings and the importance of testing.

What is corrosion and how to prevent it?

Corrosion is the process of corroding or breaking down of metal, stone, or other materials. The most common form of corrosion is rust a chemical reaction of metal which occurs when iron combines with oxygen and water. Rust most commonly occurs to unprotected metal when exposed to wet or humid environment.

Corrosion Control or (CPC) Corrosion Prevention and Control is the prevention of the deterioration of a substrate by reducing the chemical reactions that cause corrosion by changing the environment, changing the material or by placing a barrier between the material and its environment. This can be accomplished by establishing a system to delay, reduce or prevent corrosion through materials selection, non-destructive inspections, protective coatings, finishes and surface preparation such as cleaning, repairs, and other maintenance.


Why use protective coatings?

Protective coatings such as paint, resin or powder coatings are frequently utilized for the protection of composite, metal or alloy surfaces against a corrosive environment. The protective coating is a simple method to reduce this corrosion, by limiting the exposure of the surface against corrosive elements such as oxygen, water, electrolytes and UV light. Different types of protective layers can be applied to provide a barrier which is dependent on the quality of the coating, the selection, application, and handling of painted and powder coated materials.

There are four types of protective coatings for metal: barrier, inhibitive, sacrificial, and combination. Barrier coatings protect metal by preventing water, oxygen, and electrolyte from coming in contact with the underlying metal. Inhibitive coatings contain chemicals that hinder corrosion. Sacrificial coatings provide galvanic protection. In other words, they corrode or sacrifice themselves instead of underlying metal, providing electrons, which reverses the corrosion cell and transforms anodes to cathodes. For an aluminum alloy surface, three layers of the protective coatings, such as chromate conversion coating, primer coating and a top layer coating are generally applied.

Care should be taken during application of these protective coatings to prevent damage and ensure durability. Proper coating application demands special fabrication and surface preparation procedures for equipment, like tanks and pipes, to be properly coated for corrosion control. Surface preparation is the most important step for the ultimate performance of all coating systems. Contaminants like oil, grease, mud, or salts must be cleaned before blasting operation with proper chemical and mechanical applications. If contaminants smudge blasting media, there will be a high risk of coating delamination. The coating must be well adhered, remain in contact and be regularly repaired or recoated to protect the metal.

Importance of Physical Testing

Physical tests performed on coatings are crucial to determine its quality, durability and service life. By testing corrosion, surface finish requirements, wear resistance, lubrication and aesthetics you can ensure that your coating or barrier will protect the substrate. Without proper testing coating failures are a high probability.

Physical Tests to Evaluate the Integrity of Protective Coatings:
Abrasion Test: Abrasion Test that determine the wear resistance and coating reliability
Holiday Test: Continuity Test that detects unacceptable discontinuities such as pinholes and voids
Bond Adhesion Test: Adhesion Test that measures the required tensile force to break or crack the coating
Bend Test: Adhesion Test that detects if coating is peeling or flaking
Coating Mass Test:
  • Contamination Test that detects unacceptable free iron, oxide, scale, rust, grease, oil, carbonaceous or other residual non-volatile deposits
  • High Humidity Test that determine how components, subsystems and complete systems behave in severe environments that involve elevated temperatures and high or fluctuating relative humidity
  • Black Light Test that provides an instant visual check of surface cleanliness
  • Copper Sulfate Test verifies the effectiveness of the stainless-steel passivation treatment
  • Ferroxyl Test that detects porosity of metal coatings such as copper, nickel, lead, and tin, upon iron or steel
Coating Moisture Test: Moisture Test that measures the amount of moisture absorbed and surface wetting
Coating Thickness Test: Coating Thickness Tests, Wet Film Thickness and Dry Film Thickness determine the thickness of the coatings, weld overlay and residual bond, corroded sample wall thickness and similar applications
Embrittlement Test: Coating Ductility Test that determines complete or partial loss of ductility in material without deformation
Cathodic Disbonding Testing: Disbonding Test that determines the resistance to cathodic bonding of a coating system between the coating and a metal substrate, resulting in loss of coating adhesion
Surface Profile Test: Ensures that the optimal surface profile for proper adhesion is achieved, can be accomplished using Testex or similar profile tape
Surface Salt Test: Measures salt contamination of a surface which can cause coatings failure. Bresle patches and Chlor*Test are two popular tests
Cross Hatch Adhesion Test: Also known as cross-cut, this test measure adhesion by cutting the surface and applying a tape to the cut surface to measure adhesion
Pull Off Adhesion Test: Tests the force needed to pull a coating from a surface
Surface Temperature: Necessary to test to ensure proper curing of a coating
Dew Point Measurement: Necessary to test before applying a coating to ensure that condensation is not trapped between the surface to be painted and the coating, leading to coating failure and corrosion

Key Industrial Applications: Shipbuilding, marine, pipeline, gas and oil, chemical, automotive, construction, architectural, electrical, farm, manufacturing, machinery, fire protection, rail, mining, bridges, water tanks, dams, tunnels, power utilities, etc.

In conclusion corrosion control and protective coatings are crucial to the protection of composite, metal or alloy surfaces by limiting the exposure of the surface against corrosive elements. The physical testing of the substrate, coating and its application is just as crucial to properly maintain the protective coating's integrity. The true cost of corrosion can be measured in terms of personal safety, investor confidence and, yes, billions of dollars. Protective coatings and coating testing are crucial to the safety, longevity, performance and aesthetics of equipment and structures, promoting health and wellbeing for all.

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