Corrosion Resistance and Anti-Corrosion Coatings – All You Need to Know

1. Corrosion – An Overview
2. Types of Corrosion on Metals
3. Anti-Corrosion Coatings – Mechanism and Types
4. Corrosion Inhibitors and Anti-corrosive Pigments
5. Testing Corrosion Resistance – Popular Methods

Corrosion – An Overview

The term ‘Corrosion’ means degradation of a material caused by chemical or electrochemical reaction with the environment. The material generally refers to metals but can also include non-metallic materials, such as ceramic, polymer and plastics.

Corrosion not only affects the strength and durability of a material but also proves to be expensive. It leads to equipment damage and product leakage, especially crucial in chemical industries, thereby posing threat to the environment.

The performance and lifespan of metals or any other substrate can be improved with the application of anti-corrosion coatings. Coating acts as a sacrificial material and serves as a “barrier layer” to the material surface in corrosion. The benefits of using coatings for corrosion resistance mainly include:

• Improving the efficiency of metals or other components
• Producing new materials’ surfaces with enhanced functional features and properties
• Refining industrial operations
• Reducing maintenance and replacement cost
• Saving on scarce natural resource
• Reducing polluting releases

Let's explore the common types of corrosion on metals & how do they occur in detail...

Types of Corrosion on Metals

To make an appropriate selection on coatings, it is necessary to identify the type of corrosion. The five common types of corrosion include:

• Galvanic Corrosion – It occurs when two metals having different electrochemical charges are connected through a conductive path.
  
  
  Source: Researchgate.net


• Stress-corrosion Cracking (SCC) – A metal component faces SCC when exposed to intense tensile stress due to factors, such as stress due to cold work, thermal process or welding. Stress cracking intensifies when these factors combines with the exposure to an environment. It is observed in fabricated parts which are subjected to mechanical stress.
  
  
  
  Source: Researchgate.net


• Crevice Corrosion – It occurs due to attack of metal surfaces in crevices, such as edges of rivet heads and nuts. Corrosive substance like dust or sand gets accumulated on the surface and create an environment for water to get collected and corrode the material.
  


• Intergranular Corrosion – This type of corrosion does not attack the metal surface rather the metal crystallites boundaries. It occurs when the grain boundaries in a metal form an anode and the interior of the grain acts as a cathode.
  


• Pitting Corrosion – It is a localized form of corrosion leading to small holes or pits on a metal surface. Its mechanism works in same manner as in crevice corrosion.
  

Other common corrosion types are filiform corrosion, exfoliation, environmental cracking, cavitation, etc.

Anti-Corrosion Coatings – Mechanism and Types

Today, anti-corrosion coatings are widely used for corrosion protection. The mechanism that allows coatings to safeguard material substrates against corrosion mainly involves:

• Decreasing the oxidation rate or reducing half-reactions of corrosion occurring on material surface.
• Improving the electrical resistance at the material electrolyte interface.
• Posing as a physical barrier against O₂,H₂O and corrosion ions, such as Cl^- and SO₄^-2.

Kinetic, thermodynamics and nature are the key factors affecting the environment, and to understand corrosion resistance, it is imperative to have comprehensive scientific knowledge and learn about factors associated as discussed here.

External Factors	Formulation / Composition-based
Change in environment like nature, thermodynamics, kinetic Effect of oxygen and oxidizers Temperature Velocity Galvanic coupling Metallurgical factors	Type of anti-corrosive agent used Anti-corrosive pigment loading Dispersing conditions Other additives, fillers and pigments in the formulation

Types of Coatings Used for Corrosion Protection

Coating used for corrosion protection are mainly of three types i.e. metallic, organic and inorganic. Let’s discuss each in detail:

• Metallic Coatings: The application of metallic coatings includes electrodeposition, flame spraying, cladding, hot dipping and vapor deposition.
• Inorganic Coatings: The application of inorganic coatings includes spraying, diffusion and chemical conversion.
• Organic Coatings: The application involves establishing a barrier between substrate material and environment. Coatings, such as paints, varnishes and lacquers safeguards metal more efficiently.

Organic corrosion inhibitors can be used alone or in combination with inorganic corrosion inhibitors thus providing dual protective modes of action and enhancing the anti-corrosive properties of a coating.

Other common types of anti-corrosion coatings comprise of:

Ceramic Coatings – These coatings improve the corrosion resistance of the system by providing a protective barrier between the part and corrosive environment. Industries, such as semi-conductor industry, fuel cell and corrosive water containing environments like gas turbine engines, heat exchangers and internal combustion engines use highly erosion-resistant ceramic coatings like TiN, CrN.

Other interesting developments in the field of anti-corrosion coatings include hybrid coatings, smart coatings, nanomaterials, biomaterials and biomimetic.

Significance of Primer and Topcoat

For any multi-layered coating systems, primer and topcoat are the key layers responsible for protection against metal corrosion. If the primer does not have good adherence to the substrate or is not compatible with the topcoat, there are chances of early failure.

• Substrate adhesion failure usually occurs between coating layer (primer) and the adherend (substrate). Learn about the adhesion basics and factors affecting this property in coatings.
• Intercoat adhesion failure occurs when the bond between the topcoat and primer do not bond. The two primary causes of this failure are an under-cured topcoat and a thick primer coat being applied.

Primer creates a highly active substrate thus, provide a stable surface that subsequent paint layers can lock onto. It gives cathodic protection and helps to inhibit or retard corrosion of the metal surface to be protected. Topcoat is applied over primer or an existing finish for protection or beautification.

When used as an anti-corrosion paint, the main components of the primer are corrosion inhibitors/anti-corrosion pigments.

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Corrosion Inhibitors and Anti-corrosive Pigments

The application of coatings comprising of anti-corrosive pigments or corrosion inhibitors is the most prevalent method of corrosion resistance. Anti-corrosive pigments provide protection against corrosion to metal substrates mainly zinc, steel and aluminum.

These pigments or additives have a physical protective action and their mechanism works on introducing barrier effect by simply extending the diffusion distance between the surface of coating and the metal surface. The main benefits of anti-corrosive pigments comprise of:

• Providing a physical barrier to the passage of water and oxygen
• Being sacrificially destroyed as an anode, thus protecting the anodic sites that have become pitted
• Providing soluble passivating ions to protect the metal
• Producing an insoluble film, which prevents active corrosion, and
• Improving adhesion of the coating to the substrate and protecting binder against photochemical breakdown by UV reflection and/or absorption

Classification of Anti-corrosive Pigments

Anti-corrosive pigments can be classified according to their chemical nature:

• Inorganic pigments such as lead, chromate phosphates, molybdates, silicates and ferrites
• Organic pigments, such as carbon chains and carbon rings, and organic polymeric materials
• Metallic pigments, such as zinc, aluminum and alloys

Lead-based – The two oxides of lead used as anti-corrosive agents are litharge (PbO) and red lead (Pb₃O₄). Sparingly soluble (solubility of < 0.001 percent) Lead pigments are not actually direct inhibitors. They can react with certain resin systems, linseed oils, or other oils to form metal soaps that are active inhibitors and appear to be the mechanism by which lead pigments inhibit corrosion.

Chromate Pigments – Generally, hexavalent (Cr⁶⁺) chromium (a strong oxidizer) and trivalent (Cr³⁺) chromium ions provide high corrosion resistance to chromate coatings. When under corrosive attack, the hexavalent chromium undergoes active corrosion protection and reduces to form trivalent chromium. The insoluble trivalent chromium can then end the attack.

Though, lead and chromate pigments have excellent corrosion resistance, but they are highly toxic in nature. Over the time, their application in coatings formulations have declined due to their harmful affect on the environment.

In recent years, a significant amount of research and development has been conducted to find a replacement for lead and chromate pigments in corrosion inhibiting coatings. There are some additional pigments and technologies available that offer corrosion protection without the harmful health and environmental effects include:

Phosphates (Orthophosphates, polyphosphates) – It is a non-toxic and anti-corrosive pigment used in paints frequently. These pigments show improved anti-corrosion efficiency when used in high concentration. Phosphate-based pigments have nearly completely replaced lead/chromate pigments in high-end applications, such as coil coatings and aircraft primers.

• Orthophosphates are cost-effective anti-corrosives compatible with a wide range of resin types and offer improved long-term protection.
• Polyphosphates are products based on acidic aluminum tripolyphosphate modified with zinc, strontium, and calcium compounds. These compounds have high electrochemical effectiveness due to altered chemical structure design.

Zinc orthophosphate dihydrate – It has superior corrosion resistance properties and offers several benefits, such as better durability and excellent intercoat adhesion. Other phosphates pigments offered are aluminum phosphate, calcium magnesium phosphates, barium phosphates, aluminum zinc phosphates and molybdenum phosphate.

Other corrosion inhibiting materials include:

Calcium, strontium and zinc molybdates – These pigments are white and can be used as a primer in paints by mixing it with any other color. Their use has grown considerably in recent years on account of their more favorable physiological properties.

Zinc oxide - Powdered zinc oxide is used as an inhibitive and anti-corrosion pigment. It contributes to successful anti-corrosion protection of metallic structures exposed to marine atmospheres.

Silicates – Silicates, such as calcium borosilicate, calcium barium phosphosilicate, calcium strontium phosphosilicate, and calcium strontium zinc phosphosilicate also offer anti-corrosion properties when used in paint formulation.

Titanates - Calcium titanate of perovskite structure is a highly efficient anti-corrosion pigment for paints.

Ferrites - Ferrites refer to pigments composed of Fe₂O₃ and another metal, typically magnesium, calcium, strontium, barium, zinc, or manganese. These pigments help in corrosion protection by forming an alkaline environment at the interface between the coating and the substrate. This alkaline environment promotes the passivation of the metal.

 »  Also Read: Expert Tips to Select the Right Surface Treatment Method and Anti-corrosion Additives

Testing Corrosion Resistance – Popular Methods

Several test methods are available to evaluate corrosion resistance of paints surface. Here are listed the popular corrosion resistance testing methods:

ASTM D2803 – Standard Guide for Testing Filiform Corrosion Resistance of Organic Coatings on Metal

Some organic coatings applied to metal substrates exhibit filiform corrosion when there is a break in the coating film and relative humidity is in the 70 to95 % of range. This guide can be used to determine the susceptibility of organic coated metal substrates to the formation of filiform corrosion.

ASTM D7893 - Standard Guide for Corrosion Test Panel Preparation, Testing, and Rating of Coil-Coated Building Products

Coil-coated metals are subjected to a wide range of environmental stresses. Corrosion at cut edges, damage points and fabricated areas can occur leading to premature failure.

This article applies to preparation, testing and rating of line-coated and laboratory-coated test panels for the purpose of comparing and ranking the panels for corrosion resistance and other related properties.

ASTM D1654 - Standard Test Method for Evaluation of Painted or Coated Specimens Subjected to Corrosive Environments

This test method covers the treatment of previously painted or coated specimens for accelerated and atmospheric exposure tests and their subsequent evaluation in respect to:

• Corrosion
• Blistering associated with corrosion
• Loss of adhesion at a scribe mark, or
• Other film failures

 » Learn About Cyclic & Static Corrosion Test Method in Detail!

To conclude, adhesion of the coating to the substrate is also an important factor with respect to corrosion protective properties. If the coating does not adhere well to the substrate, the coating can easily delaminate thereby increasing the exposed substrate surface.

Corrosion Inhibitors for Paints, Coatings and Inks

View the full range of corrosion inhibitors available today, analyze technical data of each product, get technical assistance or request samples.