Understanding corrosion types

By gaining a deeper understanding of common corrosion patterns, operators of wastewater systems can choose the right surface coating to extend component life.

By gaining a deeper understanding of common corrosion patterns, operators of wastewater systems can choose the right surface coating to extend component life.

When components of wastewater systems corrode prematurely, the consequences can be costly. Operators may have to deal with unplanned stoppages, replace parts before the end of their reasonable lifespan, or pay to repair environmental damage caused by leaks and overflows.

Protective surface coatings for wastewater systems offer a cost-effective solution to premature corrosion. They can extend the life of components and reduce the risk of failure. However, the variety of components and materials used in wastewater management and the broad spectrum of surface treatments available have made choosing an appropriate product difficult.

Understanding common corrosion types in wastewater systems is good for determining what coating is suitable. By learning more about the various corrosion mechanisms and the forces involved, operators can select a surface coating that addresses specific challenges, enhancing effectiveness.

Understanding types of corrosion on metal surfaces

It all comes down to electrochemistry when understanding the different types of corrosion and the reactions between metals and surfaces. An anodic reaction is one where metal is oxidised, releasing electrons into the metal. A cathodic reaction is one where electrons are removed from the metal.

Corrosion occurs when these two reactions occur in the same location on metals. Here are some types of corrosion that can occur on metal surfaces within wastewater treatment facilities.

Pitting Corrosion can occur on carbon steel and cast-iron alloys when the ferric oxide layer is disturbed. The covered areas remain cathodic while the exposed areas take on an anodic reaction, which eventually forms ‘pitting’ and worsens over time. Similar pitting corrosion can also happen with stainless steel, although this is due to variances in levels of oxygen and chloride.

Graphitic Corrosion is seen most when cast iron or ductile iron is buried in acidic soils or exposed to mildly acidic waters. In these conditions, the iron leaches out of the casting into the water or soil, leaving behind a weakened matrix of graphite and iron oxides.

Galvanic Corrosion is caused when two different metals of differing electromotive potential meet each other and a common electrolyte. The less noble metal becomes anodic and sacrifices itself to protect its more noble cathode. Treatment plant operators should look for any zinc galvanised surfaces in contact with unprotected carbon steel where this type of corrosion can occur.

Other types of corrosion that can affect concrete and metals

Microbiologically Influenced Corrosion (MIC) of metals and concrete can commonly occur in wastewater treatment collections and treatment facilities in environments, with and without oxygen. The primary form of MIC in wastewater treatment is biogenic sulphide corrosion. A series of chemical reactions lead to the creation of gases which, in humid environments, form weak thiosulphuric and carbonic acid and a corrosive environment. The corrosive reactions can happen faster for exposed concrete substrates due to their alkaline nature.

Acid Attack on concrete appears like a MIC attack. However, this corrosion type is caused by process chemicals used in wastewater treatment encountering unprotected concrete. These are usually in and around containment and dose mixing stations.

Each type of corrosion can readily occur in wastewater treatment plants and facilities. It is good to know what the types of environments are or what early corrosion damage can look like. By reacting quickly to damaged surfaces and areas, you can prevent or delay more significant damage from occurring.

Making the right choice for surface coating

Understanding how corrosion occurs is essential when choosing a long-lasting and effective surface coating. To be effective against pitting corrosion, a coating must have high tensile adhesion to resist under-film corrosion. It also needs high abrasion resistance to resist exposing regions that can become anodic. A suitable coating should have low permeability and be generally resistant to thermal and chemical exposure.

Similar conclusions can be drawn by examining problem areas such as graphitic corrosion, galvanic corrosion, microbiologically influenced corrosion (MIC) in metal and concrete, and acid attack in concrete. However, these will each have different tensile, adhesive and permeability properties.

The correct protective surface coating can extend the working life of components within wastewater systems. By understanding the types of corrosion that parts are prone to, operators can avoid using inappropriate products and zero in on the suitable coating for each application. Coating experts at sealing solutions provider A.W. Company can provide further advice based on individual circumstances and requirements.

The Chesterton ARC range of coatings has a four-decade-long track record of enhancing critical industrial equipment and structures. Its products are engineered to protect metal and concrete surfaces from the damaging effects of abrasion, erosion, and chemical attack.

For more information and to read their white paper on corrosion, go to https://www.chestertoncustomseal.com.au/

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