6 Types of Corrosion on Metal Materials and How to Recognize Them
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6 Types of Corrosion on Metal Materials and How to Recognize Them

  • Katarina Knafelj Jakovac

    December 8, 2023

The presence of seawater is a common cause of issues in the operation of machinery in production facilities located near the sea, due to the intense corrosion of metal surfaces of structural materials.

The most common types of corrosion include galvanic and pitting corrosion, as well as corrosion that removes material from the surface. Other types include erosion, fracture, cavitation, and cyclic fatigue.

Gibraltar Refinery on the shores of the Mediterranean Sea
Picture: Gibraltar Refinery on the Mediterranean Coast (Source)
Advancements in the development of metal materials represent progress in the application of technology aiming to improve system performance, energy efficiency, safety, and the extended lifespan of machinery.

Metals are primary materials for constructing numerous machines and equipment in the processing and manufacturing industry, transportation, and construction.

Material selection involves assessing cost, mass, availability, productivity, maintenance, lifespan, and the absence of failures.

Corrosion often causes equipment, machinery, and component failures, ranging in intensity from simple to catastrophic.

For example, complete failure of metal components often occurs due to excessive stress, such as the breaking of bolts due to excessive tightening torque or loading.

Most failures caused by material flaws are much subtler and involve wear, decay, frequent stress or fatigue, creep, cracks, corrosion, or a combination of corrosion and fatigue.

Due to such events, when one part fails in a complex system of a specific machine, it is often not easy to determine the cause of the failure.

Therefore, the selection and application of corrosion-resistant materials for constructing machines are determined by specifications and standards prescribed by legal regulations and international technical norms. In this article, we will discuss 6 types of corrosion on metal materials caused by the presence and action of seawater.

Corrosion caused by seawater is the most common type of chemical corrosion, i.e., electrochemical attack on metal material in the presence of an electrolyte.

Seawater in the form of liquid or aerosol mist is the most common electrolyte causing problems and complications with electrochemical corrosion.

The reason for this is the high chemical activity and simultaneous high conductivity of seawater, compared to electrolytes such as fresh water, and the wide range of seawater's influence on the characteristics of metal materials.

Other causes of electrolyte occurrence leading to electrochemical corrosion usually include different environmental conditions and the internal structure of crystal lattice of the material.

Characteristics and the extent of action of metal corrosion occurring when exposed to seawater depend on the alloy itself and its chemical composition and heat treatment process.

The impact of the environment, such as the quantity and quality of air, duration of exposure to seawater, impurity deposits, combinations of different metals, presence of cavitation, temperature variations, and crack formation, is equally important.

Due to the action of seawater, the following types of corrosion occur:

1. General Corrosion. Represents uniform corrosion on the metal surface or the surface of an alloy.

Materials such as copper alloys and various steels generally have evenly distributed corrosion on the outer surface.

Uniform occurrence of general corrosion is opposite to electrochemical corrosion of materials, characterized by degradation at localized locations.

The following image shows general corrosion on the outer surface of a pump base. Corrosion occurred after the paint peeled off, causing a potential difference between the metal base and the steel bolts or anchors.

Surface corrosion of the base will not cause a malfunction that stops the pump, but if not prevented in time, it will gradually accelerate the degradation of the base and pump housing, thus shortening their lifespan.

Image: Corrosion on the outer surface of the base, Source: author's archive

Image: Corrosion on the external surface of the pedestal (Source: Author's archive)

2. Galvanic Corrosion. Occurs when two different types of metals are together in the presence of an electrolyte like seawater, as shown in the image 1.

Current will flow through the electrolyte from the anode metal to the cathode metal.

Usually, corrosion of the anode metal is accelerated while corrosion of the cathode metal slows down.

The image shows an example of the occurrence of galvanic corrosion due to the interaction of a copper backing and an aluminum plate with different potentials.

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Image: Formation of galvanic corrosion

Galvanic series is an effective way to assess the tendency of occurrence of galvanic corrosion between two different types of metals.

The galvanic series is a series of potentials in an open circuit of metals and alloys immersed in an electrolyte, measured with a specific reference electrode.

The galvanic series is arranged in approximate order, starting with metals that behave most like anodes (e.g., magnesium alloys, noble metals) to metals that behave most like cathodes (e.g., graphite and cast iron containing graphite, base metals).

The farther two metals are in the galvanic series, the greater the potential for the occurrence of galvanic corrosion.

The table shows galvanic series and their relationships. Red fields represent pairs of metals with an increased risk of corrosion, e.g., a pair of zinc alloy and stainless steel, titanium and nickel, silver and cast iron.

Such pairs of metals with red-marked fields should be avoided when designing machines, equipment, and various constructions.

Green fields indicate compatible pairs of metals where there will be no corrosion when used in an anode-cathode pair.

Pairs like cadmium and lead alloys, stainless steel and nickel/chromium alloys, and various combinations of magnesium alloys with other metals are examples of such pairs, and their use is highly recommended.

Tablica parova katoda anoda za metale

Table of cathode-anode pairs for metals (Source)

3. Pitting Corrosion. Pitting corrosion is a localized form of corrosion characterized by the formation of small pits or holes on the metal's surface.

Once the passive layer is breached, a small electrolyte cell is created between the small anode surface, where the layer is disrupted, and the large surface of the remaining passive layer.

The small electrolyte cell has a surface area like a point, which enlarges over time.

Pitting corrosion is a severe form of material deterioration because predicting the intensity of corrosion impact is extremely challenging.

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Image: Corrosion caused by removal of metal from the external surface (pitting) (Source)
Factors influencing the breakdown of the passive layer include variations in oxygen levels, temperature, pH values, chloride ion concentration, flow rate, and physical and chemical heterogeneities of the passive layer.

The removal of metal from the surface of aluminum alloys in seawater is often associated with the formation of local electrolyte cells between the aluminum anode matrix and the cathodic alloy made of heavier elements such as copper, nickel, and iron.

Therefore, aluminum alloys with lower heavy metal content are generally more resistant to pitting corrosion.

However, even these alloys can be susceptible to accelerated corrosion if seawater contains dissolved heavy metal ions, such as copper from anti-fouling paint on the hull.

Stainless steel is often more susceptible to this type of corrosion than less noble metals, such as carbon steel.

This happens because its corrosion resistance is associated with a protective oxide film, so any local cracking of the protective film creates an anodic area.

If the protective film cannot reform, potential differences between the active and passive areas cause accelerated corrosion within the crack area.

Chloride ions in seawater particularly aggressively act on this material.

4. Crevice Corrosion. Crevice corrosion is a severe form of localized corrosion resulting from the concentration of stress cells.

It occurs due to material geometry resulting from welding or at the junction of two materials, where the metal material is in contact with a non-metallic material, and due to deposits accumulated on the metal surface.

The deposits are products of other types of corrosion, sand particles, or impurities. Local corrosion occurs inside or outside the crevice.

Crevice corrosion in some metals, such as austenitic stainless steels and nickel-based alloys, occurs inside the crevice.

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Picture: Cross-sectional view of AISI 316L steel and pitting corrosion with a depth of 75-100 µm (Source)

Chloride ions in seawater can penetrate the oxide protective film and create an active surface inside the crevice area, which tries to become passive again by recombining oxygen atoms dissolved in seawater.

When oxygen atoms in the crevice are depleted, repairing the protective film is no longer possible, resulting in galvanic cells forming between the active surface inside the crevice and the passive surface outside. Both cells tend to accelerate corrosion of the alloyed material within the crevice area.

Crevice corrosion on other metals, such as some copper alloys, occurs only outside the crevice area.

In this case, corrosion in the crevice saturates trapped water with copper ions, accelerating corrosion in other areas.

The difference in copper ion concentration between the inner and outer surfaces of the crevice results in cells accelerating corrosion at the outer edges of the crevice.

5. Erosion Corrosion. Erosion corrosion is a general term for corrosion associated with increased material surface wear, resulting from the abrasive action of fluid flowing over the surface.

Continuous turbulent flow of fluid over the protective film on the metal surface for an extended period results in the corrosion film being torn apart, leaving the metal surface unprotected and directly exposed to abrasive action, as shown in the image.

Formation of erosion corrosion

Image: Formation of erosion corrosion (Source)

Irreversible mechanical damage occurs in this way.
Material fracture and cavitation are forms of erosion conditioned by the speed of the working medium.

Metal fracture occurs where there is local breakage caused by the movement of fluid over the solid metal surface. Cavitation damage occurs when vapor bubbles are created and implode near the metal surface.

Parts of machines in direct and continuous contact with the working medium, such as rotors and pump casings for pumping seawater or fresh water, blades of steam turbines, and pipelines for water transport, are most susceptible to cavitation.

rotor pumpe oštećen djelovanjem kavitacije

Picture: Pump rotor damaged by cavitation (Source)
As the fluid flow rate increases, protective coatings on the surface of non-noble metals such as austenitic stainless steels and many nickel alloys disappear, leading to crevice corrosion that increases over time.

Contrary to noble metals, the protective capabilities of copper alloys disappear as the flow rate increases, leading to enhanced corrosion.

When copper alloys form with elements like nickel or aluminum, the alloy's resistance to corrosion caused by the flow rate dramatically increases.

Copper alloys are often used to make machine housings, rotors, and screws where the wall thickness allows tolerable corrosion within acceptable limits.

In challenging working conditions where tolerance for modifying working conditions is limited, the use of protective coatings, special coatings, the use of noble metals, and specific non-metallic construction materials and their costs need to be considered.

6. Intergranular or Interstructural Corrosion. Also known as selective corrosion, it occurs in several forms:

Graphitic Corrosion is common in gray iron castings. It does not occur in austenitic cast iron with added nickel.

Iron corrosion occurs when corroded iron leaves the material surface, leaving free carbon (graphite) in gray iron castings.

The remaining material retains the original structure shape but loses mechanical strength.

Graphitic corrosion on the external surface of the pipe, microscope image

Picture: Graphitic corrosion on the external surface of the pipe, microscope view (Source)

The removal of metal particles is another process of selective corrosion typical for copper materials containing more than 15% zinc.

In this type of corrosion, copper particles dissolve when zinc ions remain in the solution, while copper continues to deposit.

The result is a metal piece that retains its shape and consists of porous layers of copper without internal strength.

Copper metals and aluminum bronzes are susceptible to the removal of metal particles in seawater, unless inhibition classes are specified, where a metal inhibitor, such as arsenic, antimony, or phosphorus, is added to copper.

The loss of aluminum particles through selective phases is a type of corrosion that occurs in some aluminum bronzes, especially cast bronzes, containing more than 8% aluminum.

Intergranular corrosion of austenitic stainless steels is another type of selective corrosion.

Carbides in steel that are sensitive to this form of damage occur at the boundary layers of crystal grains when the steel is reheated to temperatures from 430°C to 870°C.

Such temperatures are achieved in the heating zones near metal welding sites. Corrosion can occur at grain boundaries.

This type of corrosion can be avoided by using low-carbide alloys (maximum 0.03%) and using stable metals containing titanium or by returning hardened carbides back into the solution by cooling and rinsing above 900°C.

Peeling (loss of particles) is a specific form of selective corrosion that occurs along a narrow path parallel to the material surface.

In general, corrosion occurs at grain boundaries, causing corrosion products to carry particles away from the surface and create layered appearances. If corrosion products have a larger volume, internal pressure can cause the appearance of bubbles that burst on the external surface.

How to Prevent Corrosion on Metal Materials?

General corrosion can be prevented by using appropriate paints, metal coatings, cathodic protection (anode consumption or an imposed current system), and choosing a more resistant type of material.

External surfaces of machines in processing industries such as compressors and centrifugal pumps are as sensitive as steel reinforcement and must be appropriately protected.

Corrosion protection involves the application of certain methods and technologies resulting in long-term protection of metal materials from the destructive effects of various types of corrosion.

The first step in performing corrosion protection is to create a technical specification for selected coatings with quantities, types of coatings, and an overall budget for works and materials.

The specification also includes a quality control plan in all stages of preparing metal surfaces and applying all types of coatings, with the preparation of a final report certified by authorized inspectors.

Standards HRN EN ISO 12944 (paints and varnishes) and EN ISO 14713 (metal coatings) define the most common working environment conditions leading to corrosion and the duration of the protective coating from the first application to the moment when it will be necessary to reapply the coating during preventive maintenance.

Protective coatings contain corrosion inhibitors, protecting the surface of metal materials from the effects of electrochemical corrosion.

The table shows the predicted duration of the coating in years, defined according to the norm HRN EN ISO 12944:

Screenshot 2024-02-03 at 14.43.05.png

Adequate surface preparation is the most important part of the corrosion protection procedure.
Surface preparation must include sandblasting when there is a need for long-lasting corrosion protection. Then, a primer, an expanding coating, and a protective coating are applied to the prepared surfaces, as shown in the image.

Screenshot 2024-02-03 at 14.31.07.png
Image: Anti-corrosion protection system with 3 coatings (Source)

The surface corrosion protection system must include all 3 steps and a final coating thickness control.

In the end, all data on the condition of external metal surfaces of machines and equipment should be regularly recorded in asset management software.

Every failure due to corrosion, every work order for corrosion protection, and every report should also be entered into the software under the corresponding asset to have an overview and up-to-date status in one place.

Katarina Knafelj Jakovac
Katarina Knafelj Jakovac social media icon
December 8, 2023

Katarina Knafelj Jakovac holds Master degree in Mechanical engineering with long term work experience in Oil industry. She is Certified Reliability Leader specialized for mechanical equipment and operational excellence. Author of blog Strojarska Radionica (Mechanical Workshop) where she shares professional knowledge and personal experience in maintaining various rotating machines, machine systems and process equipment. Adores mechanics, thermal engineering and internal combustion engines. She is dedicated to the continuous improvement of machine maintenance and quality management of physical assets.