Your Ultimate Guide to Corrosion

April 12, 2022

Corrosion in industrial machinery, plant, structures, and equipment comes with risks to human safety and the environment, and major possible costs.

For example, corrosion in mining pumps, if left unchecked, could affect other mining equipment due to leakage. This could mean there would be a high rate of replacement in other mining equipment as well as in the maintenance of pumps and related machinery. Not only that, in operations that involve people and typically come with safety risks, such concerns are heightened when certain equipment and structures are defective or damaged due to corrosion (or some other cause).

Moreover, corrosion destroys or damages equipment prematurely – meaning the equipment that could have remained functional for several more years will be rendered useless or unsafe, leading to unnecessary wastage and costs.

This is why certain hard-facing materials and coatings are used to prevent mechanical and corrosive damage in moving parts.

But what exactly is corrosion? Why does it happen? Are there materials that are resistant to corrosion?

In this ultimate guide to corrosion, we’ll talk about corrosion, and answer all your questions about it and more.

What Is Corrosion?

Before answering the first question ‘what is corrosion?’, let’s go over some basic information first.

Elements are mainly divided into two groups depending on their physical and chemical properties, namely metals and non-metals.

Although it is destructive, corrosion is actually a natural phenomenon. It works by transforming a refined material (usually a type of metal) into an oxide, hydroxide, or sulphide which is the material’s more chemically stable form.

Corrosion Meaning and Definition

In chemistry, one definition of corrosion is the deterioration of certain metals due to a chemical reaction on their surface or because of the air or moisture in the atmosphere. It refers to the quick, immediate destruction of materials (mostly metals) because of either a chemical or electrochemical reaction with the environment, or a combination of both.

Because of corrosion, the most critical properties of metals and structures are rendered useless, including their appearance, strength and permeability to liquids and gases.

A very common example of corrosion is the rusting of iron. Here, a brown flaky material starts to form on iron objects or materials when they are exposed to moist air or water.

Corrosion is a serious problem in mining and many other industries, as equipment and plant typically comprise metal components. This is why the field of corrosion engineering is solely dedicated to the control and prevention of corrosion.

Corrosion vs Erosion vs Abrasion: What’s the Difference?

While the terms ‘corrosion’, ‘erosion’ and ‘abrasion’ sound somewhat similar, they couldn’t be more different.

To eliminate the confusion, here, we compare corrosion vs erosion vs abrasion:

• Corrosion: It is a chemical process that occurs on the surface of materials. Although corrosion primarily involves metals, other materials that can get corroded include polymers and ceramics. Corrosive agents like sulphates and oxygen cause corrosion. There are several types of corrosion, including crevice, galvanic, intergranular pitting and selective leaching. A common preventive measure against corrosion is the application of a protective layer on metal surfaces.
• Erosion: It is a degradation of material surface due to mechanical action, impact of particles, often by impinging liquid, particles suspended in fast flowing liquid or gas, bubbles or droplets, cavitation, etc
• Abrasion: Closer in context to erosion, abrasion is to the act of rubbing off or wearing away because of friction, and hard particle rubbing over a surface.

Corrosion, therefore, is totally different from erosion and abrasion, whilst the latter two are both used in describing types of metal degradation.

Causes of Corrosion and How it Occurs

So, what causes corrosion? How does corrosion occur?

These are just a few of the questions we set out to answer in this guide.

There are many possible causes of corrosion, including the following:

• When there is too much or uncontrolled humidity, moisture, vapour or water condensation on metal surfaces, corrosion is likely to occur. When humid conditions persist combined with polluted air, the corrosion rate can increase and degrade steel structures, metal guarding, sensitive and expensive equipment.
• The presence of corrosive gases such as ammonia, chlorine, hydrogen oxides, sulphur oxides, etc., can lead to the corrosion of electronic equipment, corroded or rusted parts, etc. Hydrogen and oxygen exposure can also cause corrosion, such as when metals containing iron come into contact with oxygen, thereby leading to the formation of iron oxide or rust.
• When a material like steel is subjected to too much stress and develops a crack, corrosion can follow.
• Metals periodically or usually exposed to electrical currents could be subjected to electronic corrosion.
• Metals that get exposed to dirt and bacteria can get corroded.

There are many different reasons why certain materials get corroded, so there may be times when certain rust inhibitors won’t work for every situation.

Effects of Corrosion

Whatever the equipment or industry, corrosion has certain negative direct effects including the following:

• It leads to the premature damage of commercial airplanes, infrastructure, plant equipment, vehicle electronics, etc.
• It can degrade server rooms and data centres, as well as hard disks and computers that are necessary to control complicated processes or manage programs in power plants, mining operations, petrochemical facilities, etc.
• It can cause metallic equipment to lose their efficiency.
• It reduces the functional lifespan of metal objects and equipment.
• It causes changes in the appearance and chemical properties of metal, as well as the loss of metal from the surface.
• It can make workplaces unsafe or hazardous.
• Repairing or replacing damaged items or equipment can be costly.

The cost of the effects of corrosion varies depending on the industry. In general, however, the costs tend to be monumental when it is left undetected and untreated.

Where Does Corrosion Take Place?

Nearly all types of general corrosion occur in the presence of oxygen.

At times, atmospheric corrosion produces difficult-to-explain patterns, such as those found on rusted galvanized roofs.

Factors That Affect Atmospheric Corrosion

There are a variety of factors that contribute to atmospheric corrosion, all of which must be understood and addressed in a systematic manner to avoid detrimental consequences.

What exactly are these variables?

1. Condensation, Dew, Moisture

When it comes to atmospheric corrosion, moisture, whether in the form of dew, rain, or condensation, is a major influence.

Rain can wash away dangerous air pollutants that have been deposited in exposed regions, such as in a marine environment, but it also collects in fissures and pockets in the structures. Rain, especially in locations where galvanized bolts and steel parts or structures are used, can speed up the corrosion process by persistent moisture.

Furthermore, when condensation and dew are not washed away by recurring rains, which may remove or dilute the pollution, they become the undesired types of moisture. Saturated dew films containing acid sulphates, sea salt, and other acids may generate an aggressive electrolyte environment which is conducive to corrosion.

2. Temperature

Atmospheric corrosion is influenced by temperature. Every 10°C (50°F) rise in temperature effectively doubles corrosion activity.

The surfaces of metal objects or structures appear warmer as the ambient air temperature drops in the evening in relation to the humid air surrounding them. As a result, condensation won’t start until dew point is reached.

When the ambient air temperature rises, the lag temperature in certain metals rises, turning them into condensers that keep a moisture deposit on their surfaces. When the ambient air temperature is below the dew point, the wetness time lengthens. This is also dependent on the thickness of the metal, its structure, air currents, and solar radiation.

In tropical climates, cyclic temperature can cause significant metal corrosion, particularly in unheated warehouses, things in plastic bags, metal tools, etc. The dew point in the surrounding air is an indicator of the evaporation and condensation equilibrium.

As a result, maintaining a temperature of 10°C to 15°C (50°F to 59°F) above the dew point is recommended where possible.

3. Relative Humidity

Relative humidity is defined as the amount of water vapor in the atmosphere in relation to the amount of saturation at a given temperature. In most cases, it’s given as a percentage.

The presence of an electrolyte in the form of a thin film that can grow on steel or metal surfaces exposed to crucial humidity levels is one of the most important criteria in the process of atmospheric corrosion. Despite its invisibility, the film can contain corrosive pollutants in high quantities, especially in settings where drying and wetting is alternated.

The critical level of humidity is a variable that is determined by the substance being corroded. It also depends on the product’s corrosive tendency, moisture absorption by surface deposits, and the presence of contaminants.

When there are no pollutants in the environment, for example, the necessary amount of humidity is greater than 45% and as it rises to 60% or more, the rate of corrosion increases accordingly. When electrolytes are present in thin films, atmospheric corrosion increasingly occurs as a result of a combination of cathodic and anodic processes. Metals are attacked by anodic oxidation, which causes corrosion.

4. Deposition of Aerosol Particles

Chemical and physical activities in the atmosphere can produce aerosol particles, which are a major contributor to atmospheric corrosion. Examples of common aerosols are wind dust and sea spray.

Secondary aerosols are produced by the condensation and reaction of atmospheric gases, as well as the conversion of gas into cooling condensation particles.

Aerosols do not last permanently in the environment, with an average lifetime ranging from days to one week, albeit this is dependent on the location and size of the particles.

Despite advancements in metal coating technologies, aerosol particles remain a contributing factor in the dissolution of (seemingly) corrosion-resistant surfaces.

5. Pollutants

Atmospheric corrosion is influenced by the presence of contaminants. Sulphur dioxide, for example, is one of the most damaging pollutants that can cause metal corrosion when it is formed by the burning of gasoline, diesel fuel, natural gas, and sulphur.

Nitrogen oxides, which are also combustion products, are another contaminant. These are prevalent in vehicle exhaust fumes and can react with UV light and moisture to produce new chemicals that can be transported as aerosols.

When metals are exposed to the pollutants, atmospheric corrosion can occur more quickly and in a variety of ways. Understanding these fundamental factors can therefore substantially aid in the management of corrosion and its negative consequences.

Prevention of Corrosion

There are certain methods to prevent corrosion due to pollutants that can be eliminated at the source.

Here are some of the most common prevention methods:

• Using corrosion-resistant metals (Stainless steel or aluminium)
• Metals can be subjected to surface treatments, such as barrier coatings (paint, plastic, or powder).
• Metals can be galvanized to make them more corrosion resistant.
• High-efficiency air filters (compact filters, scrubbers, and media) can be used to improve indoor air quality and eliminate dangerous particles that lead to corrosion.

You can also look at maintaining a specific relative humidity (usually below 40% relative humidity) and temperature in a regulated environment to prevent rusting. As a result, the products or materials in the controlled space are unable to absorb moisture from the environment.

Corrosion Testing

Corrosion testing is a term used to describe the techniques applied to inspect, solve, prevent, and minimise corrosion problems. These procedures are commonly used in failure analysis and can be applied to industrial materials and infrastructure items.

Corrosion testing falls under two distinct types: laboratory tests and field tests, each with its own set of advantages and disadvantages. The ambient conditions in real-world applications, for example, differ from those in laboratory settings. As a result, extrapolating the outcomes of laboratory tests to industrial settings can be problematic.

On the other hand, in laboratory experiments, the corrosivity of the environment can be accelerated to produce faster findings, which is difficult in field testing.

At Asset Management Engineers (AME), we deploy various corrosion testing and management methods. In our team, we have several engineers which have internationally recognised AMPP certifications. AMPP stands for The Association for Materials Protection and Performance (Formerly known as NACE – National Association of Corrosion Engineers).

Our corrosion testing engineers are equipped to help clients with corrosion assessments, preventive maintenance, and effective corrosion management. Our services include, but are not limited to:

• Visual Inspection
• Non-Destructive Testing
• Thickness Testing
• Hardness Testing
• Plant & Equipment Condition Reports
• Failure Investigation & Root Cause Analysis
• Asset Condition Monitoring
• Coating Inspections
• Corrosion Consulting

Types of Corrosion

Although these all fall under the ‘corrosion’ umbrella term, there are various types of corrosion:

Atmospheric Corrosion (General Corrosion)

General or atmospheric corrosion results from moisture in the atmosphere. It is the most common form of corrosion. No other corrosion type affects materials and equipment more than atmospheric corrosion.

The most severe atmospheric corrosion occurs in maritime areas as well as damp contaminated industrial sites, with airborne contaminants causing severe atmospheric corrosion of metal structures as well as plant and equipment.

Corrosion Under Insulation (CUI)

Corrosion under insulation or CUI is characteristic of piping, pressure vessels and structural components. It occurs from water being trapped under insulation or fireproofing.

It usually involves metals like carbon steel, low alloy steels, 300 Series SS, and duplex stainless steel.

Boiler Water Condensate Corrosion/Oxygen Pitting

Boiler water condensate corrosion (or oxygen pitting) involves the general corrosion and pitting of boiler systems and condensate return piping.

CO2 Corrosion

When carbon dioxide (CO2) dissolves in water and forms carbonic acid, it causes corrosion (H2CO3). The acid can reduce the pH, and in large enough concentrations, it can cause general and/or pitting corrosion in carbon and low-alloy steels.

Microbiologically Induced Corrosion (MIC)

Living organisms such as bacteria, algae, and fungi induce this type of corrosion. It’s frequently accompanied with tubercles or slimy organic material.

It affects common construction materials like carbon and low alloy steels, 300 Series SS and 400 Series SS, aluminium, copper, and some nickel-based alloys.

Flue-Gas Dew-Point Corrosion

In combustion products, sulphur and chlorine species in fuel will generate sulphur dioxide, sulphur trioxide, and hydrogen chloride. These gases, along with the water vapor in the flue gas, condense at low enough temperatures to generate sulphurous acid, sulfuric acid, and hydrochloric acid. These intrusive acids can lead to condensation on carbon and stainless steel.

Caustic Corrosion

Refers to localized corrosion caused by a build-up of caustic or alkaline salts, which is common in evaporative or high-heat-transfer environments. Depending on the strength of the alkali or caustic solution, widespread corrosion can occur.

Oxidation Corrosion

At high temperatures, oxygen interacts with carbon steel and other alloys, turning the metal to oxide scale.

This phenomenon affects iron-based materials (such as carbon steel and low-alloy steels) and 300 Series SS, 400 Series SS and nickel base alloys depending on the composition and temperature.

Fuel Ash Corrosion

Fuel ash corrosion occurs when impurities in the fuel form deposits and melt on the metal surfaces of burned heaters and boilers, resulting in increased high-temperature material wastage.

Ashes are generated and carried through the furnace and combustion gas paths when coal and oils are burned in water-tube boilers.

Chloride Stress Corrosion Cracking (Cl- SCC)

Environmental cracking on the surface of 300 Series SS and some nickel base alloys under the combined action of tensile stress, temperature, and an aqueous chloride environment causes surface cracks. The presence of dissolved oxygen raises the cracking potential in these metals.

Surface cracking has a branched/spider web pattern and can be readily distinguished by its crazed-cracked appearance.

Amine (NH2) Stress Corrosion Cracking

When carbon and low alloy steels are exposed to lean amine solutions, stress corrosion cracking occurs.

This type of corrosion primarily affects carbon steel and low alloy steels.

Ammonia Stress Corrosion Cracking

Stress corrosion cracking (SCC) can be caused by ammonia-containing aqueous streams in some copper alloys and carbon steel.

Surface oriented, branching, tiny cracks are all characteristics of SCC which affects carbon steel in anhydrous ammonia and some copper alloys in environments with ammonium compounds.

Pitting Corrosion

Pitting corrosion, commonly known as pitting, is a type of corrosion that occurs on metal surfaces in a confined manner. Pitting usually appears as small holes or cavities on the object’s surface, with the rest of the metallic surface remaining unaffected.

This type of corrosion is also very penetrative, and it is regarded as one of the most dangerous types of corrosion since it is difficult to forecast and causes unexpected and dramatic failures.

Dry Corrosion

When there is no water or moisture to aid the corrosion, dry corrosion occurs, and the metal oxidises on its own. Metals are corroded by electron transfer, which involves two processes: oxidation and reduction.

Wet Corrosion

Wet corrosion occurs in the presence of a liquid containing ions, an electrolyte. Problems with wet corrosion attacks in stainless steels occur in mineral acids, process solutions, seawater and other chloride containing media.

Galvanic Corrosion

When two dissimilar metals come into direct or indirect contact with each other, bimetallic corrosion, also known as galvanic corrosion, occurs. This form of corrosion is visually distinguished by the rapid deterioration of one metal while the other remains intact.

Electrolytic Corrosion

Electrolytic corrosion is an accelerated form of corrosion where a metallic surface is continuously corroded by another metal it is in contact with. This is caused by an electrolyte and the flow of electrical current between the two metals, caused from an external source of electromotive force (EMF).

How to Measure the Rate of Corrosion

The rate of corrosion refers to how quickly a metal deteriorates in a given environment. The rate or speed is determined by environmental factors as well as the metal type and its condition.

Corrosion rates are typically calculated by the number of millimetres of corrosion penetration per year.

The following data is used to assess the rate of corrosion:

• Loss of metal weight during the reference time period
• Density of the metal
• Total initial surface area of the metal piece
• The passage of time within the reference time period
• Loss of wall thickness within the reference time period

Corrosion rate assessments are important to predict the lifespan of metal-based plant, equipment and structures. It also helps to determine the suitability and choice of metals for specific purposes and environmental conditions they are deployed in. The rate of corrosion also impacts the necessary corrosion testing and maintenance regime.

Corrosion of Different Metals

Although corrosion also affects polymers and ceramics, it is known to mostly impact metallic materials.

Below, we talk about metal deterioration based on type:

• Corrosion of Steel: The chemical reaction of some substances, such as sulfuric acid, can cause the degradation of steel. Steel corrosion is influenced by a variety of elements, including ambient air temperature, the presence of chemical fumes and vapours, and humidity.
• Corrosion of Aluminium: Aluminium corrosion, unlike purposeful operations like laser etching, anodising, or brightening, is a lengthy process that takes months or years to complete. However, what distinguishes aluminium from other metals is the variety of corrosion paths (e.g. atmospheric, galvanic, pitting, etc.) available. Understanding the various corrosion phenomena is the first step toward implementing control measures that will decrease or eliminate their occurrence.
• Corrosion of Stainless Steel: Corrosion or rust on a stainless-steel structure or object is known as stainless steel corrosion. Although stainless steel does not quickly corrode, stain, or develop rust, being ‘stain less’ does not imply ‘stain impossible’, so when working conditions are conducive to corrosion, stainless steel is not safe from degradation.
• Corrosion of Copper: Copper corrosion occurs when materials consisting of copper or copper alloys corrode. Copper oxidises when exposed to the air, causing ordinarily bright copper surfaces to tarnish. After a few years, the tarnish fades to a dark brown or black colour, then to green.
• Corrosion of Iron: When iron is exposed to moisture and oxygen, it corrodes, which is an oxidation reaction where electrons are lost. This reaction is also known as ‘rusting’ as it produces a reddish-brown hydrated iron oxide. In the presence of oxygen and moisture, iron corrosion results in the creation of FeO(OH) or Fe(OH)3. An electrolyte (e.g., water particles) and an oxygen-rich atmosphere are the bare essentials for this electrochemical process. Corrosion is accelerated by pollutants.

Corrosion Resistant Metals

There are various corrosion-resistant metals available, with each having their own advantages and disadvantages.

Stainless Steel

Perhaps the most commonly thought of metal when it comes to corrosion-resistance, stainless steel is not created the same. It comes in different types, and there are environments where it is effective and ones in which it is less so.

Stainless steel can be divided into three main or popular families: austenitic, ferritic and martensitic.

Austenitic stainless steels are extremely corrosion-resistant, contain high amounts of nickel and are generally the priciest of the three. It is commonly used in food preparation equipment, medical devices and process vessels.

Ferritic stainless steels are less corrosion-resistant compared to their austenitic cousins. They contain high amounts of chromium, without the nickel in austenitic stainless steels. Ferritic stainless steels are more affordable than austenitic stainless steels and popularly used in making automotive components.

Martensitic stainless steels are also less corrosion-resistant and have a high carbon and chromium content, with some having nickel. Their carbon content enables them to be heat treatable. They are also stronger and harder than ferritic or austenitic stainless steels, and are used for making tools and cutlery.

Aluminium

Aluminium is a corrosion-resistant metal frequently used in harsh environments. It is resistant to corrosion because it forms an aluminium oxide layer when its surface is exposed to oxygen.

The aluminium oxide is the one actually protecting the aluminium itself so the remaining aluminium does not degrade further. It is commonly used in the aerospace industry for the manufacture of airplane body components. It is also used in the food and beverage industry as it’s being corrosion resistant means food contamination is less likely.

Titanium

Titanium is the most expensive metal in this discussion, but it is also a very useful corrosion-resistant material. Through the formation of a passive oxide film on its surface, the remaining interior of the metal is protected from further oxidation.

It is also incredibly strong and can survive very harsh conditions for extended periods, which is probably why the aerospace industry is probably the most abundant user of titanium. It is used in making jewellery, marine applications, and in the manufacture of medical devices.

Protect Your Equipment and Plant from Corrosion

The process of corrosion can render the most expensive tools, equipment, and infrastructure useless once it has taken its hold.

The best way to protect your business and plant equipment from corrosion is by staying on top of its maintenance and care.

Asset Management Engineers can assist you with various corrosion testing and management services to prevent and curb corrosion issues in your operation.

You’ll also be assured of a safe workplace and avoid possible damage, repair and replacement costs.

At AME, we also conduct plant and equipment audits, root cause analysis and other related inspection activities to help our clients ensure structural integrity in their mining sites and other site-based operations.