Uniform corrosion of steel and metals – mechanism, corrosion rate and material selection

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Uniform corrosion, also known as general corrosion, is one of the basic forms of metal destruction. In contrast to pitting corrosion or crevice, it does not concentrate in single points of the surface, but causes a relatively uniform loss of material over a large area.
A typical example is the gradual rusting of unprotected carbon steel exposed to a humid atmosphere. However, uniform corrosion can also occur in stainless steels, high-alloy steels, and nickel alloys if the material is used in an environment that exceeds its corrosion resistance.
In the case of stainless steels, general corrosion primarily occurs when the passive layer protecting the metal surface becomes unstable in a given environment. This can occur in environments such as strong acids, hot alkaline solutions, or an unfavorable combination of temperature, chemical concentration, and contaminants.
Uniform corrosion is usually more predictable than localized corrosion. This allows for the determination of the rate of material loss and the estimated lifespan of the component.

What is uniform corrosion?

In uniform corrosion, corrosion reactions occur over a large portion of the metal surface. The material gradually releases into the environment in the form of ions or corrosion products, causing a reduction in the component's thickness.
In the case of carbon steel, this may be visible as a layer of rust covering a large surface. In stainless steel, the process can be different – ​​the surface may dull, lose its shine, or gradually etch away without the characteristic red rust.
The most important characteristic of uniform corrosion is that the material loss is relatively evenly distributed. This distinguishes it from pitting corrosion, where a small surface area can be attacked to a very great depth.

Uniform corrosion and localized corrosion

Paradoxically, uniform corrosion is often less dangerous than localized corrosion. This doesn't mean, of course, that it can be ignored. If the rate of material loss is high, it can lead to the wall thickness of tanks, pipes, or heat exchangers being reduced below safe limits.

Its advantage from the designer's point of view, however, is the ability to relatively well monitor changes in material thickness and predict the durability of the installation.

Uniform corrosion rate – mm/year

The resistance of a material to uniform corrosion is often described by the corrosion rate expressed in millimeters of material thickness loss per year – mm/year.
In laboratory testing, a material sample is exposed to a specific medium for a specified period of time. After removing corrosion products, the mass loss is determined. The corresponding corrosion rate can then be calculated based on the sample surface area, exposure time, and material density.
For stainless steels, a corrosion rate below approximately 0,1 mm/year is often considered very good resistance to uniform corrosion. However, this is not a universal design limit. The allowable loss depends on the type of device, its required durability, wall thickness, pressure, temperature, and safety requirements.

The values ​​provided are indicative. When designing a specific installation, data for the actual medium, temperature, and concentration should be used, and design requirements should be taken into account.

Why might stainless steel corrode evenly?

The resistance of stainless steels stems from the presence of a very thin passive layer on their surface, rich in chromium oxides. This layer separates the metal from the environment and can self-regenerate under the right conditions.

However, if the environment is too aggressive, the passive state can become unstable. Instead of localized damage to the layer, as in pitting corrosion, it may destabilize over a large area and initiate general dissolution of the material.

Therefore, even stainless steel 1.4301 (AISI 304) or 1.4404 (AISI 316L) can corrode quickly in certain acids or at high temperatures.

What influences the rate of uniform corrosion?

Corrosion rates are not a constant property of a given grade of steel. The same material may exhibit virtually no corrosion in one environment and deteriorate very rapidly in another.

The most important factors influencing the corrosion rate are:

• type of chemical medium,
• acid or base concentration,
• temperature,
• pH,
• the presence of oxygen and other oxidizing substances,
• presence of chlorides and other impurities,
• medium flow speed,
• aeration of the solution,
• surface condition of the material,
• material microstructure,
• heat treatment and welding,
• presence of other metals in the installation.

Temperature influence

Temperature is one of the most important parameters influencing corrosion. In many environments, even a relatively small increase in temperature can significantly increase the rate of corrosion reactions.
For this reason, information that a given species is resistant to a specific acid, without specifying its concentration and temperature, has little technical value.
A material that is suitable for a 10% acid solution at room temperature may not necessarily be suitable for use with the same solution at 80 or 100°C.

Effect of chemical concentration

The relationship between acid concentration and corrosion rate is not always linear. Higher concentrations do not automatically mean higher corrosion rates.
Some environments, as their concentration changes, change from a reducing to a more oxidizing character or cause the formation of a more stable passive layer. Therefore, the resistance of materials in, for example, sulfuric acid should be analyzed based on isocorrosion diagrams or resistance tables for a specific concentration and temperature.

Uniform corrosion in sulfuric acid

Sulfuric acid H₂SO₄ is one of the classic environments in which the differences between the different grades of stainless steel are very clearly visible.
Standard steels 1.4301 (304) and 1.4404 (316L) have limited use in sulfuric acid. As the environment becomes more aggressive, it may be necessary to use steels containing higher amounts of nickel, molybdenum, and copper.
One of the most well-known examples is 1.4539 steel – AISI 904L / UNS N08904. This grade was developed, among other things, for use in sulfuric acid. Its nickel, molybdenum, and copper content significantly increases its resistance compared to standard 304 and 316L steels.
Even higher resistance in many aggressive environments is provided by 1.4529 / UNS N08926 steel, containing about 6% molybdenum, a large amount of nickel and additions of nitrogen and copper.

Uniform corrosion in hydrochloric acid

Hydrochloric acid (HCl) is a particularly aggressive medium for stainless steels. It is a reducing acid and simultaneously introduces chloride ions into the environment, destabilizing the passive layer.
For this reason, popular stainless steels such as 304 and 316L have very limited suitability in hydrochloric acid solutions. Even high-alloy stainless steels cannot automatically be considered suitable for all HCl concentrations and temperatures.
For more demanding conditions, alloys with very high nickel and molybdenum contents are used. An example is alloy 2.4819 – Alloy C-276, which exhibits significantly higher resistance to general corrosion in hydrochloric acid than typical austenitic steels.
The selection of material for HCl should always be made based on the concentration, temperature, presence of oxygen and other components of the medium.

Uniform corrosion in phosphoric acid

Phosphoric acid H₃PO₄ is generally less aggressive than hydrochloric acid, but its actual corrosivity in industrial applications may be much higher due to the presence of impurities.
Of particular importance are chlorides, fluorides and sulfuric acid found in industrially produced phosphoric acid.
In milder conditions, 316L steel can be used. In more demanding installations, 904L, 1.4529 / Alloy 926, 254 SMO, and appropriately selected nickel alloys are used.

Organic acids

Stainless steels are also widely used in installations containing organic acids, including acetic and formic acid.
However, resistance depends on the type of acid, its concentration, temperature, and the presence of impurities. Formic acid can be significantly more aggressive than many other organic acids.
In mild conditions, 304 or 316L steel may be sufficient, while in more demanding processes, high-alloy steels and nickel alloys are used.

304 and 316L steels – what is the difference?

1.4301 / AISI 304 steel is one of the most commonly used stainless steel grades. It offers good resistance in many atmospheric, aqueous, and food environments, as well as in contact with numerous moderately aggressive chemicals.
1.4404 / AISI 316L steel contains molybdenum, which makes it much more resistant to localized corrosion, especially in environments containing chlorides.
However, don't assume that 316L will automatically be resistant to every acid. Molybdenum improves resistance to many environments, but both 304 and 316L can exhibit high rates of general corrosion in strong reducing acids.

904L - 1.4539

1.4539 / AISI 904L / UNS N08904 is a high-alloy austenitic steel with increased nickel and molybdenum content and copper addition.
This grade was developed for environments much more aggressive than those typically used in 304 and 316L. One of its classic applications is sulfuric acid installations.
904L is used, among others, in:

• chemical and petrochemical industry,
• sulfuric acid installations,
• heat exchangers,
• process tanks,
• flue gas desulphurization installations,
• hydrometallurgy.

1.4529 / Alloy 926

Steel 1.4529 / UNS N08926 / Alloy 926, belongs to the group of high-alloy superaustenitic steels.
The high content of molybdenum, nickel and chromium as well as the addition of nitrogen and copper provide very high resistance to both general corrosion and many forms of local corrosion.
The material exhibits excellent resistance to both oxidizing and reducing media. It is used, among others, in systems containing dilute sulfuric and phosphoric acid, even in the presence of chlorides.

254 SMO – 1.4547

1.4547 / UNS S31254 / 254 SMO is a super-austenitic steel containing approximately 6% molybdenum and increased nitrogen content.
The grade is primarily associated with very high resistance to pitting and crevice corrosion, but at the same time it exhibits high resistance to uniform corrosion in many aggressive chemical environments.
It is used in chemical installations, seawater systems, desalination, the pulp and paper industry and gas purification devices, among others.

Constantly duplex and superduplex

Duplex steels, such as 1.4462 / Duplex 2205, and superduplex steels, such as 1.4410 / Superduplex 2507, combine an austenitic and ferritic structure.
They are characterized by high mechanical strength and very good resistance to localized corrosion and chloride stress corrosion cracking.
They also exhibit very good resistance to general corrosion in many environments. However, this does not mean that superduplex steels are always more chemically resistant than all austenitic steels. In the presence of strong reducing acids, high-nickel superaustenitic steels or nickel alloys may be a better solution.

Nickel alloys in highly corrosive environments

If the aggressiveness of the environment exceeds the capabilities of stainless steels, alloys with high nickel, chromium and molybdenum contents are used.
Examples are:

• 2.4856 / Alloy 625 / UNS N06625,
• 2.4858 / Alloy 825 / UNS N08825,
• 2.4819 / Alloy C-276 / UNS N10276,
• Alloy 22 / UNS N06022,
• Alloy 59 / UNS N06059.

Ni-Cr-Mo alloys can provide very high resistance to general corrosion in sulfuric acid, phosphoric acid and many other aggressive media.
High-molybdenum nickel alloys such as Alloy C-276 exhibit particularly high resistance to reducing acids, including hydrochloric acid. However, this does not mean that one alloy is best in every environment. The nature of the medium, oxidation potential, temperature, and contamination can completely alter the order of resistance of individual materials.

Comparison of sample materials

How is uniform corrosion resistance tested?

One of the basic methods is laboratory immersion testing. A sample of known surface area and mass is placed in a specific chemical medium at a set temperature and for a specified time.
After the test is completed, the corrosion products are removed and the mass change of the sample is determined. From this, the corrosion rate can be calculated.
The procedures for performing laboratory immersion tests are described, among others, in ASTM G31, while the preparation, cleaning and evaluation of samples after corrosion tests are described in ASTM G1.
However, laboratory test results should be interpreted with caution. Real-world installations may contain contaminants, fluctuating temperatures, flow, deposits, welds, and other factors not present in a simple laboratory test.

Isocorrosion diagrams

Isocorrosion diagrams are often used to compare materials in acid environments. They show combinations of temperature and medium concentration corresponding to a specific corrosion rate, e.g., 0,1 mm/year.
This allows you to quickly see that the safe operating range of a material can change dramatically with temperature or acid concentration.
Such charts are particularly useful when selecting materials for sulfuric acid, hydrochloric acid, phosphoric acid and other aggressive industrial media.

Can the durability of an element be predicted?

If corrosion is truly uniform and its rate is known, the expected loss of material thickness can be approximately estimated.
For example, at a corrosion rate of 0,1 mm/year, the theoretical loss after 10 years is approximately 1 mm.
This simple calculation can be useful in designing a corrosion allowance, but it should not be used indiscriminately. Corrosion rates can vary over time with changes in temperature, fluid composition, flow, or surface condition.
This calculation also does not apply to pitting, crevice or stress corrosion cracking, where local damage may develop much faster than the average mass loss would suggest.

How to reduce uniform corrosion?

The basic method is to select the correct material for the operating conditions. If the corrosion rate of standard steel is too high, using a more alloyed grade may be a solution.
Depending on the application, the following can also be used:

• protective coatings,
• chemically resistant floor coverings,
• corrosion inhibitors,
• cathodic protection,
• temperature control,
• change in pH of the medium,
• limiting the content of aggressive pollutants,
• appropriate corrosion allowance,
• regular wall thickness measurements.

Selecting the steel grade for a corrosive environment

There is no single universal order of material resistance that would allow us to say that one type is always better than another.
For example, superduplex steels may outperform austenitic steels in chloride environments, while high-nickel steels like 904L or Alloy 926 may perform better in certain acidic environments. In very aggressive acids, it may be necessary to switch from stainless steels to nickel alloys like Alloy 625, C-276, or Alloy 22.

Therefore, before selecting a material, you should determine at least:

• the exact composition of the medium,
• concentration of individual substances,
• minimum and maximum temperature,
• content of chlorides and other pollutants,
• flow conditions,
• the presence of oxygen or other oxidants,
• expected operating time,
• permissible thickness loss.

Uniform corrosion is a process that causes gradual loss of metal over a significant portion of its surface. Compared to pitting or stress corrosion cracking, it is usually easier to detect, measure, and predict.
This doesn't mean, however, that it's a harmless phenomenon. In aggressive media, the rate of corrosion can be so rapid that it significantly reduces the wall thickness of devices and installations in a relatively short time.
Material resistance depends primarily on the chemical composition, temperature, and concentration of the medium. Standard 304 and 316L stainless steels are sufficient for many moderate environments, while for more aggressive conditions, materials such as 904L, 1.4529 / Alloy 926, 254 SMO, duplex and superduplex steels, and, for the harshest environments, high-alloy nickel alloys are used.
When selecting a material, one should not rely solely on the term "stainless steel." It is crucial to match the specific grade to the specific medium, its concentration, temperature, and anticipated corrosion mechanism.

Corrosion can occur via various mechanisms, depending on the material, environment, and operating conditions. Learn about other types of corrosion:

Types of corrosion of steel and metals

Uniform corrosion of steel and metals
Pitting corrosion of stainless steels
Crevice corrosion of stainless steels
Intergranular corrosion of stainless steel
Stress corrosion cracking SCC
Galvanic corrosion
Erosive corrosion
Cavitation corrosion
Corrosion fatigue
Fretting corrosion
Microbiological corrosion MIC
High-temperature corrosion

Division of corrosion according to the environment

Marine corrosion – corrosion of steel and metals in seawater
Atmospheric corrosion
Ground corrosion
Corrosion in fresh and industrial water
Corrosion in chloride environments
Corrosion in acidic environments
Corrosion in alkaline environments
Corrosion in gases and exhaust gases
Corrosion in sewage and sewer systems
Corrosion in chemical and petrochemical environments

Related topics

CPT – critical pitting corrosion temperature of stainless steels and nickel alloys
PREN – pitting corrosion resistance index for stainless steel
CCT – Critical crevice corrosion temperature of stainless steels and nickel alloys