Do you know about the corrosion resistance of stainless steel in various environments?

The corrosion resistance of stainless steel generally increases with higher chromium content. The basic principle is that when there is sufficient chromium in the steel, a very thin, dense oxide film forms on the surface, which prevents further oxidation. Oxidising environments can strengthen this film, whereas reducing environments inevitably destroy it, leading to corrosion of the steel.

Corrosion Resistance in Various Environments

Atmospheric Corrosion

The resistance of stainless steel to atmospheric corrosion is essentially dependent on the chloride content of the atmosphere. Consequently, corrosion of stainless steel in the vicinity of the sea or other sources of chloride contamination is of paramount importance. Rainwater is only a significant factor when it affects the chloride concentration on the steel surface.

Rural Environments 1Cr13, 1Cr17 and austenitic stainless steels are suitable for a wide range of applications without significant changes to their appearance. Consequently, the choice of stainless steel for outdoor use in rural areas may be based on price, market availability, mechanical properties, workability and appearance.

Industrial environments In industrial environments free from chloride contamination, 1Cr17 and austenitic stainless steels can operate for long periods whilst remaining essentially rust-free. A film of dirt may form on the surface, but once removed, the original bright appearance is retained. In industrial environments containing chlorides, stainless steel will corrode.

Marine Environment 1Cr13 and 1Cr17 stainless steels form a thin rust film within a short period, but this does not cause significant dimensional changes. Austenitic stainless steels such as 1Cr17Ni7, 1Cr18Ni9 and 0Cr18Ni9 may exhibit some corrosion when exposed to a marine environment. This corrosion is usually superficial and can be easily removed. 0Cr17Ni12Mo2 molybdenum-containing stainless steel is essentially corrosion-resistant in marine environments.

In addition to atmospheric conditions, there are two other factors that influence the atmospheric corrosion resistance of stainless steel: surface condition and manufacturing process. The finish grade affects the corrosion resistance of stainless steel in chloride-containing environments. Matt surfaces (rough surfaces) are highly susceptible to corrosion, whereas standard industrial finishes are less susceptible. The surface finish grade also affects the removal of dirt and rust. It is easy to remove dirt and rust from highly polished surfaces, but difficult to do so from non-polished surfaces. For non-polished surfaces, frequent cleaning is required to maintain the original surface condition.

Freshwater

Freshwater can be defined as water from rivers, lakes, ponds or wells, regardless of whether it is acidic, saline or slightly brackish.

The corrosiveness of fresh water is influenced by the water’s pH, oxygen content and tendency to form scale. Hard water. Its corrosiveness is primarily determined by the amount and type of scale formed on the metal surface. The formation of this scale is a result of the minerals present and the temperature. Soft water is generally more corrosive than hard water. Its corrosiveness can be reduced by raising the pH or reducing the oxygen content.

1Cr13 stainless steel is significantly more resistant to freshwater corrosion than carbon steel and exhibits excellent performance when used in freshwater. This steel is widely used in applications such as shipyards and dams, where high strength and corrosion resistance are required. However, it should be noted that in certain circumstances, 1Cr13 may be susceptible to moderate pitting corrosion in freshwater. Nevertheless, pitting corrosion can be entirely prevented by cathodic protection methods. 1Cr17 and austenitic stainless steels are almost completely resistant to freshwater corrosion at room temperature (ambient temperature).

Acidic Water

Acidic water refers to contaminated natural water leached from ores and coal. As it is strongly acidic, its corrosiveness is far greater than that of natural freshwater. Due to the leaching of sulphides from ores and coal, acidic water typically contains large amounts of free sulphuric acid. Furthermore, this water contains significant quantities of ferric sulphate, which plays a major role in the corrosion of carbon steel.

Carbon steel equipment exposed to acidic water is usually corroded very rapidly. Test results using various materials exposed to acidic river water indicate that austenitic stainless steel exhibits high corrosion resistance in such environments.

Austenitic stainless steel exhibits excellent corrosion resistance in both fresh water and acidic river water; in particular, its corrosion film impedes heat transfer to a lesser extent, which is why stainless steel pipes are widely used in heat exchange applications.

Saline Water

The characteristic of corrosion in saline water is that it frequently manifests as pitting corrosion. In the case of stainless steel, this is largely due to the localised breakdown of the passivation film that provides corrosion resistance. Other causes of pitting corrosion in these steels include the formation of potent concentration cells by marine algae and other marine organic matter adhering to the stainless steel equipment. Once formed, these cells are highly active and cause significant corrosion and pitting. Under conditions of high-velocity flow in saline water, such as within pump impellers, corrosion of austenitic stainless steel is generally minimal.

For condensers using stainless steel tubes, the water flow velocity should be maintained at over 1.5 m/s to minimise the accumulation of marine organic matter and other solids within the tubes. When designing stainless steel equipment for handling saline water, it is advisable to minimise crevices and utilise thick-walled components.

Soil

Metals buried in soil are subject to constantly changing and complex conditions, depending on weather and other factors. Experience has shown that austenitic stainless steels generally exhibit excellent resistance to corrosion in most soils, whereas 1Cr13 and 1Cr17 are prone to pitting in many soils. 0Cr17Ni12Mo0 stainless steel has been found to be completely resistant to pitting in all soil tests.

Nitric Acid

Ferritic and austenitic stainless steels containing not less than 14% chromium exhibit excellent resistance to nitric acid corrosion. 1Cr17 stainless steel has been widely used in processing equipment for nitric acid plants. However, as 0Cr18Ni9 generally offers better formability and weldability, it has largely replaced 1Cr17 stainless steel in the aforementioned applications.

The resistance to nitric acid corrosion of other austenitic stainless steels is similar to that of 0Cr18Ni9. 0Cr17 stainless steel generally exhibits a slightly higher corrosion rate than 0Cr18Ni9, and higher temperatures and concentrations have a more detrimental effect on it.

If the steel is not heat-treated appropriately, hot nitric acid will cause intergranular corrosion in both austenitic and ferritic stainless steels. Therefore, this type of corrosion can be prevented by appropriate heat treatment or by using stainless steels resistant to this type of corrosion.

Sulphuric acid

Standard stainless steel grades are rarely used in sulphuric acid solutions because their service range is very narrow. At room temperature, 0Cr17Ni12Mo2 stainless steel (the standard grade with the highest resistance to sulphuric acid) is corrosion-resistant when the sulphuric acid concentration is less than 15% or greater than 85%. However, in the higher concentration range, carbon steel is generally used. Martensitic and ferritic stainless steels are generally not resistant to corrosion by sulphuric acid solutions.

As with nitric acid, sulphuric acid can cause intergranular corrosion if the stainless steel is not treated appropriately. For welded structures where post-weld heat treatment is not possible, low-carbon grades such as 00Cr19Ni10 or 00Cr17Ni14Mo2, or stabilised grades such as 0Cr18Ni11Ti or 0Cr18Ni11Nb stainless steel, should be used.

Phosphoric Acid

Austenitic stainless steels exhibit good resistance to corrosion by phosphoric acid solutions and are widely used in equipment for the production and processing of phosphoric acid. They demonstrate effective corrosion resistance at temperatures up to 107°C and at various concentrations. At temperatures up to approximately 95°C, equipment made from 0Cr17Ni12Mo2 stainless steel can effectively handle phosphoric acid concentrations exceeding 100% H₃PO₄.

It should be noted that trace impurities of fluoride or chloride salts are sometimes present in phosphoric acid produced by wet processes. The presence of these halides in the acid may have a detrimental effect on the corrosion resistance of the stainless steel.

Martensitic and ferritic stainless steels exhibit significantly poorer resistance to phosphoric acid corrosion than austenitic stainless steels and are therefore generally not used with this acid.

Hydrochloric Acid

Even at room temperature, hydrochloric acid solutions of various concentrations corrode stainless steel rapidly. Consequently, it is not possible to use stainless steel in this acid.

Other Inorganic Acids

Austenitic stainless steels generally exhibit good resistance to corrosion by boric acid, carbonic acid, chloric acid and chromic acid at almost all concentrations and temperatures, with the exception of 100% chloric acid. 1Cr13 and 1Cr17 stainless steels exhibit significantly poorer resistance to chromic acid than austenitic stainless steels, but they possess relatively good resistance to boric acid and carbonic acid corrosion.

Acetic acid

Austenitic stainless steels generally exhibit excellent resistance to acetic acid corrosion, whereas martensitic and ferritic stainless steels are unsuitable for most applications involving acetic acid. Austenitic stainless steels are fully resistant to acetic acid of all concentrations at room temperature; at higher temperatures, 0Cr17Ni12Mo2 and 0Cr19Ni13Mo3 demonstrate better resistance to acetic acid corrosion than other austenitic stainless steels.

Formic acid

At room temperature, any austenitic stainless steel can be used to handle formic acid without issue. However, when formic acid is hot, it can rapidly corrode stainless steels that do not contain molybdenum; therefore, the use of 0Cr17Ni12Mo2 and 0Cr19Ni13Mo3 is required. Formic acid rapidly corrodes martensitic and ferritic stainless steels at all temperatures.

Oxalic acid

Generally speaking, at room temperature and at concentrations of at least 50%, stainless steel exhibits good resistance to corrosion by oxalic acid. However, at higher temperatures, oxalic acid solutions are just as corrosive to all types of stainless steel as a 100% solution is at room temperature.

Lactic acid

0Cr18Ni9 stainless steel may be used for lactic acid storage equipment at temperatures up to approximately 38°C. At higher temperatures, molybdenum-free austenitic stainless steels are susceptible to pitting corrosion; therefore, 0Cr17Ni12Mo2 and 0Cr19Ni13Mo3 are preferred. Martensitic and ferritic stainless steels generally exhibit lower resistance to lactic acid corrosion.

Alkalis

Stainless steels generally exhibit good resistance to corrosion by weak alkalis, such as ammonium hydroxide. For strong alkalis, such as sodium hydroxide and potassium hydroxide, austenitic stainless steels demonstrate good corrosion resistance at temperatures up to approximately 105°C and concentrations up to approximately 50%; at higher temperatures and concentrations, the corrosion rate may become significant. Stress corrosion cracking may occur in austenitic stainless steels when temperatures exceed the boiling point at atmospheric pressure (and at slightly lower temperatures with concentrations approaching 50%).

Hydrochloric Acid Solutions

With the exception of halide solutions under certain conditions, stainless steels generally exhibit excellent resistance to corrosion by hydrochloric acid solutions. For acidic salts, the corrosion resistance of stainless steels is inevitably influenced to some extent by the specific acids formed by the hydrolysis of the salt. For acidic salt solutions at higher temperatures, molybdenum-containing austenitic stainless steels (0Cr17Ni12Mo2 and 0Cr19Ni13Mo3) generally exhibit better corrosion resistance than other grades of stainless steel.

When stainless steel is used in halide solutions, particularly chloride solutions, it should be borne in mind that although corrosion rates are generally low, pitting corrosion and/or stress corrosion cracking may occur under certain conditions. Although there are many instances where stainless steel has been used with excellent results in the presence of chlorides (such as in food processing equipment and seawater flowing at relatively low temperatures), each application must be considered individually. Whether pitting or stress corrosion cracking occurs depends on many factors, including the environment, equipment design and operation.