Corrosion in alkaline environments – resistance of steel and metals to alkalis

Alkaline environments occur in many industries, including chemical, petrochemical, paper, food, energy, pharmaceutical, and water treatment plants. The most common strong bases are sodium hydroxide (NaOH) and potassium hydroxide (KOH), but alkaline environments can also result from the presence of carbonates, ammonia, and other compounds that cause pH increases.
A high pH does not automatically imply a non-corrosive environment. Many steels exhibit good resistance in moderately alkaline solutions, but as the base concentration and temperature increase, severe general corrosion, localized corrosion, or stress corrosion cracking can occur. Of particular importance in industrial installations is the exposure to concentrated, hot NaOH solutions, commonly referred to as caustic soda or soda lye.

Why can an alkaline environment cause corrosion?

The behavior of a metal in an alkaline environment depends on its ability to form stable surface products, or a passive layer. For many materials, increasing pH can promote passivation, but this resistance exists only within a specific range of potential, temperature, and solution chemical composition.
In highly alkaline environments, this equilibrium can be altered. Some metal oxides and hydroxides can become soluble, and high temperatures can significantly accelerate electrochemical processes. Furthermore, the presence of chlorides, sulfides, oxidizing compounds, and other impurities can completely alter the material's behavior.

The most important factors influencing corrosion in an alkaline environment

Factor	Importance for corrosion
Type of rule	NaOH, KOH, ammonia and other alkaline compounds can affect materials in different ways.
Concentration	an increase in concentration may change the stability of protective layers and increase the aggressiveness of the environment
Temperature	is one of the most important parameters; hot concentrated bases can be much more aggressive than cold dilute solutions
Chlorides	may increase the risk of local corrosion of stainless steels even in an alkaline environment
Oxidizing ingredients	change the electrochemical potential and may affect the stability of the passive state
Stresses	under certain conditions they can lead to stress corrosion cracking in caustic environments
Local concentration	evaporation or boiling may locally increase the concentration of the base on the metal surface
Flow	influences the transport of reagents, surface temperature and the possibility of environmental concentration

Sodium hydroxide NaOH – caustic soda

Sodium hydroxide is one of the most commonly used alkaline compounds in industry. It is used in chemical, pulp and paper, refinery, food, and pharmaceutical production, as well as in plant cleaning and purification processes.
The resistance of metals to NaOH depends strongly on its concentration and temperature. A material that behaves normally in a cold, dilute solution may exhibit a significantly higher corrosion rate when the temperature or concentration is increased.
In systems containing hot NaOH, special attention should be paid to the heating, evaporation, and heat transfer areas. The metal surface may locally experience a higher temperature than the bulk solution, and water evaporation can further increase the local alkali concentration.

Potassium hydroxide KOH

Potassium hydroxide is a strong base used in the chemical industry, detergent production, electrolyte production, and various technological processes. As with NaOH, the resistance of a material cannot be determined solely by its name.
Concentration, temperature, presence of contaminants, and design conditions are all important factors. At high temperatures and concentrations, the capabilities of standard stainless steels may be limited, and it is necessary to use materials selected based on corrosion data for a specific process.

Caustic corrosion

The term caustic corrosion refers to corrosion caused by highly alkaline environments, primarily NaOH solutions. It can occur in chemical, boiler, and process installations, particularly in areas where localized alkali concentrations increase.
One dangerous mechanism is the concentration of alkali in crevices, under deposits, or on intensely heated surfaces. Even if the average alkali concentration in the system is not very high, local water evaporation can lead to the formation of a much more concentrated solution near the metal surface.

Under such conditions, corrosion may develop much more intensively than would be expected from the analysis of a sample taken from the main stream of the medium.

Caustic SCC – corrosion cracking in an alkaline environment

Highly alkaline environments can, under certain conditions, cause stress corrosion cracking (SCC), also known as caustic stress corrosion cracking (CCC). This phenomenon requires the simultaneous presence of a susceptible material, a suitably aggressive environment, and tensile stresses.
Stresses can arise from operational loading, welding, bending, forming, or other technological processes. Areas of localized concentration of hot NaOH can be particularly dangerous, as the actual conditions on the metal surface can be much more aggressive than those measured in the bulk medium.
Caustic SCC should be distinguished from chloride SCC. Both phenomena are stress-corrosion cracking, but the environment, material susceptibility range, and mechanisms may be different.

Carbon steel in an alkaline environment

Carbon steel can exhibit good resistance in many moderately alkaline environments by forming surface layers that limit corrosion rates. For this reason, it is used in many installations containing NaOH solutions.
However, this does not mean universal resistance. Increased temperature, very high base concentrations, local concentrations, and stresses can lead to increased corrosion or stress-corrosion cracking.
In the case of carbon steel, not only the average NaOH concentration is of particular importance, but also the possibility of local concentration of the solution and the temperature of the metal surface.

Stainless steels in alkaline environments

Stainless steels exhibit high resistance to many alkaline environments due to their ability to passivate. Standard austenitic grades 304L and 316L can be used in many alkaline solutions, especially at moderate temperatures and concentrations.
However, resistance may deteriorate as temperature and strong base concentration increase. Under these conditions, increasing chromium and molybdenum content does not always produce the same effect as in chloride environments. Therefore, material selection for hot, concentrated alkalis requires separate corrosion data.
Contaminants are also important. The presence of chlorides can introduce an additional risk of pitting and crevice corrosion, which would not occur to the same extent in a pure alkaline solution.

304L and 316L in alkaline solutions

304L, or 1.4307 / UNS S30403, and 316L, or 1.4404 / UNS S31603, are among the most commonly used austenitic stainless steels. They exhibit good corrosion resistance in many dilute and moderately hot alkaline environments.
In chloride environments, the molybdenum present in 316L provides a distinct advantage over 304L in terms of localized corrosion resistance. In pure, strongly alkaline solutions, this relationship is not as straightforward. Therefore, it should not be automatically assumed that 316L will always be significantly more resistant to NaOH than 304L.
At high temperatures and concentrations, the corrosion data for the specific material should be checked, as both the general corrosion rate and the risk of stress corrosion cracking may become limiting factors.

Duplex steels in alkaline environments

Duplex steels, such as 1.4462 / UNS S32205, combine ferritic and austenitic structures and high mechanical strength. They are valued primarily for their high resistance to localized corrosion in chloride environments and greater resistance to chloride SCC than standard austenitic steels.
This doesn't mean, however, that Duplex 2205 or Superduplex 2507 are automatically the best choice for every highly alkaline environment. For hot, concentrated alkalis, their suitability should be assessed based on data regarding the specific concentration, temperature, and composition of the medium.

Nickel alloys in alkaline environments

Nickel and alloys with a high nickel content can exhibit very high resistance to many highly alkaline environments. For this reason, nickel materials are used in demanding installations containing hot and concentrated alkalis.
Of particular importance are commercially pure nickel grades such as Nickel 200 and Nickel 201. Nickel 201 is a low-carbon variety of nickel intended, among others, for applications requiring good resistance in caustic environments at elevated temperatures.
The selection between stainless steel, nickel and nickel alloy should take into account not only resistance to the main base but also all impurities and process components.

Aluminum in an alkaline environment

Aluminum is an example of an amphoteric metal. Its natural oxide layer provides good protection in many environments, but it can dissolve in both strongly acidic and strongly alkaline conditions.
For this reason, aluminum can react aggressively with strong bases such as NaOH and KOH. When designing systems containing strong alkalis, it should not be assumed that the natural oxide layer of aluminum will provide protection similar to that found in milder pH environments.

Zinc and zinc coatings in strong bases

Zinc also exhibits amphoteric properties and can dissolve rapidly in highly alkaline solutions. Therefore, zinc coatings, while providing excellent protection for steel in many atmospheric conditions, are not universally effective for installations exposed to concentrated, strong bases.
Zinc resistance depends on pH, temperature, and solution composition. At very high pH, the surface layer of products may lose its protective properties and the zinc dissolution rate may increase.

Effect of temperature and NaOH concentration

Temperature and concentration must be considered simultaneously. Claiming a material is "NaOH-resistant" without specifying these two parameters is of little technical value.
As temperature increases, the electrochemical reaction rate, substance transport, and stability of surface layers change. Simultaneously, increasing NaOH concentration can alter the physicochemical properties of the solution and the material's behavior.
Therefore, when designing tanks, pipelines and exchangers, it is necessary to analyze not only the nominal operating parameters, but also the maximum conditions, start-up, shutdown, cleaning and the possibility of local overheating and concentration of the solution.

Base concentration under deposits and on heating surfaces

One of the most dangerous phenomena is localized alkali concentration. It can occur under sediments, in crevices, and on surfaces subject to intense water evaporation.
For example, in a heat exchanger, the bulk of the medium may contain a relatively dilute NaOH solution, while the concentration directly at the hot surface may be much higher. These local conditions can cause corrosion or cracking, even though analysis of the average composition of the medium would indicate a much more benign environment.

The importance of pollutants in an alkaline environment

Real industrial media rarely consist solely of water and a single hydroxide. They may contain chlorides, sulfides, sulfur compounds, carbonates, organic substances, oxidants, and process products.
The presence of chlorides is particularly important for stainless steels, as it can increase the risk of localized corrosion. As a result, steel that exhibits very good resistance in a pure NaOH solution may behave differently in a process solution containing additional chlorides and other impurities.

Selection of materials for alkaline environments

Material	General characteristics in alkaline environments
carbon steel	may show good resistance in many alkaline solutions, but may be limited by high temperature, concentration and the risk of caustic SCC
304L/1.4307	Good resistance in many moderate alkaline environments; range of application depends on temperature and concentration
316L/1.4404	good resistance in many alkaline solutions; the presence of Mo is particularly advantageous when the environment also contains components causing local corrosion
Duplex / superduplex	high strength and resistance to many environments, but for hot concentrated alkalis, data verification for specific conditions is required
Nickel 200	very good resistance to many caustic environments; the scope of application depends on the temperature and process conditions
Nickel 201	low-carbon nickel used, among others, in demanding alkaline environments and at elevated temperatures
Aluminium	limited resistance to strong bases due to the amphoteric nature of aluminum and its oxides
Zinc	limited resistance in highly alkaline environments; zinc coating is not a universal protection against concentrated alkalis

The table is general in nature and does not specify permissible concentrations or operating temperatures. Material selection for NaOH, KOH, or other alkaline media should be based on corrosion data for the specific concentration, temperature, and composition of the medium.

Does PREN determine resistance to NaOH?

PREN is an index designed primarily to compare the resistance of stainless steels to pitting corrosion, particularly in chloride-containing environments. It is not a universal indicator of chemical resistance and should not be used to predict a material's resistance to NaOH or KOH.
A steel with a higher PREN rating is not necessarily more resistant to hot, concentrated caustic soda than a material with a lower PREN rating. Such a comparison should be made using actual corrosion data for the specific alkaline environment.

Alkaline corrosion and chloride corrosion

In pure alkaline environments, the mechanisms limiting material durability may be different than in neutral chloride solutions. In the case of chlorides, one of the primary problems is localized damage to the passive layer, leading to pitting and crevice corrosion. In hot, concentrated alkalis, general corrosion and caustic SCC may become more significant.
In real installations, however, both types of hazards can occur simultaneously. If the NaOH solution contains significant chloride content, the material must be resistant to both a strongly alkaline environment and the local effects of Cl⁻ ions.

How to reduce corrosion in alkaline environments?

select the material according to the actual base concentration and temperature,
control the maximum metal surface temperature, not just the temperature of the main medium flow,
take into account the possibility of NaOH or KOH concentration due to evaporation,
limit the formation of deposits and crevices that allow local concentration of the medium,
analyze the content of chlorides and other pollutants,
take into account residual stresses after welding and plastic processing,
in environments at risk of caustic SCC, control the material, temperature, concentration and stress level simultaneously,
take into account the conditions for starting, stopping, flushing and cleaning the installation,
use corrosion data for the actual medium instead of basing the selection solely on the material type name,
in critical applications, confirm the selection by operating experience or appropriate corrosion tests.

How to select material for NaOH and KOH?

The selection process should begin by determining the minimum, nominal, and maximum base concentrations and temperature. Next, the presence of chlorides, other salts, oxidizing substances, and process contaminants should be determined.
It's also important to consider the heating method of the medium, the possibility of boiling or evaporation, the presence of deposits and gaps, and the stress level in the structure. In the case of welded installations, residual stresses remaining in the material after joints are made may be significant.
Only under such defined conditions can carbon steels, stainless steels, duplex steels, nickel, and nickel alloys be compared. In particularly demanding installations, material manufacturers' corrosion charts and tables, which show corrosion rates as a function of concentration and temperature, are helpful.

Alkaline environments are not automatically benign for metals. A material's resistance depends on the type of base, its concentration, temperature, impurities, flow conditions, local solution concentration, and stresses.
Carbon steel and 304L and 316L stainless steels can exhibit very good resistance in many alkaline solutions, but their performance changes with increasing temperature and concentration. High-nickel materials, including Nickel 200 and Nickel 201, are also used in demanding caustic environments.
Hot, concentrated NaOH solutions and areas of local evaporation and concentration of alkali require special attention, as they can lead to both increased corrosion and caustic SCC. Therefore, the correct material selection should take into account the entire system: material – type of base – concentration – temperature – impurities – stresses – local concentration – construction – operating conditions.

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