Corrosion in various environments – environmental corrosion of metals and steel

Environmental corrosion encompasses the degradation processes of metals and alloys that occur under the influence of environmental conditions such as the atmosphere, seawater, freshwater, soil, and chloride, acid, and alkaline environments. Metal corrosion depends not only on the type of material but also on the environment in which it operates. The same steel grade can maintain high resistance for many years in mild atmospheric conditions, but in environments containing chlorides, acids, elevated temperatures, or deposits, it can experience intense local or general corrosion. The corrosive environment determines, among other things, the stability of the stainless steel's passive layer, the rate of electrochemical reactions, and the potential for pitting, crevice, galvanic, microbial (MIC), and stress corrosion cracking (SCC). Therefore, the correct material selection requires analysis not only of the steel grade and its chemical composition but also of the actual operating conditions.

Corrosive environment and corrosion mechanism

Terms such as marine corrosion, atmospheric corrosion, soil corrosion or corrosion in water describe primarily corrosion environmentThey should not be confused with mechanisms or forms of destruction such as pitting, crevice, galvanic, or intergranular corrosion.
Several corrosion mechanisms can coexist in a single environment. For example, the presence of chlorides in seawater can promote pitting and crevice corrosion of stainless steel, dissimilar metal contact can cause galvanic corrosion, biofilm can contribute to the development of microbial corrosion (MIC), and the appropriate combination of stress, temperature, and a chloride environment can lead to stress corrosion cracking (SCC).

Environment	The most important factors	Possible forms of corrosion
Morskie	chlorides, temperature, oxygen, biofilm, sediments, flow	pitting, crevice, galvanic, MIC, erosive, SCC
Atmospheric	moisture, wetting time, salts, impurities, temperature	general, local, pitting, crevice, galvanic
Ground	humidity, salts, pH, oxygen, microorganisms, stray currents	general, local, pitting, MIC, stray current corrosion
Fresh and industrial water	chlorides, hardness, pH, oxygen, temperature, sediments, chemical additives	pitting, crevice, galvanic, MIC, erosive
Chloride	Cl⁻ concentration, temperature, pH, potential, gaps	pitting, crevice, SCC
Sour	type of acid, concentration, temperature, impurities, redox potential	general and, depending on conditions, local forms of corrosion
Alkaline	type of base, concentration, temperature, impurities	general, local, stress corrosion cracking under specific conditions

Corrosion in the marine environment

The marine environment is one of the most demanding operating environments for many structural materials. Seawater contains significant amounts of chlorides, which can destabilize the passive layer of stainless steel and promote the development of pitting and crevice corrosion.

However, corrosion conditions are not uniform throughout a marine structure. Materials permanently submerged in water behave differently, operating in the intertidal zone, and in the splash zone or coastal atmosphere. Temperature, water velocity, oxygenation, biofilm, sediments, and contact with other metals also play a role.
In more demanding marine applications, instead of standard austenitic steels, duplex and superduplex steels, high-alloy austenitic steels of the 6Mo type and appropriately selected nickel alloys are used.

Atmospheric corrosion

Atmospheric corrosion occurs on metal surfaces exposed to the atmosphere and the moisture layer that forms on them. The intensity of the process depends primarily on the surface wetting time, humidity, temperature, and the presence of salts and contaminants.
A rural atmosphere with low levels of pollution is typically much less aggressive than an industrial or coastal atmosphere. Near the sea, salt spray contributes chlorides, while in industrial environments, sulfur compounds and other pollutants may play an additional role.
Dust, salt, and other contaminants can retain moisture and create localized conditions conducive to corrosion. Therefore, the condition of the surface, its design that allows for natural flushing, and periodic cleaning can significantly impact the durability of components.

Ground corrosion

The ground is a highly complex and heterogeneous environment. Corrosion of underground structures is influenced by factors such as humidity, soil chemical composition, chloride and sulfate content, pH, electrical conductivity, oxygen availability, microbial activity, and the presence of stray currents.
Soil heterogeneity can lead to local differences in electrochemical conditions on the surface of the same component. Particular attention should be paid to pipelines, tanks, underground structures, and transition points between different soil types.
For buried structures, material selection is just one element of protection. Depending on the structure and conditions, appropriate coatings, cathodic protection, and stray current control are also used.

Corrosion in fresh and industrial water

The term "water" does not describe a single corrosive environment. Drinking water, river water, well water, cooling water, boiler water, and process water can vary significantly in chemical composition and aggressiveness.
The most important parameters include chloride content, pH, temperature, oxygenation, hardness, conductivity, the presence of sediments, and the presence of inhibitors, biocides, and other technological additives. Flow conditions are also important. Stagnation can promote the formation of sediments, biofilm, and local oxygenation differences, while very high flow rates under certain conditions can increase the risk of erosion and erosive corrosion.
In the case of stainless steels, particular attention should be paid to chlorides. As their concentration and temperature increase, the risk of localized corrosion increases, and the material's resistance to pitting and crevice corrosion, as well as parameters such as PREN, CPT, and CCT, become increasingly important.

Corrosion in chloride environments

Chlorides are among the most important factors limiting the use of stainless steels. They can locally destabilize the passive layer and lead to the initiation of pitting and crevice corrosion. Under appropriate conditions, chlorides can also contribute to stress corrosion cracking (SCC).
Risk cannot be assessed solely based on chloride concentration. Temperature, pH, electrochemical potential, the presence of crevices and sediments, and surface condition are all important factors. Therefore, the permissible chloride concentration for a given species is not a single, universal value applicable under all conditions.
The resistance of stainless steels to localized corrosion in chloride environments increases primarily with the appropriate content of chromium, molybdenum, and nitrogen. The PREN index is used for initial material comparison, while laboratory resistance to localized corrosion initiation can be additionally characterized using CPT and CCT.

Corrosion in acidic environments

Acidic environments can cause intense general corrosion if the material's passive layer is not stable under the given conditions. However, there is no single, universal "acid resistance." Material behavior depends on the type of acid, its concentration, temperature, the presence of oxygen and other oxidants, and contaminants, including chloride ions.
Sulfuric, hydrochloric, phosphoric, nitric, acetic, and formic acids create different corrosive environments. A material highly resistant to one acid may not necessarily exhibit similar resistance to another. Hydrochloric acid is particularly aggressive to many stainless steels because it combines an acidic environment with high chloride activity.
When selecting a material for use in acid environments, it is therefore necessary to consider the specific system: material – acid – concentration – temperature – contaminants. In practice, isocorrosion diagrams are used, among other things, to determine the corrosion rate depending on the temperature and concentration of the given medium.

Corrosion in alkaline environments

Many stainless steels exhibit good resistance to moderate alkaline environments, but highly concentrated alkalis, especially at elevated temperatures, can be aggressive. The type of alkali, its concentration, temperature, and the presence of additional impurities are important factors.

An example is sodium hydroxide (NaOH), used in many chemical processes. Depending on the concentration and temperature, an improperly selected material can be susceptible to accelerated corrosion or certain forms of stress-corrosion cracking. As with acidic environments, the resistance assessment should be based on the specific process conditions, not simply the term "alkaline environment."

Corrosion in gases and exhaust gases

Gaseous environments and exhaust gases can cause corrosion of metals both in the presence of moisture and condensate, and at elevated temperatures. The aggressiveness of the environment is determined by factors such as temperature, the content of water vapor, oxygen, sulfur compounds, chlorine, and other gaseous components, and the potential for their condensation on the metal surface. Areas where the surface temperature drops below the dew point and condensate containing dissolved combustion products forms can be particularly dangerous. Depending on the conditions, general, local, condensation, and high-temperature corrosion can occur. Material selection requires consideration of both the gas composition, the operating temperature, and the component's surface temperature.

Corrosion in sewage and sewer systems

Sewage and sewage systems create a complex corrosive environment, where chlorides, sulfates, sulfides, organic compounds, variable pH, sediments, and microorganisms can coexist. Of particular importance is the potential for microbial corrosion (MIC) and the formation of hydrogen sulfide (H₂S) under anaerobic conditions. In the spaces above the sewage, H₂S can be oxidized by biological processes to compounds that create a highly acidic environment, which is a particularly significant problem for concrete and metal components of the sewage infrastructure. Stagnant media, sediments, biofilm, and differences in oxygenation can further promote local corrosion. Material selection should take into account the actual wastewater composition, temperature, pH, chlorides, sulfur compounds, flow conditions, and microbiological activity.

Corrosion in chemical and petrochemical environments

Chemical and petrochemical installations are among the most diverse corrosive environments. Materials may come into contact with acids, bases, chlorides, hydrogen sulfide (H₂S), carbon dioxide (CO₂), sulfur compounds, solvents, process water, and mixtures of many chemicals. These conditions can lead to general and localized corrosion, stress corrosion cracking (SCC), corrosion under deposits, and other process-dependent mechanisms. Material resistance is determined not only by the chemical composition of the medium but also by its concentration, temperature, pressure, water content, flow, contaminants, and stresses within the structure. In more aggressive environments, duplex and superduplex steels, high-alloy austenitic steels, and appropriately selected nickel alloys are used in addition to standard stainless steels. Material selection should always be based on actual process conditions, as the general terms "chemical environment" or "petrochemical" do not allow for a single assessment of corrosion risk.

The most important factors determining the aggressiveness of the environment

Regardless of the type of environment, there are several parameters that have a particularly strong influence on the course of corrosion processes:

temperature – an increase in temperature often accelerates electrochemical reactions and may reduce resistance to local corrosion,
pH – influences the stability of the passive layer and the course of electrochemical reactions,
chlorides and other halides – are among the most important factors initiating local corrosion of stainless steels,
oxygen and other oxidants – affect the electrochemical potential and the ability to maintain a passive layer,
medium flow – may improve mass transfer, but at high speeds and in the presence of particles it may also increase the risk of erosion corrosion,
sediments and crevices – create local environments that differ from the main medium and promote crevice corrosion,
microorganisms – biofilm and microbiological activity may influence local electrochemical conditions and MIC development,
contact of different metals – in the presence of an electrolyte it can lead to the formation of a galvanic cell,
stress – in combination with a certain environment can lead to stress corrosion cracking (SCC).

Material selection for corrosive environments

There's no single stainless steel grade that's best for all environments. Popular 304L and 316L stainless steels are suitable for a wide range of applications, but their resistance may be inadequate in warm chloride environments, seawater, strong acids, or structures particularly susceptible to crevice corrosion.

As environmental aggressiveness increases, high-alloy steels such as 904L, duplex and superduplex steels, 6Mo austenitic steels such as 1.4547 and 1.4529, and appropriately selected nickel alloys are used. However, a higher alloying level does not automatically mean that a material will be the best solution in every environment.

The selection should take into account the chemical composition of the medium, temperature, pH, chloride concentration, flow conditions, presence of cracks and sediments, stresses, construction technology and expected operating time.

The environment and the material must be analyzed together

Corrosion resistance is not a property that can be adequately described by a single term such as "corrosion-resistant steel." A material is either resistant or susceptible to corrosion in relation to specific environments and operating conditions.

Therefore, when designing an installation, it is necessary to analyze the entire system: material – environment – temperature – design – construction method – operating conditions . Only this approach allows for a proper assessment of corrosion risk and the selection of materials with the required durability.

Corrosion of metals in various environments

Individual corrosive environments require separate analysis. The following papers provide detailed descriptions of corrosion mechanisms, risk factors, and material selection principles for the following conditions:

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

The operating environment is one of the fundamental factors determining the durability of metals and alloys. Atmosphere, soil, seawater, industrial water, chlorides, acids, and bases can create completely different corrosive conditions and trigger different material degradation mechanisms.
Therefore, knowledge of the material type alone is not sufficient to assess its durability. Proper selection requires determining the actual operating conditions and identifying the corrosion mechanisms that may occur. Only a combination of knowledge about the material and the environment allows for a proper assessment of the corrosion resistance of a structure.

Types of corrosion of steel and metals

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