Stainless Steel - Corrosion Resistance

Topics Covered

Introduction
General Corrosion
Pitting Corrosion
    Pitting Resistance Equivalent number (PRE)
Crevice Corrosion
Stress Corrosion Cracking (SCC)
Sulfide Stress Corrosion Cracking (SSC)
    Stress Level
    Environment
    Temperature
Intergranular Corrosion
Galvanic Corrosion
Contact Corrosion
Passivation and Pickling

Introduction

Corrosion resistance is one of the main advantages of the application of stainless steels. However, in some cases, stainless steels suffer certain types of corrosion and therefore precautions should be taken in choosing a suitable steel grade for any application. Based on the applications, corrosion results in a number of problems as follows:

• Pitting or corrosion products can deteriorate the appearance of a product by affecting the decorative surface finish.
• Corrosion leads to the formation of rust or scale, which contaminates the material. This usually occurs in food processing systems.
• Corrosion reduces the cross section of structural members, causing loss of strength of the structure.
• Perforation of pipes or tanks, leading to leakage of gases or fluids.
• Stainless steels are subjected to one of the following corrosion phenomena:
• Contact corrosion
• Galvanic corrosion
• Intergranular corrosion
• Stress corrosion cracking
• Sulfide stress corrosion cracking
• Crevice corrosion
• Pitting corrosion
• General corrosion

General Corrosion

Corrosion is a process of uniform removal of material through dissolution - e.g., while using stainless steel in a chemical plant containing strong acids. In this instance, design and life span of the material is based on published data.

Description on the gradual removal of metal can be obtained from the published data list. Further, the list contains tables of corrosion resistance to various chemicals provided by several organizations, a large collection of charts, and technical papers published by stainless steel suppliers and manufacturers.

Pitting Corrosion

In some cases - which involve acidic conditions, moderately high temperatures and high chloride concentration - localized corrosion takes place causing perforation of fittings and pipes. This type of corrosion can penetrate the cross section of the material. Steel grades containing high amounts of chromium, nitrogen and molybdenum have high resistance to pitting corrosion.

Pitting Resistance Equivalent number (PRE)

The pitting resistance equivalent number (PRE) indicates the rate of the pitting resistance of stainless steels. The PRE can be calculated using the following equation:

PRE = %Cr + 3.3 x %Mo + 16 x %N

Pitting corrosion needs to be taken seriously due to the fact that when a pit is formed on a surface of the steel, it will continue to develop in the surrounding area.

The tendency for a particular steel to be attacked by pitting corrosion can be evaluated in the laboratory. A number of standard tests have been devised - the most common of which is that given in ASTM G48. A graph can be drawn giving the temperature at which pitting corrosion is likely to occur, as shown in Figure 1.

The extent to which a particular steel can be affected by pitting corrosion can be determined in the laboratory. Although a number of standard test procedures have been devised, the most common procedure is given in ASTM G48. Figure 1 shows a graph providing the possible temperatures at which pitting corrosion can occur.

Figure 1 - illustrates the temperature at which pitting corrosion is likely to occur

This graph is based on a conventional ferric chloride laboratory test.

Crevice Corrosion

A protective oxide layer present on the surface of stainless steel determines its corrosion resistance. However this oxide layer is vulnerable to damage under certain conditions, such as in reducing acids, or in certain combustion mechanism having a reduced atmosphere. Certain areas of the component, such as sharp re-entrant corners or overlapping surfaces, have the tendency to form crevices that promote corrosion.

Crevices need to be of sufficient width to allow entry of the corrodent, and at the same time narrow to ensure that corrodent remains in its place. Accordingly crevice corrosion generally occurs in gaps of a few micrometers width, and it does not occur in groves that allow circulation of the corrodent. This problem can be resolved by avoiding the formation of crevices, or by keeping them open. Crevice corrosion is similar to pitting corrosion, and alloys resistant to one type of corrosion will be resistant to the other.. Crevice corrosion occurs at lower temperatures when compared to pitting corrosion.

Stress Corrosion Cracking (SCC)

Stainless steels can be subjected to rapid and very serious corrosion under the conditions of stress and corrosive environments. The stresses can be tensile, resulting from the application of loads, or residual stresses from the method of fabrication. The most damaging environment is the chloride environment with high temperatures. Austenitic group of steels is more commonly affected by corrosion due to chlorides. The duplex stainless steels, on the other hand, are more resistant to stress corrosion cracking (SCC) than austenitic grades. The ferritic grades are free of SCC.

It has been found that SCC resistance can be improved in some cases with the application of compressive stress to the material at risk. This can be performed by shot peening the material surface, or by annealing the surface to relieve tensile stresses.

Under certain conditions chloride stress corrosion failures occur at lower temperatures of 30 to 40°C. Such failures take place in the warm, moist environment above indoor chlorinated swimming pools, where stainless steel fixtures are often used to suspend items such as ventilation ducting. Failures due to stress corrosion at higher temperatures with chloride levels as low as 10 ppm have also been reported.

Sulfide Stress Corrosion Cracking (SSC)

Material resistance to sulfide stress corrosion cracking is of prime importance for oil and gas industrial applications. Sulfide stress corrosion cracking occurs due to conjoint action of chloride and hydrogen sulfide, presence of tensile stresses and non-linear relationship with temperature.

The three main factors of sulfide stress corrosion cracking are stress level, environment and temperature.

Stress Level

In some cases a threshold stress can be observed in each material. Some reports show a continuous reduction threshold stress with increasing concentration of H2S. According to NACE specification MR0175 for sulfide environments, maximum hardness of common austenitic grades is limited to 22HRC to avoid sulfide stress corrosion cracking.

Environment

Hydrogen sulfide, chloride and pH are the principal agents of sulfide stress corrosion cracking.

Temperature

The concentration of chloride increases with increasing temperatures, while the effect of hydrogen decreases - owing to its increased mobility in the ferrite matrix. As a result, maximum susceptibility can be observed in the range of 60-100°C. Heat tint at welds, presence of cold work, surface condition and amount of ferrite are some of the observed secondary factors that promote sulfide stress corrosion cracking.

Intergranular Corrosion

Intergranular corrosion is a process of rapid and localized corrosion, resulting from a defective microstructure such as carbide precipitation. Upon exposing austenitic steels to temperatures of 425 to 850°C for a time period, or heating the steel to high temperatures followed by slow cooling, chromium carbide particles are formed along the grain boundaries of the steel. This deteriorates the surrounding environment of the metal surface, thereby reducing the corrosion resistance of the steel.  Steel is termed as “sensitized” in this condition.

Carbide precipitation generally depends on the time, temperature and carbon content. The most critical temperature range is about 700°C, at which 0.06% carbon steels tend to precipitate carbides within 2 min. Steels subjected to carbide precipitation can be reclaimed by heating them above 1000°C followed by water quenching, to retain chromium and carbon in solution thereby preventing the formation of carbides. However, this treatment is not suitable for welded or heated structures, and hence special steel grades have to be designed to avoid this problem. Such special grades are grade 321 stabilized with titanium and grade 347 stabilized with niobium.

Use of extra low carbon grades, such as grades 304L and 316L, is another method to overcome intergranular corrosion. These grades have extremely low carbon levels and hence they have more resistance to carbide precipitation.

Sensitized austenitic stainless steels are free of intergranular corrosion in most of the environments, such as alkaline salt solution, glacial acetic acid at room temperature, and fresh water. It should also be noted that high temperature strength of stainless steels remains unaffected by grain boundary carbides. Grades used for high temperature applications often have high carbon contents, which in turn enhances their creep resistance and high temperature strength.

Galvanic Corrosion

Galvanic corrosion is an electrochemical process, which involves flow of electric current. It occurs due to the galvanic effect caused by the contact of two dissimilar metals in an electrically conductive liquid. The following are the three major conditions for galvanic corrosion:

• Surfaces of the metal should be bridged by an electrolyte.
• Two dissimilar metals must be in electrical contact with each other.
• Two metals should be separated from each other on the galvanic series.

Figure 2 - illustrates galvanic series of metals in flowing seawater

Avoiding mixed metal fabrications can prevent galvanic corrosion. However, this is not practically possible and, hence, cutting the electrical contact between metals is usually recommended for the prevention of galvanic corrosion. Electrical contact can be cut by using rubber or plastic washers or sleeves, or by using protective hoods or draining the electrolyte.

Galvanic corrosion effect is also based on the areas of dissimilar metals. The corrosive effect may be negligible if the area of less noble material is larger than that of more noble material. By contrast, contact of large area noble metal with small area noble metal will increase the rate of galvanic corrosion. For instance, aluminum sheets are generally fastened using stainless steel screws, but aluminum screws fastened to a large area of stainless steel will tend to corrode at a rapid rate.

Contact Corrosion

This type of corrosion combines galvanic, crevice and pitting corrosion, and it occurs at places where small foreign particles such as carbon steels are left on the surface of stainless steel. Contact corrosion occurs as a galvanic cell is anodic in nature and hence it tends to rapidly corrode resulting in the formation of pitting corrosion. The most common reason for this type of corrosion is debris formation due to grinding of carbon steel, or use of tools contaminated with carbon steel.

It is essential that all warehouses and workshops storing or handling stainless steels must take precautions to prevent this potential problem. Contact between stainless steels and carbon steel storage racks can be prevented using carpet strips, wood or protective plastic. Crane lifting fixtures and forklift tynes are other handling instruments that need to be protected. Clean fabric slings are often considered as a useful alternative.

Passivation and Pickling

Pickling using a mixture of nitric and hydrofluoric acids, or passivation with dilute nitric acid, can be used to remove contamination due to carbon steel debris on the surface of stainless steels.