Revisiting the Selection of Stainless Steel in Water and Wastewater Treatment Environments: Part 1

Metals such as bronze, copper and iron have been used for thousands of years by man for both peaceful and non-peaceful purposes. One of the most useful purposes for metal is the production of steel.

Steel is defined as iron with a small amount of carbon (usually less than 0.5 percent) and other elements in small quantities. Grades of steel are determined by the amounts of non-ferrous components such as carbon, chromium, nickel, molybdenum, copper and titanium present. Depending on the specific content and manufacturing process, steels can be formed for a range of properties including the following.

•               Overcome brittleness

•               Cohesive strength

•               Corrosion resistance

•               Tensile strength

•               Fatigue protection

•               Ductility

•               Hardness

•               Malleability

•               Shear strength

•               Torsional strength

•               Electrical conductivity

•               Thermal conductivity

•               Thermal expansion (or resistance)

•               Magnetic properties

•               Heat treatability

The selection of the best material for a specific application is complicated but very important to the success of the project. Steel has many uses and, therefore, is one of the largest industries worldwide today. Advantages of steel include

•               High strength,

•               Uniformity,

•               Elasticity under high stress,

•               Ductility,

•               Adaptations for connections and manufacture,

•               Fatigue strength and toughness,

•               Resistance to heat deformation, and

•               No ultraviolet light degradation.

Disadvantages of steel use include

•               Maintenance costs due to susceptibility to corrosion by air, water and microbiological elements, and

•               Fatigue if subjected to stress reversals (pressure on, then off).

The most critical drawback in using steel is corrosion. Common types of corrosion in steel include galvanic, dezincification, pitting, crevice, intergranular, stress, cracking, erosion and microbiologically induced corrosion (MIC). A number of solutions have been pursued to address problems with corrosion. The development of stainless steel is one answer.

There are more than 40,000 steel grades manufactured, depending on heat treatment, chemical composition and application. Steel can be divided into several groups: carbon, alloy, tool, high temperature, stainless, super-stainless, and high alloy with chromium, nickel and cobalt alloys included.1 Stainless steel typically is chosen over painted carbon steel and galvanized steel to reduce the maintenance and replacement costs associated with less corrosion-resistant metals.2

Economical production of stainless steel has occurred since the middle of the nineteenth century. The chemistry, mineralogy and metallurgy of steel is very important in the proper manufacture of steel. The American Society for Testing Materials (ASTM) and American Petroleum Institute (API) specify exact maximum percentages of carbon, manganese, silicon, etc. in their steel standards. Of concern here are the grades of stainless steel that are commonly specified for water treatment and membrane systems. The commonly available commercial types of stainless steel include

•               Martensitic Grades (Cr 12–17 percent, high carbon),

•               Ferritic Grades (Cr 18–30 percent, lower carbon),

•               Austenitic Grades (304 and 316, including low carbon forms (L), Cr 18–30 percent, 2–6 percent Ni),

•               Duplex Grades (higher corrosion resistance than Austenitic grades),

•               Precipitation Hardening Grades,

•               High Performance Grades (6 Mo), and

•               Nickel Rich Grades.

Duplex and austenitic are the two grades most commonly used in water treatment applications.3 The difference between them is the crystalline structure of the metal (face or block centered—Figure 1). Nickel is an austenitic stabilizer. This means the addition of nickel to iron-based alloys promotes a change in the crystal structure of stainless steel from body centered to face-centered cubics. This change improves corrosion resistance.

Stainless Steel Used in Membrane Facilities

Types 304, 304L, 316 and 316L are austenitic stainless steel with 18 percent chromium and 8 percent nickel. Types 304L and 316L are low carbon steels designed to be resistant to intergranular corrosion after welding without requiring further heat treatment. Removal of the oxide film (heat tint) and the thin, chromium-depleted layer just beneath the heat tint that occurs in areas adjacent to the welds when welding is performed is required. Unless removed, these areas of the pipe will corrode very easily. Removal of the heat tint and the thin, chromium depleted layer just underneath will restore the integrity of the stainless steel.

The duplex stainless steels are a family that combines good corrosion resistance with high strength and ease of fabrication. The early grades of duplex stainless steels were developed more than 60 years ago in Sweden. Duplex stainless steels have the physical properties of austenitic and ferritic stainless steels but tend to be closer to ferritic and carbon steel. They have an improved resistance to chloride pitting and crevice corrosion because of the combination of molybdenum, chromium and nitrogen.4 Duplex steel was developed to reduce stress corrosion cracking problems that existed in the early high-carbon austenitic stainless steels.4 Duplex stainless steel can have a range of ferrite to austenite from 30–70 percent but typically have a microstructure of equal proportions of ferrite and austenite. A stable duplex structure must form. Development of inter-metallic phases at high temperatures must be avoided during the manufacturing process to ensure corrosion resistance. When formed in large quantities and under conditions where the chromium depleted areas do not have time to repair themselves during annealing, the precipitates may adversely affect corrosion resistance.

A minimum of 10.5 percent chromium is necessary to form a passive film that is sufficient to protect stainless steel against mild atmospheric corrosion. Corrosion resistance increases with increasing chromium content. Chromium is a ferrite-former, meaning that the addition of chromium stabilizes the body-centered cubic structure of iron. Iron has two unpaired electrons in its outer orbital. Chromium shares electrons to complete the pairing (and create a stable form) of electrons. Higher chromium content requires more nickel to form an austenitic structure. The first generation duplex stainless steel provided good performance but had limitations in the welded areas. The welds have a low toughness as a result of excessive ferrite and lower corrosion resistance than the base metal so such steel use was restricted to unwelded applications.4

Second generation duplex stainless steel usually is comprised of at least 22 percent chromium. Molybdenum is added to support the chromium in providing corrosion resistance. When the chromium content is at least 18 percent, the addition of molybdenum makes the steel much more corrosion resistant to chloride pitting. Molybdenum is a ferrite-former but also has the tendency to form detrimental intermetallic phases. Therefore, it is restricted to less than 7.5 percent. Ferritic stainless steels contain little or no nickel, while duplex stainless steels contain at least 4 percent nickel.4

In 1968, argon oxygen decarburization was developed, making the addition of nitrogen as an alloying element possible. This process makes field welds more corrosion resistant. It also reduces the rate at which the detrimental inter-metallic phases form. Since then, the super duplex grades have been developed to maximize corrosion resistance.4

The use of nitrogen as an alloying element in stainless steel allows the chromium nitrides to be present on ferrite-ferrite grain boundaries, on austenite-ferrite boundaries and in heat-affected zones of welds. Nitrogen also

•               increases resistance to pitting and crevice corrosion while increasing strength and toughness;

•               raises the temperature at which austenite is formed and allows for accelerated cooling; and

•               reduces the rate at which detrimental inter-metallic phases form.

Table 1 outlines the chemical composition of various grades of stainless steel. Experience has shown that for optimum corrosion resistance, and to avoid inter-metallic phases, the chromium, nitrogen and molybdenum should be kept to 22–23 percent, 0.14–0.2 percent and 2.5–3.5 percent, respectively. Duplex stainless steels have much higher strength than austenitic grades while exhibiting good ductility and toughness. All duplex stainless steel have chloride stress corrosion cracking resistance that is much greater than the 300 series austenitics. Super duplex stainless steel has 25–26 percent chromium, with higher levels of nitrogen and molybdenum.4

Problems

Stainless steel has provided good service in many applications. However, use of the appropriate grade is important to the ultimate success of the steel. Corrosion should be an expected by-product of any steel use, including stainless steel. The question is, how much corrosion is acceptable in the intended application?

Corrosion is the deterioration of a metal or alloy as the result of exposure to and reaction with its environment. It is an electrochemical process, like a battery, with an anode and a cathode reaction, and ions migrating through an external circuit, even in single metal steel. Positive electrolytes migrate through the liquid electrolyte toward the cathode, whereas negative ions are attracted to the anode. The corrosion reaction cannot occur without a simultaneous cathodic reaction. The typical reactions are

Fe0 Æ Fe2+ + 2e–

2H+ + 2e– Æ H2O

2H+ + 1/2O2 + 2e– Æ H2O

H2O + 1/2O2 + 2e– Æ 2OH–

The first equation is anodic and the rest are cathodic. Corrosion of stainless steels is dependent almost entirely on chlorides, sulfates and dissolved oxygen content. Chlorides also can be adsorbed or chemisorbed in films on the metal surfaces (silicates, aluminates, iron oxides and calcareous deposits). In potable water, which may be soft (Hardness = 0–75 ppm), moderate (Hardness = 75–150 ppm) or hard (Hardness = 150–300 ppm), the calcareous deposits can affect how stainless surfaces react to chlorides, regardless of bulk water chemistry.3 There are several corrosion types that are common to various grades of stainless steel.

•               Crevice corrosion is most likely to occur on the outside of the pipes used for sludge, Vitaulic couplings in membrane facilities and buried pipe in damp soils. The tightness of the crevice will affect the speed with which crevice corrosion occurs.3 Corrosion occurs in tight, stationary crevices in the presence of oxygen and chlorides greater than 1,000 mg/L. The initial step in crevice corrosion is depletion of oxygen in the crevice solution. The dominant mechanism is the presence of bacteria that will accelerate initiation of localized attack.5 In membrane plants, crevice corrosion often will be noted in the J-bends at the Vitaulic couplings. Type 304 will pit 1/4* in 2 years. Type 316 is half as likely to have crevice corrosion as Type 304, but is still considered very susceptible to crevice corrosion. Crevice gaps smaller than 0.4 µm will initiate crevice corrosion.3 If the sulfate/chloride ratio is 3 or better, there is less chance of crevice attack, but this ratio normally is not present in water treatment applications.3 Second generation Duplex grades including Zeron 100 and 2507 show significantly better resistance to chloride pitting and crevice corrosion.3

•               Chloride pitting—When the chlorides exceed 50 mg/L, 304 stainless steel should be used instead of carbon steels. Where the chlorides exceed 200 mg/L, 316L would be preferred.2 Manufacturers often recommend a chloride ceiling for Type 316L of 500 mg/L. This level is below all brackish water sources. Chloride induced pitting is another problem resolved by metal selection. Inappropriate metal selection can lead to serious damage to the pipe, as well as microbial colonization.

•               Pinhole leaks—Often with microbiological components, pinhole leaks will occur as a result of the field welding process when the heat-affected zone is not properly removed in lower grade stainless steels. Heat tint removal is a tedious field process, but failures can occur quickly. At the pinholes, slimy spots that appear to be microbiological in nature, often are found. MIC is a phenomenon that occurs as a result of prolonged exposure of the pipe wall to raw water that contains microbes. These growths are permanent in nature and must be controlled. Figure 2 shows critical pitting and critical crevice corrosion temperatures of various grades of stainless steel. 316L is at the lower range, well within the expected temperature of the raw and finished water supplies to most treatment facilities.

•               External corrosion in the form of rust stains often is found in multiple locations on the exterior of the pipe. Bands of rust streaks on the exterior of the pipes generally result from the inappropriate use of an unpadded steel sling on the crane to lift the pipes into place. They generally are cosmetic in nature.

•               Stagnant water always presents a risk for the occurrence of corrosion and microbiological activity. The ability to completely drain the pipeline during periods of non-use is important as static water encourages microbial activity.6 A minimum velocity of 3 fps is required to prevent corrosion.5 Conversely, there is no adverse effect of velocity. Stainless steel performs better at high velocities that replenish dissolved oxygen and minimize the possibility of deposits unless the water has a severe scaling potential. Piping using austenitic stainless steel has withstood more that 100 fps in seawater with no evidence of erosion corrosion.3

•               Heat tints near welded areas have been shown to make the welds susceptible to biocorrosion.5 Mill pickling is used to control surface corrosion on stainless steel pipe. In the pickling process, scale is removed from prior annealing treatments that can cause a depleted chromium layer at the annealing or inclusion points.2 This is the same problem that occurs with field welds. The pickling process provides a passivated, corrosion resistant surface as a result of the thin oxide film.2

•               Operations issues such as scaling, over chlorination, poor fabrication and MIC can initiate corrosion in 316L stainless steel. 3

The appropriate choice of stainless steel grade and accompanying appurtenances, plus the appropriate construction and treatment techniques should allow the metal to provide a reasonably long life by resolving most of these corrosion issues. However, the microbiological problem can only be controlled, and where biological problems can reasonably be anticipated such as wells and membrane plants higher grade stainless steels should be specified when steel strength is required.

Microbiological Concerns

Due to the lack of corrosion products on its surface, corrosion resistant stainless steel can provide a good substratum for biofouling. Biofilm formation and structure are more easily observed by electron microscopy.5 When the stainless steel surface is passivated, the biological mechanisms become dominant.5 Pitting and crevice corrosion can be the immediate result of stainless steel in contact with aqueous solutions containing microorganisms. Microbiologically induced corrosion is not in itself a corrosion form; instead, corrosion is initiated and/or aggravated by microorganisms. The involvement of microorganisms in metal corrosion has led to the question of how the biological agents affect the corrosion process and how they may modify the electrochemical nature of the reaction.5 Both aerobic and anaerobic bacteria can cause corrosion in stainless steel.3

It should not be a surprise to anyone in the water industry that organisms that can cause microbiologically induced corrosion are present in raw water supplies, regardless of the source. The average dimensions of bacteria, fungi and yeast involved in corrosion processes are small (measured in micrometers). This allows the microorganisms to colonize inaccessible areas such as the interior of crevices and pits where they can avoid the shear of fluid velocity. The small size also facilitates the rapid and easy dispersion of microbial cells.5

The typical agents for microbiological fouling include iron bacteria, sulfate-reducing and slime-producing organisms, although fungi and algae that typically occur only in surface water sources also may exist. Some of these organisms are pathogens and/or opportunistic pathogens yet do not fall under the group of bacterial species most commonly analyzed (coliform bacteria and the subset of fecal coliform bacteria). A problem occurs when enough of these orgnisms come together and form a slime matrix or biofilm.

Active participation of microorganisms in the corrosion process introduces several inherent features, the most relevant of which is the formation of biofilms at the metal/solution interface.5 There are six steps in the formation of a biofilm.

1.             Free floating cells are transported in the bulk water.

2.             Cell absorption on the surface (that can be controlled by fluid shear forces).

3.             Permanent attachment after a critical residence time.

4.             Adsorption of nutrients by cells.

5.             Growth of biofilms.

6.             Shedding of biofilm to colonize other parts of the pipe.5

Generally, a two-stage sorption model is suggested for modeling bacterial attachment. In such models, the bacteria are rapidly attracted to the surface, but held only weakly. While the first stage is reversible, the attachment in the second phase is irreversible. The second phase includes synthesis of extracellular polymetric glue, a mechanism of cell metabolism. However, the study of pseudomonas aeruginosa has indicated that the second stage occurs almost immediately.7

Biofilms affect the interaction between metal surfaces and the environment, not only in biodeterioration processes such as corrosion but also in several biological processes applied to materials recovery and handling. These adhesion processes are mediated by extracellular polymetric substances of a polysaccharide nature.5 The bacteria in biofilms bind together in a sticky web of tangled polysaccharide fibers that connect cells and anchor them to the surface and each other. If the cells of a different species form colonies in the same area, an aggregate will form.5 Within this aggregate, aerobic and anaerobic bacteria thrive along side each other sharing water passageways that deliver nutrients and remove wastes like a circulatory system and may interact on an intimate metabolic level.8

The advantages to forming a microbial consortium include enzymatic interactions, concentration and exchange of nutrients, resistance to velocity currents and maintenance of a stable biofilm.5 Chemistry of the water can facilitate microbiological colonization and reduce the time for the permanent attachment of the biofilm.5 The mass of the extracellular gel may exceed the mass of the microorganisms, which leads to a macroscopic appearance of the biofilm.5 Direct measurement of the biofilms is restricted by the thickness of the biofilm, the diffusion limitations of concentration profiles and heterogeneous nature of the biofilm. Biofilms are 95 percent water and are very porous.

The synergistic effects of microorganisms such as growth rates and metabolisms and high surface to volume ratios allow them to be active in the pipe environment, resulting in corrosion rates 1,000 to 100,000 times greater than when these factors are absent.5 The concentration of dissolved oxygen decreases to zero at a distance of approximately 180 µm from the metal surface. The depleted oxygen layer beneath the biofilm creates an environment suitable for the growth of harmful, anaerobic bacteria and creates differential anoxic cells that lead to pipe corrosion. The implication is that the bottom of the biofilm is anaerobic, and corrosive sulfate reducing bacteria can proliferate regardless of dissolved oxygen in the flowing water. Acid- (acetic acid) producing organisms may accumulate enough acid within the biofilm to reduce the pH dramatically, thereby liberating metallic ions.5

Bacteria that can attach to inanimate surfaces are important not only as agents of biofouling but also as contamination sources for any material that affects the surface.7 Bacteria can cause corrosion directly by their metabolic processes, forming specific chemical species such as ammonia, hydrogen sulfide, dissolved sulfate, ferric or manganic chlorides.3 Anaerobic bacteria in biofilms reduce sulfur to hydrogen sulfide, increasing the susceptibility to pitting, while aerobic bacteria can corrode metals by oxidation. Biofilms act to protect the bacteria from the shearing effect of turbulent flow. However, periodic sloughing occurs when the biofilm becomes too thick, releasing bacteria into the treatment systems.