This post will discuss two forms of corrosion totally diferent from each other, regarding their causes. Intergranular corrosion or attack happens when the material is corroded selectively in the grain boundaries and neighbour region, in a combination of susceptible material with a determined environment. On the other hand, microbial corrosion is associated to the metabolism of living organisms, such as bacteria and fungi that can release chemicals which raise the corrosion rate.

INTERGRANULAR CORROSION

The higher energy state of the grain boundary turns this region of the material slightly more reactive than the bulk metal (Fontana, 1986). However, the combination of a susceptible microstructure and an active environment might result in intergranular corrosion, i.e. corrosion localised in the grain boundaries (Revie, 2000).

A classical example of a material which microstructure may facilitate the intergranular corrosion is the austenitic stainless steel. The simplified microstructure of this material consist of a polycrystalline austenite matrix which dissolves the alloying elements (chromium, carbon, nickel, molybdenum, etc.), with grain boundaries between the crystals. Nevertheless, the precipitation of chromium or chromium/molybdenum carbides at grain boundaries causes a depletion of these elements in the regions adjacent to the grain boundaries, as shown in the Figure 1 (Talbot and Talbot, 1997). Considering the facts that these elements are not available in the matrix increasing the corrosion resistance, the further corrosion occur selectively in the grain boundaries (Shreir, Jarman and Burstein, 1993).

Figure 1 Schematic representation of the carbide precipitation at grain boundaries in austenitic stainless steel. Source: http://www.hindawi.com/journals/ijelc/2013/970835/fig10/

Classes of austenitic stainless steels such as AISI 304 and AISI 316 are supplied by the steelmaker with the carbon retained in the matrix, but the subsequent reheating and slow cooling through a critical temperature range lead to the precipitation of carbides in the grain boundaries, phenomenon also known as sensitization (Talbot and Talbot, 1997). Figure 2 represents a temperature-time diagram for sensitization, also demonstrating that the carbon content can shift the precipitation to occur in shorter times.

Figure 2 Temperature-time-sensitization diagrams for three Cr-Ni steels with different carbon content solution treated at 1050°C. Source: Shreir, Jarman and Burnstein, 1994.

When the heating source in associated to a welding procedure, the heat-affected zone (HAZ) of the component can be submitted to the cycle of heating and slow cooling, therefore resulting in a sensitization of this zone, which is attacked intergranularly, also denominated as weld decay (Fontana, 1986).

One strategy to avoid the sensitization-associated problems in austenitic stainless steels is easily concluded from Figure 2: reducing the carbon content in the alloy will increase the time required in the temperature range for the carbide precipitation or, in other words, will reduce the amount of carbon available for carbides. This strategy is successfully used in the L-grade AISI stainless steels, with L meaning low carbon content, e.g. AISI 316-L (Roberge, 1999).

Another strategy is to add elements in the steel that form carbides prior to chromium and molybdenum, such as titanium, niobium and tantalum, in the stabilised class of austenitic stainless steel, e.g. AISI 321 and 347 (Talbot and Talbot, 1997). However, even the stabilised steel can be attacked intergranularly, as the titanium and niobium added might not combine with carbon, because they dissolve in the high temperature that the region adjacent to the weld is subjected to. In these occasions, the knife-line attack (KLA) occur (Fontana, 1986). The conditions and consequences are very similar between knife-line attack and weld decay, as they appear in welded joints and lead to intergranular corrosion, but the differences are that the knife-line attack occurs in a narrow band immediately adjacent to the weld and only arise in stabilised steels (Shreir, Jarman and Burstein, 1993).

A further strategy to avoid sensitization that does not require change in the composition of the stainless steel is the control of the heat input in the weld process, minimizing the effect of thermal cycle on carbide precipitation (Roberge, 1999).

Intergranular corrosion can occur in some high-strength aluminum alloys, which depends of phase precipitation to achieve their strength, as well as some magnesium and copper alloys. Cast zinc alloys that contain aluminum also can exhibit intergranular attack, specially in steam and marine atmospheres (Fontana, 1986).

MICROBIAL CORROSION

According to Marcus (2002), the microbially influenced corrosion (MIC) is defined as corrosion associated with action of microorganisms in a system. Fontana (1986) states that MIC is not a type of corrosion itself, but it is corrosion occuring as result of metabolism of these living organisms, either directly or indirectly. These microorganisms include bacteria and fungi found in several environments as soils, freshwater and seawater, natural petroleum products, and some industrial fluids.

Pope in Bianchetti (2001) presents some statements explaining how can MIC be feasible as follows:

• microorganisms are small (ranging from less than 0,2 µm in length by up to 2-3 µm in width), meaning that they can easily penetrate crevices and unreachable areas;
• bacteria have the capacity of motion, being them able to move towards another food sources and away from toxic environments;
• bacteria have receptors for certain chemicals which allow them to “see” higher concentrations of nutrients that mean a food source. Alongside this fact, there is the ability of materials, specially metals, on adsorbing nutrients in their surfaces, thus creating a natural source for these organisms;
• microorganisms can live in a wide range of environmental conditions: temperatures from -10 to 99°C, pH from 0 to 10,5, and oxygen concentrations from 0 to almost 100%;
• they can grow in colonies, creating an advantage of maintaining their life cycle in adverse conditions;
• they reproduce very exponentially quick;
• individual cells can be widely and quickly dispersed by external agents, increasing the potential for some cells on finding more favourable environments;
• many are adaptable regarding the use of nutrients as energy sources. For example, Pseudomonas fluorescens can use more than 100 different organic compounds as food source;
• many form an extracellular polysaccharide protection, known as biofilm, against biocides and corrosion inhibitors, also attaching more efficiently these organisms to the surface;
• many bacteria and fungi produce spores which are very resistant to high temperature and toxic systems. Spores can “hibernate” for hundred of years until they find favourable conditions;
• microorganisms are resistant to many chemicals, by the degrading these substances or preventing them to penetrate their inner structure. Resistance may also be acquired by mutation.

The first report of MIC in metals was published in 1891 by Garrett, consisting of corrosion accelerated by ammonia, nitrite, and nitrate of biological origin. Around two decades later, Gaines concluded that sulfate-reducing, sulfur-oxidizing, and iron bacteria were partially responsible for corrosion of steel in soils. Since then, a great number of reports of MIC in metallic and nonmetallic materials has been reported (Marcus, 2002).

Marcus (2002) states that the cost of MIC is very significant. For example, estimatives point that the annual cost of MIC of buried pipelines in the United States was between $500 million to $2 billion. And in the United Kingdom, at least 50% of the corrosion occurring on buried metals had a microbial origin.

Microbial corrosion has been proved to affect mostly pipelines, whatever the enviroment is a soil, freshwater or seawater. Bacteria may enter in a system and stand long periods. In some situations, the system of pipelines receive contaminated fluids (Jackman and Smith, 1999). As a consequence, all materials involved in a pipeline, metals and polymers in the majority, are affected by MIC. Microorganisms weakens the linkages of polymers by using the carbon in their metabolisms, resulting in depolymerization and even production of inorganic compounds, such as CO2, H2O, CH4 and H2S; for metals, micoorganisms are capable of using their free electrons (Revie, 2000).

In concern with metals strictly, Marcus (2002) summarises the main ways in which organisms may increase the corrosion rate and/or the potential of localised corrosion in aqueous environments as follows:

• Formation of concentration cells at the metal surface and in some oxygen concentration cells. This effect is more pronounced when there is production of a biofilm, which is heterogeneously distributed. For example, Gallionella sp. bacteria create tubercles as they oxidise iron. Other bacteria are able to retain heavy metals such as copper and cadmium in the biofilm structure, thus forming a ionic concentration cell;
• Modification of corrosion inhibitors. As stated before, some bacteria can destroy corrosion inhibitors. For example, nitrite, a corrosion inhibitor for iron and mild steel, is transformed to nitrate.
• Production of corrosive metabolites. Organic and inorganic acids, sulfides and ammonia are often produced by bacteria and are corrosive to metallic materials.
• Destruction of protective layers. Various organisms may attack organic coatings depositated in the metal to protect.
• Stimulation of electrochemical reactions. For example, the evolution of cathodic hydrogen from hydrogen sulfide produced by microorganisms.
• Hydrogen enbrittlement. Microorganisms may play a role in the enbrittlement of metals by hydrogen, by acting as a source of hydrogen and/or producing hydrogen sulfide.

Regarding the reactions that lead to a corrosive process, the classification of the microorganisms according to their ability of living in presence of oxygen is usual (Fontana, 1986). Organisms that do not require oxygen to live are anaerobic; those that need oxygen are called aerobic.

 (a) Anaerobic bacteria

According to Revie (2000), the sulfate-reducing bacteria (SRB) are among the most thoroughly studied group of microorgarnisms associated to MIC. Figure 3 shows a population of SRB on a metal surface.

Figure 3 Scanning electron micrographs of a population of SRB on a surface of a metal coupon (stainless steel 316). Source: Revie, 2000.

Under anaerobic conditions, oxygen is not available to accept electrons. This usually occurs in environments such as wet clay, boggy soils, and marshes (Fontana, 1986). Alternatively, sulfate anion or other compounds are used as electron acceptors. The reactions below demonstrate the corrosion of a ferrous alloy enhanced by SRB (Revie, 2000).

Fontana (1986) states that the sulfide has a tendency to retard cathodic reactions, particularly hydrogen evolution, and to accelerate anodic dissolution of metals.

(b) Aerobic bacteria

The most common example of aerobic bacteria are the Thiobaccillus thiooxidans species, which are capable of oxidizing elemental sulfur or sulfur-bearing compounds to sulfuric acid according to the reaction below (Fontana, 1986):

These organisms succeed in low pH environments and are able to increase the sulfuric acid concentration to a value up to 5% wt, thus creating a highly corrosive condition (Fontana, 1986).

In some occasions, the gelatinous structure of biofilm allows the existence of zones with presence and absence of oxygen, what results in a colony with aerobic and anaerobic bacteria living simultaneously. Aerobic processes consume oxygen which is toxic for the anaerobic organisms; therefore, the respective corrosive process also occur simultaneously (Revie, 2000).

The prevention of microbial corrosion have different approaches. Fontana (1986) recalls the use of asphalt, enamel, plastic tape, or concrete on the protection of buried pipelines; also, avoid wet soils in the construction of pipeline networks is a potential solution.

Marcus (2002) reports the use of cathodic protection to avoid microbial corrosion. For example, a practical advice to control anaerobic corrosion of iron is to reduce the potential of the structure to at least -1 V versus Cu/CuSO4, instead of -0.85 V in the absence of SRB.

Marcus (2002) also regards the use of biocides as an efficient method. For this purpose, biocides are divided into two categories: (a) oxidizing agents such as chlorine, ozone, and chlorine dioxide and (b) nonoxidizing agents such as bisthiocyanate, isothiazolines, acrolein, dodecylguanidine hydrochloride, formaldehyde, glutaraldehyde, among others. However, some bacteria may develop enzymes that are able to revert the toxicity of the biocide. Also, the environmental damage for macroorganisms caused by the use of biocides is sometimes critical.

Revie (2000) suggests reducing relative humidity and temperature in order to simply control bacterial growth, as their active metabolism require an appropiate temperature and humidity. However, some fungi are capable of developing under these conditions. The regular cleaning is so a good practice to prevent biofilm formation and further degradation of the material.

References:

BIANCHETTI, Ronald L. (2001). Peabody’s Control of Pipeline Corrosion. 2nd ed., NACE International.

FONTANA, Mars G. (1986). Corrosion Engineering. 3rd ed., McGraw-Hill.

JACKMAN, P.S. and SMITH, L.M. (1999). Advances in Corrosion Control and Materials in Oil and Gas Production. European Federation of Corrosion (EFC).

MARCUS, Philippe (2002). Corrosion Mechanisms in Theory and Practice. 2nd ed., Marcel Dekker, Inc.

REVIE, R. W. (2000). Uhlig’s Corrosion Handbook. 2nd ed., John Wiley & Sons, Inc.

ROBERGE, Pierre R. Handbook of Corrosion Engineering. McGraw-Hill.

SHREIR, L.L., JARMAN, R.A. and BURSTEIN, G.T. (1993). Corrosion: Metal/Environment Reactions. vol. 1, 3rd ed., Butterworth Heinemann.

TALBOT, David and TALBOT, James (1997). Corrosion Science and Technology. CRC Press.