Volume 7 geothermal energy 7 08 – corrosion, scaling and material selection in geothermal power production

Volume 7 geothermal energy 7 08 – corrosion, scaling and material selection in geothermal power production Volume 7 geothermal energy 7 08 – corrosion, scaling and material selection in geothermal power production Volume 7 geothermal energy 7 08 – corrosion, scaling and material selection in geothermal power production Volume 7 geothermal energy 7 08 – corrosion, scaling and material selection in geothermal power production Volume 7 geothermal energy 7 08 – corrosion, scaling and material selection in geothermal power production

SN Karlsdóttir,Innovation Center Iceland, Iceland

© 2012 Elsevier Ltd All rights reserved

Comprehensive Renewable Energy, Volume 7 doi:10.1016/B978-0-08-087872-0.00706-X 241

Glossary Geothermal power plants Power plants that use steam Corrosion Process of deterioration of material into its from high temperature geothermal wells to produce constituent atoms due to chemical reactions with its electricity They usually also produce hot water

Scale substances that form on

Corrosion film A film made of corrosion products that components, e.g., pipes, when precipitated from a liquid form on the surface of corroded metal as a by-product of a Scaling This occurs when minerals dissolved in

geothermal fluid precipitate from the liquid

Geothermal fluid Geothermal liquid, steam and gas, onto the surface of the geothermal wells and equipment. together or separately The state that the fluid is in, i.e., Stainless steel This is classified as steel that contains a

minimum of 10.5% chromium by mass which makes

7.08.1 Introduction

Materials used in high-temperature geothermal wells and equipment in direct contact with geothermal fluid can be subjected to corrosion; this results in high costs associated with the materials, labor, and production efficiency of wells Corrosion is described

as the natural process of deterioration of metals and alloys in corrosive environments The corrosion aggressiveness of geothermal fluids depends on the chemical composition and physical characteristics of the fluid, for example, acidity (pH level), and on the exploitation parameters such as temperature, pressure, and flow rate The principal corrosive agents in geothermal fluids are the dissolved gases hydrogen sulfide (H2S) and carbon dioxide (CO2), and chloride ions (Cl−) Other corrosive components that can

be present in geothermal fluids are dissolved ammonium (NH3), methane (CH4), and sulfate ions (SO42−) [1, 2] Dissolved hydrogen (H2) and nitrogen (N2) gases can also be present In uncontaminated, high-temperature geothermal fluids, there is no free oxygen If oxygen gets into wet geothermal steam systems, corrosion is accelerated In some systems, hydrogen chloride (HCl)

is present; if condensation and reboiling occurs, localized enrichment of hydrochloric acid can cause severe corrosion of materials

in the systems [3]

There can be significant variation in the physical characteristics and the chemical composition of geothermal fluids (geofluids) in geothermal systems Thus materials used in geothermal energy production can be subjected to a wide variety of corrosive environments related to the geological conditions under which the geofluids are produced There can be different conditions in wells within the same geothermal system, which can result in corrosion problems in one well but not in other wells within the same geothermal system It is therefore not always easy to predict whether corrosion will occur before geothermal well drilling is commenced, even though the surrounding geothermal system is well known Corrosion of materials inside geothermal power plants is dependent on the design of the power plant and the point of production because these factors influence key parameters such as temperature, velocity of the fluid, and even the composition of the geofluid [3–5] Table 1 shows the composition and physical characteristics of geofluid from different high-temperature geothermal fields This is an example of how the chemical composition and characteristics of geothermal fluids can vary between locations and wells

Because the composition of geothermal fluids can vary greatly between locations and within a single geothermal system, the fluid can be corrosive at one point but passive and show a trend toward scaling at another, due to a change in its physical and chemical parameters Scaling occurs when minerals dissolved in geothermal fluid precipitate from the liquid and deposit onto the surface of the geothermal wells and equipment (due to a change in pressure, temperature, or pH value, which disturbs the equilibrium of the system) Thus, geothermal systems can undergo corrosion, scaling, or both simultaneously When scaling

Temperature Fluid

Concentration of key species in the fluid (ppm)

Total Total Total Location (°C) (location) descriptiona pH Cl− CO2 H2S NH3 SO4 2− CH4

Salton Sea, California, USA 250 (borehole) Unflashed 5.2 115 000 1000 10–30 300 20

wellhead fluid Baca (Valles Caldera), New 171 (wellhead) Flashed fluid 6.8 3 770 128 6 59

Mexico, USA

Bjarnarflag, North Iceland, 171 (wellhead) Flashed fluid 7.9 283 529 1333 1.7 7.6 Iceland

Reykjanes, Southwest 295 (borehole) Unflashed 4.7 19 319 1779 53 2.0 12.2 0.2

a Measurements were made at different points of production, before or after flashing; thus, the source fluids cannot be directly compared because often during flashing Cl concentration increases while the concentration of CO and HS decreases and increase e in pH will occur

occurs in geothermal wells and systems, it can create major problems for geothermal operations Deposition by scaling on the surface of geothermal wells and equipment can result in plugging at these locations, inhibiting production and incurring expensive cleaning costs High-temperature geothermal resources that have higher water ratios often have an increased level of silica that causes difficult scaling problems Scaling problems do not usually occur in dry steam fields, but can still entail serious corrosion problems Unfortunately, at some fields, both scaling and corrosion problems are encountered at the same time [8–10]

It should be noted that corrosion or scaling in geothermal systems is not the limiting factor in the production of geothermal power However, when it does occur, it can be costly and delay production These issues can be avoided by correct material selection, good engineering design, and proper corrosion and scaling control methods

In this chapter, forms of corrosion and scaling that can occur in geothermal systems are presented and discussed Material selection for geothermal systems is discussed in relation to corrosion, and different materials and their performance in geothermal environments are compared The mechanism of scaling and the problems associated with it are also a topic in this chapter Finally, solutions to corrosion and scaling problems are presented and discussed

7.08.2 Corrosion Films and Processes

The corrosion of a piece of metal may be summarized as the transformation from a metal to a metal ion, or as the loss of one or more electrons from the metallic atom All corrosion reactions produce by-products, called corrosion products These are, for example, insoluble hydroxides, carbonates, oxides, sulfides, silicates, and borates that form films on the surface of the corroded metal Some of the films are porous and loose, allowing diffusion to and from the metal surface These types of films do not protect the metal surface and allow further corrosion On the other hand, corrosion films can be nonporous, tight, and adherent These are substantially more protective toward further corrosion, mainly because they limit access of corrosives to the metal surface In some environments, the corrosion products are very soluble and no corrosion film forms on the surface of the corroding metal This is called active corrosion Some alloys are unreactive, meaning that they form corrosion films made up of mixed oxides that are so nonreactive that they protect the base metal after a short period of active corrosion This type of corrosion process is called passive corrosion and the films are called passive films [11] This type of film can occur on metal alloys such as titanium and stainless steels These alloys can, however, experience active corrosion if the environment or conditions are severe enough, for example, in very corrosive fluids with low pH values

7.08.3 Forms of Corrosion in Geothermal Environments

There are several forms of corrosion that can occur in metals in geothermal environments Some of these forms rarely occur, whereas others are more common The following sections describe these forms

7.08.3.1 Uniform Corrosion

In general, uniform corrosion is the most common type of corrosion It can be defined as the attack of the entire metal surface exposed to the corrosive environment resulting in uniform loss of metal from the exposed surface The metal becomes thinner and eventually fails Uniform corrosion generally increases when acidity increases (decrease in pH) It is often promoted by oxygen, carbon dioxide, chloride, hydrogen sulfide, or ammonia In geothermal systems, it is generally promoted by carbon dioxide, hydrogen sulfide, and in some cases chloride [4] Rapid failure of equipment in geothermal environments due to uniform corrosion

is not common Uniform corrosion is commonly quantified by measuring the corrosion rate (mm yr−1) of the metal by using corrosion tests where specimens are immersed in the corrosive environment, such as geothermal liquid, and the weight change is measured (weight loss) [11]

7.08.3.2 Pitting Corrosion

Pitting is a form of localized corrosion where a small portion of the metallic structure is corroded at a rate much faster than the bulk material It is a localized form of attack where pits develop on the metal surface The pits are holes that can be small or large in diameter, but in most cases they are relatively small They can be isolated or close together so that they look like a rough surface [11] Pits can deepen due to the breakage of a passive film that forms on some metals [5] Pitting is one of the most destructive forms of corrosion, causing equipment to fail because of perforation, with only a small percent weight loss of the entire structure It can be difficult to detect pits because of their small size and they are often covered with corrosion products It can be hard to measure corrosion pits quantitatively and compare the extent of pitting due to variation in their size and number for identical condition It can also be difficult to predict pitting in laboratory tests because sometimes it takes a long time for them to occur on the field (it can take several months or even years) Pits most often grow in the direction of gravity, that is, they form and grow downward from horizontal surfaces Velocity can affect the extent of pitting, wherein they are more severe in stagnant conditions than in

high-velocity flow The most common cause for pitting failures is chloride and chlorine-containing ions [11] Pitting is especially fierce because its intense and localized form often results in failures that occur with extreme suddenness In geothermal environ­ ments, pitting corrosion has resulted in sudden unexpected failures in pipes and tubes [12]

7.08.3.3 Crevice Corrosion

Another form of localized corrosion is crevice corrosion It occurs within crevices of equipment and other shielded areas on metal surfaces exposed to corrosive environment In geothermal environments, crevice corrosion can, for example, occur in metals due to deposits, mill scale, and mechanical crevices It is geometrically dependent unlike most other forms of corrosion [5] Crevice corrosion is usually associated with small volumes of stagnant solution caused by gasket surfaces, holes, lap joints, surface deposits, and, as the name implies, crevices under bolt and rivet heads Deposits that can produce this form of corrosion are, for example, corrosion products, dirt, sand, and other solids The deposits can act as shields and form a stagnant condition beneath them Permeable corrosion products can also have this effect The stagnant condition promotes depletion of oxygen within the crevice due

to restricted convection This results in excess production of positive charges in the solution when the metal continues to dissolve (initially the dissolution and the reduction of oxygen are even) Both hydrogen and chloride ions accelerate the dissolution rate of most metals; these can both be present in the crevice as a result of migration and hydrolysis The increase in the dissolution increases the migration of the species, which results in accelerated corrosion [11] Figure 1 shows crevice corrosion in a stainless-steel tube from an oil cooler where geothermal steam condensate was used as cooling water [13]

7.08.3.4 Intergranular Corrosion

Intergranular corrosion can be defined as localized corrosion at and adjacent to grain boundaries, with relatively little corrosion at the grains As a consequence, the metal alloy disintegrates and/or it loses its strength This form of corrosion can be caused by impurities at the grain boundaries and depletion or enrichment of one of the alloying elements in the grain boundary area – such as the formation of chromium carbide at the grain boundary regions of stainless steel resulting in chromium-depleted zones (often called sensitization) – thereby leading to intergranular corrosion [11] This can usually be avoided by using stainless steel with low carbon content In geothermal environments, intergranular corrosion can occur in austenitic and ferritic stainless steel [5] 7.08.3.5 Galvanic Corrosion

When two dissimilar metals are immersed in a corrosive or conductive solution, usually a potential difference exists between them This potential difference produces electron flow between them when they are placed in contact or if they are electrically connected in some other way In this condition, the metal which is less noble will experience accelerated corrosion; this is called galvanic corrosion Metals can be ordered in series by increased nobility; this is called the galvanic series and can help in material selection to avoid corrosion Galvanic corrosion can occur in geothermal environments, for example, in a geothermal iron pipe section in contact with a bronze valve [12] Environmental factors such as temperature and chemistry can change the order of metals in the galvanic series The relative area of the two alloys is also an important factor in galvanic corrosion The severity of the galvanic corrosion is greater when the area of the more active alloy is small compared to the area of the noble metal Some procedures can be used to prevent galvanic corrosion, for example, selection of combinations of metals as close together as possible in the galvanic series and insulation of dissimilar metals [11]

Figure 2 Stress corrosion cracking of an AISI 304 stainless-steel float from a geothermal hot water storage tank [13]

7.08.3.6 Stress Corrosion Cracking

Stress corrosion cracking (SCC) is a catastrophic type of failure caused by the simultaneous presence of tensile stress and a corrosive environment During SCC, the metal is essentially unattacked over most of its surface area, but fine cracks progress through parts of it This kind of cracking has serious consequences because it can occur at stresses within the range of typical design stress The fine cracks often form a net of cracks that are spread out, appearing like tree branches Stress corrosion cracks have the appearance of brittle mechanical fractures and the cracking generally proceeds perpendicular to the applied stress Important variables that affect the susceptibility of metals to SCC are metal structure and composition, stress, and temperature If the metal is in a fluid, the pH value and the composition of the fluid are also very important The chloride and oxygen content in the fluid increases the susceptibility of the metal to SCC SCC is known to be accelerated by increasing temperature A ‘lower critical temperature’ exists for a given concentration of oxygen and chloride and pH level below which SCC does not occur There is no critical stress above zero stress below which SCC does not occur SCC can occur in cases where there is no applied stress, for example, when residual stresses exist from cold working and welding in the metal [5, 11] Figure 2 displays the SCC of an AISI 304 stainless-steel float from a geothermal hot water storage tank, representing another example of SCC in a geothermal environment [13] Damages due to SCC have also been observed in rotors, blades, and other components of steam turbines [14–16] as well as in heat exchanger tubes in geothermal power plants SCC has also occurred and caused problems in geothermal equipment where leaks or condensation on the outside of stainless-steel equipment has promoted SCC

7.08.3.7 Hydrogen Embrittlement

Hydrogen embrittlement (HE) refers to mechanical damage of a metal due to the penetration of hydrogen into the metal causing loss in ductility and tensile strength HE can occur due to corrosion of steel by H2S when hydrogen atoms are generated During corrosion of steel in geothermal steam, H2S reacts with the surface and forms a corrosion film (FeS, MnS) and free hydrogen ions (H+) The free hydrogen ion would normally not diffuse into the metal, but the sulfide (S2−) ion acts as a poison and promotes the uptake of the hydrogen, which gets trapped in the metal structure and results in embrittlement of the metal [11, 17]

7.08.3.8 Hydrogen-Induced Cracking

One form of HE is hydrogen-induced cracking (HIC) HIC occurs when hydrogen ions (H+) diffuse into weak interfaces (e.g., laminations, inclusions, and voids) in the metal and recombine there to form molecular hydrogen, which is many times larger in volume than H+, causing the formation of cracks (or blisters) in the metal [17–19] HIC does not require any external stress to occur The cracks or blisters caused by the accumulation of the molecular hydrogen generally run parallel to the surface of the material Under the influence of tensile stress (residual or applied), the cracks can link up and propagate in a step-like manner until catastrophic failure occurs when the effective thickness of the metal is reduced; this is called stress-oriented hydrogen-induced cracking (SOHIC) [17] The susceptibility of metals to HIC is primarily dependent on the microstructure, impurity content of the material, metallurgical processing, and heat treatments [17, 20] HIC and SOHIC usually occur in lower strength steels used in plate and pipe products with a yield strength below 700 MPa [17] HIC was blamed for the cracking of a brine accumulator and steam purifier in a geothermal power plant in New Zealand [21] Figure 3 shows HIC causing leakage in a geothermal steam pipe in Iceland [13]

Figure 3 Hydrogen-induced cracking in a weld in a geothermal steam pipe [13]

7.08.3.9 Sulfide Stress Cracking

Sulfide stress cracking (SSC) is a special type of HE and occurs in metals due to the combined effect of tensile stresses and corrosion by H2S [22, 23] SSC is a solid-state embrittlement reaction resulting from the interaction between the metal lattice and the atomic hydrogen generated from the corrosion of the metal by H2S [17] SSC is a catastrophic failure like SCC that results in a brittle fracture It can occur at stresses falling within the range of typical design stress [5] Due to the presence of H2S in geothermal fluids, there is a danger of SSC in geothermal equipment [24] Unlike SCC, the severity of SSC decreases as temperature and pH level increase, and as H2S concentration, yield strength, and stress decrease Oxygen is known to have little

or no effect on the SSC mechanism [5] The occurrence of SSC depends on the strength of the steel, stress concentration, levels of the stress, chemical composition of the steel, microstructure of the steel, and hydrogen concentration in the steel [25] High-strength steels are more susceptible to SSC than low-strength steels Because of this, it is a common industry standard to limit the hardness of these types of steels to 250 HV (Vickers hardness) [26, 27] This is not an absolute value and SSC can still occur for steels that fulfill this requirement SSC can, for example, occur in low-strength steels when they are subjected to high residual stresses derived from fabrication techniques [21], or to high stresses or high stress intensities [28] The low-carbon steel casing material, H40, with a hardness of approximately 120 HV and a relatively low tensile strength (400 MPa), cracked due to SSC in a geothermal environment when subjected to stresses above the yield strength and at high stress intensities [28] SSC also occurred in the carbon steel casing material API 5CT N-80, and in a high-strength carbon steel wire (tensile strength > 1200 MPa)

in a geothermal well with high partial pressure of H2S and high thermally induced tensile stress The material selection for this environment was not ideal, the N-80 steel grade does not have any hardness limitation, which increases the possibility of SSC, and the high strength of the wire material makes it more susceptible to SSC [29] Another example of SSC in geothermal equipment is the cracking of a brine accumulator and a steam purifier in a geothermal power plant in New Zealand because of high residual stresses in the welds attributed to the use of submerged arc welding [21] The microstructure of steel also has a considerable effect on SSC; for example, fine-grained steels are less susceptible to SSC than coarse-grained steel Martensitic and ferritic steels are susceptible to SSC, while austenitic steels are less susceptible [5] In susceptible microstructures, residual stresses can be sufficient to cause cracking

7.08.3.10 Corrosion Fatigue

Corrosion fatigue can be classified as a premature fracture when cyclic stresses are imposed on a material in a corrosive environment [5] Corrosion fatigue is most dominant in mediums where corrosion pitting occurs The pits act as stress raisers and initiate fatigue cracks, which lead to corrosion fatigue failure Corrosion fatigue can occur in pipes carrying steam

or hot liquids at varying temperatures because of cyclic stresses from vibration caused by varying pressure and periodic expansion and contraction of the pipe caused by thermal cycling [30] Corrosion fatigue can also occur in turbine parts used

in geothermal power plants due to the cyclic loading and corrosive environment This includes parts such as rotors and turbine blades [15] Corrosion fatigue testing of different types of steel in geothermal steam in high-temperature geothermal fields in Iceland showed that the fatigue lifetime of the steel was lower in the geothermal steam than in air as well as dependent on the microstructure of the steel The martensitic steels had shorter lifetime in the geothermal steam than the austenitic steels [31]

7.08.3.11 Erosion Corrosion

Erosion corrosion is an accelerated form of corrosion of a metal caused by relative movement between corrosive media and metal surfaces The corrosive medium can be one of the following: fluids, for example, water or solutions containing suspension; organics;

or gases or steam such as geothermal liquid The metal surface becomes damaged by mechanical or hydraulic wear or abrasion caused by the flow of the medium In erosion corrosion, the metal surface is not covered by corrosion products, but characterized in appearance by grooves, waves, gullies, rounded holes, or valleys, and it usually exhibits directional pattern In many cases, failures due to erosion corrosion occur in a relatively short time and they are sometimes unexpected because previous evaluation corrosion tests were run under static conditions, or because the erosion effects were not considered Most metals and their alloys are susceptible to erosion corrosion damage Metals that depend on passivity by forming a protective surface film are also susceptible

to erosion corrosion as, if the surface film is damaged, the bulk metal or alloy is attacked at a rapid rate Increased velocity usually results in increased erosion corrosion [11] Erosion corrosion can occur in equipment used in a geothermal environment that is exposed to moving fluid including piping systems, particularly elbows and tees, pumps, valves, impellers, blowers, heat exchanger tubing, condensers, nozzles, and turbine blades Erosion corrosion can also be caused by impingement; this can occur in the steam turbine blades in geothermal turbines particularly in the exhaust or wet-steam ends of the turbine [15] Moreover, another form of erosion corrosion is cavitation damage; it is caused by the formation and collapse of vapor bubbles in a liquid near a metal surface [11] It occurs in equipment where high-velocity liquid flow and pressure changes are encountered; these conditions can occur, for example, in geothermal wells and equipment Cavitation can occur in geothermal wells when the water starts to boil when the pressure decreases because of vapor bubbles that form (containing dissolved gases) and collapse at the metal surface at high speed resulting in cavitation damages, that is, holes In a high-temperature geothermal well (∼300 °C) in Iceland containing H2S, CO2, and HCl, the steel casing – grade K-55 – underwent extensive cavitation and HE that caused fracture of the steel liner

7.08.3.12 Exfoliation

Exfoliation is a form of corrosion where discrete layers of corrosion products (sometimes with metal attached that has separated from the lattice) flake off or break loose from the surface, reducing the thickness of the material The corrosion products are, for example, iron sulfide that forms as a corrosion film on steel pipes carrying steam containing H2S These films can flake off and damage other components downstream such as turbines operating directly on flashed steam by causing erosion and possibly erosion corrosion [5]

7.08.4 Variables and Corrosive Species That Affect Corrosion Rates

Variables and corrosive species that affect corrosion in geothermal environments are described here in connection with the corrosion forms previously described

7.08.4.1 pH Level

In general, corrosion rates increase with decreasing pH, that is, with increased acidity of the fluid Decreasing pH means increasing amount of hydrogen ions For example, for carbon steel, the corrosion rates generally increase in environments with pH levels below

7 As mentioned previously, the pH level influences the passivity of many metal alloys That is, the formation of the passive film for these metals depends on the pH level; if it is too low, the film cannot form and the alloy is vulnerable to corrosion This can occur in local areas on the metal surface and lead to serious forms of corrosion such as crevice corrosion, SCC, and pitting [5]

7.08.4.2 Temperature

Increased temperature generally increases corrosion rates This can be explained as being due to the common effect that increased temperature has on reaction kinetics But the effects of temperature are complicated; for example, at increased temperature, the corrosion rates can also decrease due to the decrease in solubility of gases For example, in systems with oxygen, increased temperature can lead to acceleration of corrosion rates first but then a decrease due to the lowering of the solubility of oxygen and decrease in oxygen concentration [30] As mentioned earlier, increased temperature has an opposite effect on SCC and SSC: higher temperature increases the likelihood of SCC while the chances of SSC are reduced SSC susceptibility reaches a maximum at around room temperature but then decreases with increasing temperature over the range 25–200 °C [17]

7.08.4.3 Suspended Solids and Solid Deposition

Solid deposition on equipment surfaces from the precipitation of liquid phase species (or ions) from the geothermal liquid can influence corrosion and cause erosion [5]; in geothermal energy production, this is called scaling and can influence the performance

of the geothermal system Scaling and its effects in exploitation of geothermal energy are discussed in more detail later in this chapter

7.08.4.4 Fluid Velocity

Fluid velocity has different effects on different corrosion forms as mentioned previously For example, low velocity can lead to stagnant areas, which can result in crevice corrosion, while high velocity can result in erosion corrosion Thus for every geothermal design, the velocity of the fluid has to be included in the design criteria

7.08.4.5 Hydrogen Sulfide

H2S is along with CO2 the main reason for corrosion of steel and iron alloys in geothermal fluids It is the main source of hydrogen for HE and SSC of metals in geothermal environments [2] H2S reacts with carbon steel to form corrosion films If they break down,

it can cause an accelerated corrosion attack H2S attacks certain copper/nickel alloys; therefore these alloys are practically unusable in geothermal environments that generally contain H2S

7.08.4.6 Hydrogen Ion

The effect of the concentration of hydrogen ions is partially described in the section discussing the effect of pH because the pH level reflects the concentration of hydrogen ions The general corrosion rate of carbon steel increases rapidly with increasing hydrogen ions, that is, with decreasing pH, as mentioned previously Hydrogen ions are also the key factors in HE and SSC as well as in HIC, which is closely related to HE

7.08.4.7 Chloride Ions

Increasing concentration of chloride ions (Cl−) increases uniform corrosion Chloride ions can also cause local breakdown of metals that form passive films, which results in a decrease in corrosion resistance of metal and causes localized corrosion This is usually a more serious effect than the uniform corrosion The local breakage of the film can lead to pitting and crevice corrosion, and it increases the risk of SCC The largest risk of SCC occurs when the steam condenses with chloride ions on the steel surface so that the chloride concentration builds up and increases the susceptibility of SCC dramatically The source of the chloride ions in geothermal steam can be either salt brine (NaCl) in the steam in geothermal areas close to the sea or volatile chloride transported as HCl [3, 32] HCl in geothermal steam has been reported in geothermal steam fields in different parts of the world: Larderello, Italy; Krafla, Iceland; St Lucia, Windward Islands; Tatum, Taiwan; and The Geysers, USA [33, 34] The presence of HCl in superheated geothermal steams has caused severe corrosion problems, which have led to major operating difficulties [3, 32, 34, 35] The corrosion mechanism due to the presence of HCl is believed to be connected to the partitioning of the HCl into the liquid present and the subsequent dissociation into Cl− and H+ ions Corrosion of carbon steel steam pipelines is negligible due to the presence of chlorides above the dew point, but fast pitting occurs where condensation takes place due to the acid solutions formed, potentially leading to rapid localized failure of geothermal pipes and equipment [32, 34] In the geothermal area of Larderello in Italy, chloride

in the steam (1–10 ppm) was blamed for the etching of turbine components and severe corrosion of a carbon steel liner was attributed to chloride vapor (tens to hundreds of ppm) in contact with condensate [32] In Krafla – a geothermal area in Iceland – the presence of hydrogen chloride in a well-producing dry superheated steam resulted in condensation and reboiling, which caused localized enrichment of hydrochloric acid and consequently severe corrosion of a wellhead, pipelines, and turbine materials [3]

7.08.4.8 Carbon Dioxide

The pH level of geothermal fluids is largely controlled by CO2 Increased CO2 concentration results in decreased pH level and increased acidity CO2 is very soluble in water, 100 times more soluble than oxygen [30] CO2 can accelerate uniform corrosion of carbon steels in the acidic region Along with dissolved H2S its presence is the main reason for corrosion of steel and iron alloys in geothermal fluids [2] 7.08.4.9 Oxygen

Oxygen accelerates corrosion caused by other dissolved gases, such as CO2 and H2S [30] Therefore, even the addition of part-per-billion quantities of oxygen to high-temperature geothermal systems can greatly increase the chance of severe localized corrosion of normally resistant metals In uncontaminated high-temperature geothermal fluid, there is generally no free oxygen but if a small amount of oxygen enters the systems, materials that are normally corrosion resistant in this environment can experience SCC and other forms of corrosion [4] In general, the corrosion of steel is sensitive to trace amounts of oxygen [5] Higher pressure and temperature and lower pH increase the corrosivity of oxygen [30]

7.08.4.10 Ammonia

Ammonia (NH3) can accelerate uniform corrosion of steel It can also cause SCC of some copper alloys However, it is usually found

in a very low concentration, if at all, in geothermal steam and thus is not considered a general hazard to the materials used in geothermal applications [5]

7.08.4.11 Sulfate

In most geothermal fluids, sulfate (SO4) has little effect on corrosion In some streams containing low amounts of chloride, the sulfate can be an aggressive anion but despite that it rarely causes the same severe localized attack as chloride [5]

7.08.4.12 Other Factors

In liquid-dominated geothermal resources, there are two factors that should be mentioned that can cause difficult corrosion problems First, carryover of entrained liquid provides chloride ions that often cause localized corrosion attacks and the impinge­ ment of high-velocity droplets can induce localized attacks Thus efficient steam separation is very important; it will, however, not always prevent attacks The corrosion will often depend on the chloride content and the corrosivity of the liquid stream for a given steam separation efficiency Second, areas in geothermal systems where local condensation can occur are exposed to corrosion attack by low-pH condensate containing CO2, H2S, and chlorides Areas that are subjected to this in geothermal systems are, for example, the low-pressure turbine section and stagnant or poorly insulated parts of steam transfer sections and liquid traps [5]

7.08.5 Material Selection and Performance in Geothermal Environments

In this section, the performance of different materials in geothermal environments is discussed and rated The discussion is focused on how these materials perform in relation to different corrosion forms in geothermal steam and fluid Material selection in geothermal energy exploitation is also discussed, for example, which materials can be and are used for geothermal well casings, pipes, and various components used in geothermal power plants and systems

7.08.5.1 Ferrous Alloys

7.08.5.1.1 Carbon and low-alloy steel

Carbon and low-alloy steel (i.e., containing no more than 4% alloying elements) are attractive materials for construction purposes

in geothermal power plants due to their availability, low cost, and fabrication ability Their reliability depends, however, on their applications in the power plants Carbon steel can be used for thick-walled applications in contact with most geothermal fluids [5]

It is commonly used for the wellhead and pipelines for the transportation of two-phase geofluid (mixtures of liquid and vapor, i.e., gas and steam), from the wellhead to the flash separator units, as well as for the transportation of separated liquid and geothermal steam Carbon steel has also been used for separators and flash units [3] Low-alloyed carbon steels such as 1% and 2.5% CrMoNiV are commonly used in turbine components such as turbine rotors [36, 37] The most common forms of corrosion that affect carbon and low-alloy steel in geothermal systems are localized and uniform corrosion Usage of carbon and low-alloy steel is limited in thin-wall application due to the susceptibility of these materials to localized attacks such as crevice and pitting corrosion Chloride ions are the main factors in initiating localized attack and H2S can increase the severity of localized corrosion Geothermal corrosion field tests indicate that the rate of uniform corrosion for these materials is generally 0.03–0.3 mm yr−1 when the chloride concentration is lower than 2% and the pH level is higher than 6 When the pH level is below 6 and the amount of chlorides above 2%, a rapid increase in corrosion rates is observed [5] In some cases, scales that form on the surface of the steel by precipitation from geothermal fluids are believed to protect the steel surface from corrosion so long as the scale is adherent and reaches sufficient thickness to ensure its mechanical integrity [3] On the other hand, if this scale is porous and thus prone to cracking – which is true in many cases – corrosive attacks can occur at these small exposed areas

High-strength low-alloy carbon steel can fail and brittle fractures can occur due to SSC in geothermal environments containing aqueous H2S Low-strength low-alloy steels are generally not sensitive to SSC but they can incur SSC when combined with residual stress or high stress intensities and H2S in geothermal environments Low-strength low-alloy steel can be subject to HE in geothermal environments when difficult conditions exist due to HCl and H2S gases and low pH levels, or when coarse-grained structure and residual stress exist within the material [21] Severe corrosion can occur on the outside surface of carbon and low-alloy steel wellheads just below the soil or cellar floor during a standby due to condensation of the steam when the casing is allowed to cool To avoid this, it is best to try to keep the wells hot, either by production or bleeding to a small silencer [8]

In general, carbon and low-alloyed steel are preferred for many components in geothermal systems due to economical advantage over other materials, even though their resistance against corrosion is limited, especially at low pH levels, high chloride concentration, and high flow rates Nevertheless, they serve well in many applications and thus geothermal systems are composed in large parts of them

7.08.5.1.2 Stainless steels

Stainless steel is classified as steel that contains a minimum of 10.5% chromium (Cr) by mass, which makes it corrosion resistant

in air Stainless steels often have other alloying elements to give them better properties, for example, nickel (Ni) and molybde­ num (Mo) are often added to increase the corrosion resistance Stainless steels are often classified into types corresponding to their iron alloy phases: ferritic (ferrite phase), austenitic (austenite), and martensitic (martensite) steels

In geothermal fluids, stainless steel exhibits a much lower corrosion rate due to uniform corrosion than carbon and low-alloy steel Stainless steels can, however, be subject to other forms of corrosion – often labeled as more serious forms of corrosion – such

Figure 4 Stress corrosion cracking in an AISI 304 stainless-steel plate from a plate heat exchanger in a geothermal power plant [13]

as crevice corrosion, intergranular corrosion, pitting, SSC, SCC, and corrosion fatigue Stainless steel along with carbon and low-alloy steel is the main construction material in geothermal systems [38, 39] Generally, stainless steels are used in geothermal systems in much smaller quantities than carbon and low-alloy steel because of cost

Austenitic stainless steels form a passive film (an oxide layer) which shows good corrosion resistance to geothermal condensate [40] The austenitic stainless steels AISI 304 and 316 have been used for components in geothermal power plants, for example, in condensate collection systems, heat exchangers, and parts of cooling towers AISI 304 contains 19% Cr and 9.5% Ni, whereas AISI

316 contains 17% Cr, 12% Ni, and 2.5% Mo The selection between 304 and 316 is usually based upon the combination of chloride content and the temperature of the geothermal fluid, where the 316 is more corrosion-resistant against localized corrosion than

304 However, if the geothermal fluid is heavily chlorinated, heat exchangers made of 316 stainless steel can fail due to corrosion because chloride ions easily break down the oxide layer, which leads to localized corrosion such as pitting and SCC

Stainless steel is also used in turbine components in geothermal power plants This includes, for example, 13% Cr martensitic stainless steel (AISI 403) used for turbine blades and nozzles [36] Corrosion fatigue is a potential problem in geothermal turbines and stainless steel is more resistant to corrosion fatigue than carbon and low-alloy steel Other examples of stainless steels used in geothermal systems are the ferritic steel, AISI 430 (16–18% Cr), and the martensitic steel, AISI 431 (15–17% Cr, 1.25–2.5% Ni), which are used for valve and pump components If the geothermal fluid contains high amounts of chloride ions or sulfur, the AISI

430 is more suitable because of its higher resistance against pitting and SCC [4] In general, the corrosion resistance of AISI 431 and

430 is lower than that of AISI 316, but similar to or slightly lower than that of AISI 304 The superaustenitic alloy 254 SMO (19.5–20.5% Cr, 17.5–18.5% Ni, 6.0–6.5% Mo) has shown good performance in corrosion tests in geothermal environments in Italy and Iceland [41, 42] It is currently not commonly used in geothermal equipment due to high costs, but it has been used when AISI 316 has not been adequate due to corrosion, for example, for pipes in a heat exchanger used in a geothermal plant Because stainless steel is often used in complex equipment, localized corrosion such as crevice and pitting corrosion can be a serious problem Chlorides increase the susceptibility of stainless steels to these localized corrosion problems [5] The pitting and crevice corrosion resistance of stainless steels is highly dependent on their Mo and Cr content; increased amounts of Mo and Cr increase the resistance to these localized corrosion forms [11] Nickel has a great effect on the susceptibility of stainless steel to SCC, especially in chloride solutions Immunity from SCC is usually not obtained unless the Ni content is less than 1% or greater than 42–45% [43, 44] It is most pronounced at 8–12 wt.%, but decreases at lower and higher levels Austenitic stainless steels with a Ni content of 42 wt.% and above are considered immune to cracking [44] By adding Mo and silicon (Si), the resistance to SCC can be improved [5] Ferritic steels are generally more resistant against SCC in hot chloride solutions than austenitic stainless steel, which is more susceptible Ferritic steels are, on the other hand, susceptible to SSC like martensitic steels, whereas austenitic steel tends to be more immune [5] Both ferritic and austenitic stainless steels can be subject to intergranular corrosion Figure 4 shows a picture of SCC in an AISI 304 stainless steel plate from a plate heat exchanger in a geothermal power plant The stainless steel plate was replaced with a titanium plate [13]

7.08.5.2 Nonferrous Metals and Alloys

7.08.5.2.1 Nickel alloys

High nickel containing alloys are commonly used to battle severe corrosion problems Ni-Cr-Mo-based nickel alloys have shown very good performance in high-temperature geothermal fluid [5, 38–40, 45] These include nickel alloys such as Inconel 625 and Hastelloy C-276, which are especially resistant to corrosion and can tolerate high flow rates and occasional aeration [5, 38, 39] Hastelloy C-276 along with titanium had much higher corrosion resistance than carbon and low-alloy steel, stainless steels, and other Ni-base alloys when tested in a flowing two-phase fluid in the Onikobe geothermal field in Japan The tests in Onikobe were done in geothermal fluid with velocities in the range of 70–100 m s−1, pH of 2–4.5, and temperatures of 102–137 °C [39] Nickel-base alloys containing more than 8% Mo (Hastelloy C-276 and Alloy 625) and titanium also gave the best performance in