7.1 Material Selection in Wet CO2 Environments
This section considers the materials selected for handling wet corrosive environments of various types containing CO2. The terms “sweet” and “sour” are commonly applied within the oil and gas industry to describe CO2–containing fluids respectively lacking or containingsulphide. The materials selection “rules” described in this Chapter have been drawn from other industries (principally Oil and Gas) and have formed the basis for material selection for the various carbon capture processes within this report.
So-called “sweet” environments characteristically cause corrosion which is dominated by the presence of CO2, as described in Section 6.4.1.
The addition of hydrogen sulphide to a CO2-containing stream changes not only the form of corrosion in carbon steels, but also affects the choice of materials applied for mitigating the corrosion process. ISO 15156/NACE MR0175 provides the starting point for materials selection in these “sour” conditions, although strictly the scope of the standard is restricted to oil and gas production environments. The standard does allow the use of materials outside the published limits on the basis of suitable experimental data or service experience.
ISO 15156/NACE MR0175 only covers oxygen-free environments. The combination of CO2 and H2S with oxygen present is also considered in this section. Corrosion in partially deaerated brines has been studied in connection with seawater handling and downhole injection, and also with downhole storage of natural gas in salt caverns, among other situations [44, 45]. The CCS conditions are more severe in some respects due to the high CO2 levels and acidity.
7.1.1 Carbon steel
Carbon steel is used in this report in the context of corrosion properties to refer to mild steel, C-Mn steels, micro-alloyed steels and also low alloy steels where the alloy content does not produce a significant difference in corrosion behaviour from that of mild steel. Carbon steel is common as the material of construction in non-corrosive environments; even in moderately corrosive conditions this material is still preferred owing to its relatively low price, strength and ready availability internationally.
The threshold of H2S concentration at which an environment is considered to be “sour” for carbon and low alloy steels is defined by ISO 15156/NACE MR0175. In oxygen-free, sour environments, the corrosion product (iron sulphide) can be highly protective and carbon steel can be remarkably successful as long as the guidance of ISO 15156/NACE MR0175 is followed for avoiding SSC and HIC cracking, sections 6.8.2-6.8.4.
Corrosion rates of carbon steel in wet sour conditions with trace oxygen are difficult to predict, but are typically these are very severe conditions for carbon steel. Elemental sulphur and sulphur acids may be produced by reaction of H2S and O2, leading to high rates of corrosion, often with severe localised corrosion, section 6.4.3. Carbon steel is usually unsuitable, and other materials or modification of the environment have to be considered.
7.1.2 Low temperature carbon steel
From a corrosion viewpoint, low-temperature carbon steels (LTCS) behave identically to the standard carbon steels; they have been developed chiefly for use in low-temperature equipment which may have low minimum design temperature. This usually includes higher pressure equipment especially for welded pressure vessels, high pressure flare and some pipe systems where a sudden depressurisation could result in temperature drop. Selection of particular steel grades for any item of equipment depends upon the specified low temperature properties.
7.1.3 Corrosion Resistant Alloys
Where predicted corrosion rates for carbon steel are too high and where lining or other protection of the carbon steel is not practicable, Corrosion Resistant Alloy (CRA) materials will be selected in place of CS. Corrosion Resistant Alloys (CRAs) are essential for providing long term resistance to corrosion for many components exposed to corrosive environments. There are many CRAs to select from and this discussion is limited to the common options.
Key environmental parameters influencing the corrosion properties of CRAs are:
- Chloride ion concentration
- Partial pressure CO2
- Partial pressure H2S
- Environment pH
- Presence of other contaminants, principally oxygen and other acidic or oxidising contaminants.
Between them these parameters influence
- the stability of the passive film (initiation of pitting)
- ease of repassivation of initiated pits
- rate of dissolution of metal from pits (pitting rate)
- the risk of stress corrosion cracking (SCC) initiating and propagating (or SSC in ferritic & martensitic CRAs)
The aim in selecting CRAs for a given environment is to choose the most cost-effective one for which there is no risk of passive film breakdown. So the choice of alloy should be one for which the expected operating conditions are within the safe operating envelope of no pitting or cracking. The following sub-sections discuss the safe operating envelope for several standard CRA grades. In all cases, the environment is considered to be wet.
7.1.4 Martensitic Stainless Steel
220.127.116.11 Sweet Conditions
Martensitic stainless steel is usually used for non-welded components such as forged parts in compressors or as seamless threaded pipe for downhole production/injection tubing. It is not used for welded items such as piping or vessels as it requires a lengthy 2-stage post welding heat treatment which is often inconvenient or costly. The typical 13%chromium containing grades have good resistance to carbon dioxide. Figure 7.1 represents the safe operating envelope for API 13 Cr stainless steel exposed to wet CO2 containing NaCl but without any contaminants present . Review of published data suggests that the proprietary S13Cr alloys, referred to as Super 13Cr, Hyper 13Cr or Modified 13Cr and generally stronger than the basic API 13Cr grade, are suitable up to about 30°C higher operating temperature than the standard 13Cr grades in H2S -free environments.
In high CO2 concentrations the environment pH can drop below 3.5 and then there is a high risk of pitting because of the ease of breaking down the passive film in the martensitic stainless steels. Initiation of pitting is also affected by the presence of chloride ions at high concentration, so above 200g/l sodium chloride, the 13Cr group of materials cannot be considered. Such extremely concentrated chloride levels may arise under upset conditions at times within plant (by evaporation of a water phase with high level of dissolved solids leaving a hygroscopic salt deposit), or, more commonly, at the bottom of injection wells disposing into concentrated brine aquifers.
18.104.22.168 Sour Conditions
NACE MR0175/ISO15156-3 specifies a maximum H2S content of 0.1bar (1.5 psi) and minimum pH of 3.5 for martensitic stainless steels in tubing and general equipment. This also applies to the low carbon, “supermartensitic” stainless steel grades. The latest consensus from laboratory work and field data is that standard martensitic API 13Cr L80 can tolerate a little higher H2S than early publications seemed to suggest or than NACE MR0175/ISO15156-3 allows [ii]. Figure 7.2 shows the range of conditions where data indicates the material is resistant to sulphide stress cracking (SSC) and the region at low pH or higher H2S where the material will crack in standard SSC test conditions. This figure is based on a variety of data at chloride levels >50,000ppm, and different service ranges can be expected for extremely low or extremely high chloride contents.
The critical feature of this Figure 7.2 is the transition between no cracking and cracking which arises at pH 3.5 (test data established at room temperature). Recent publications also confirm the tendency for 13Cr family of materials to depassivate at below about pH 3.5 depending upon heat treatment condition and alloying composition .
The higher strength Super-13Cr grades have been found to be more susceptible to H2S than the standard 13Cr grade, probably reflecting its higher strength. One publication indicates the influence of material yield strength on performance, illustrating that the higher strength material has a greater risk for enhanced initiation of pitting, hydrogen embrittlement and sulphide stress cracking (Figure 7.3).
22.214.171.124 O2 – containing conditions
In the presence of oxygen the 13Cr stainless steels are considered to be “just” passive, but over time the surface does rust.
In the complex environment of CO2 plus some H2S and oxygen there is a significant pitting risk because the oxidised hydrogen sulphide forms sulphur on the steel surface which is a potent pit initiator. Martensitic stainless steels would not be considered suitable for corrosive (wet) service in the presence of H2S and O2 with chloride ions as pits would readily initiate. There is also an greatly increased risk of SSC with oxygen present.
7.1.5 Austenitic Stainless Steels
126.96.36.199 Sweet Conditions
The 300 series - stainless steels are a broad range of materials based around the standard AISI 304L grade and the higher Molybdenum-containing Alloy 316L. All these materials are resistant to corrosion in sweet environments; the more Mo-rich grades having a more stable passive layer and therefore being more suitable for CO2 environments with chloride ions present. The limits of environmental parameters for Alloy 316L in terms of NaCl%, partial pressure of CO2 and temperature are shown in Figure 7.4. This graph indicates a rather high sensitivity to chloride contents when the partial pressure of CO2 is very high.
188.8.131.52 Sour Conditions
ISO 15156-3 was published in first edition on 15/12/2003. It is subject to continuous revision with Technical Corrigenda being published from time to time and these have frequently affected the limits for austenitic steel. The table below summarises the currently published operating limits of austenitic stainless steels applicable in ISO 15156-3. Values stated are maximum limits of parameters allowed. The testing required for alloys to be included in ISO15156 nowadays requires rigorous exclusion of air from the test medium. Alternatively, materials are listed because of proven long experience in service.
|Material Type||Temperature, °C||Partial pressure H2S (psi)||Chloride conc. (mg/l)||pH|
|Austenitic stainless steels (including 304L)||60||15||Any||Any|
Note that these materials are not suitable in sour conditions with elemental sulphur present.
The limits of use of Alloy 316L in environments containing H2S and high chloride levels were established in an extensive review by TWI . In principle these tests were made with oxygen purging, but they pre-dated rigorous laboratory controls and it is considered that the lower limits they obtained are indicative of the impact of some (undefined small quantity) of air. This indicated that for chloride contents of 10 g/l the limiting partial pressure of H2S above which there was a likelihood of sulphide stress cracking was 0.9 bar over a wide temperature range from room temperature to about 225 °C.
At higher chloride contents the amount of H2S which could cause cracking was much less and is also sensitive to temperature; increasing the temperature increased the risk of cracking up to about 100 °C. Thus the maximum sensitivity to cracking was found to be with chloride contents above 100g/l at a temperature of 100 °C. At these conditions only 0.009 bar H2S was sufficient to cause sulphide stress cracking. Between room temperature and 100 °C the maximum tolerable partial pressure of H2S decreased from 0.9 bar to 0.009 bar when the chloride content is 100 g/l.
At temperatures continuously higher than 100 °C laboratory test data shows that the material can tolerate exposure to higher levels of H2S. However, in applications at temperatures above 100 °C it is considered dangerous to assume a higher limit for H2S because the equipment may experience lower temperatures at certain times, and cracking could arise in these periods, even if only fairly short in duration.
So for temperatures at or above 100 °C, the maximum allowed level of H2S is taken to be 0.009 bar when the chloride content is above 100 g/l.
184.108.40.206 O2 – containing conditions
In the complete absence of chloride ions the AISI 300-series materials remain passive in environments rich in CO2 and with oxygen and hence there can be useful niches where AISO 304L stainless is appropriate. However, such environments are rare as chloride ions are ubiquitous.
Where there is any substantial amount of oxygen present in the environment, along with chloride ions, AISI 300-series materials readily pit and crevice corrode at a high rate, even at temperatures as low as 10 °C. This class of materials is considered unsuitable for application where there is oxygen in the environment with chlorides, and particularly where there is also H2S or other sulphide species or acids.
Possible austenitic grades which may be considered for conditions with oxygen and low chloride-ion concentrations are those with increased molybdenum content relative to 316L (2.0%Mo), such as AISI 317 (3.0%Mo) and AISI 904L (4.5%Mo).
7.1.6 22 Cr and 25Cr (Duplex Stainless Steel)
220.127.116.11 Sweet Conditions
In sweet environments the resistance of 22Cr duplex stainless steel is very good. There is no risk of pitting or stress corrosion cracking of duplex stainless steels at up to 200 °C, even with sodium chloride content in the brine of 200 g/l (200,000 ppm). This equates to a chloride ion concentration of about 125 g/l.
The Superduplex 25Cr steels have a greater resistance to pitting than the 22Cr duplex steels, adding probably around 30 °C generally at any set of conditions compared to the 22Cr grade (Figure 7.5).
At more extreme chloride ion concentrations made from mixed sodium, magnesium and calcium chloride salts evaporated to form a concentrated brine slurry (230 g/l Chloride ion concentration), it was shown that both 22Cr and 25Cr duplex stainless steels were susceptible to stress corrosion cracking in the absence of oxygen at 140°C. These conditions were estimated to have arisen in some duplex stainless steel topside piping downstream of a choke valve with a high pressure drop and carrying a small volume of concentrated brine in the gas stream. Internal stress corrosion cracking was observed associated with the concentrated brine formed by evaporation of the produced water . The cracking problem was mitigated by upgrading material to Alloy 625. In the CCS context, similar conditions might occur within the injection well.
18.104.22.168 Sour Conditions
ISO15156-3 / NACE MR0175 allows 22Cr duplex alloys (Pren > 30) with upto 0.1 bar H2S and 25Cr superduplex alloys (Pren > 40) with up to 0.2 bar H2S, and without limits on chloride content or pH. Various published data has shown that the H2S levels can be extended with restricted pH or chloride levels . On the other hand, at low pH values below pH 3 (which are unusual in oil and gas production), the materials are less resistant than ISO15156-3 / NACE 0175 suggests iii.
Duplex stainless steels are most sensitive to sulphide stress cracking at around 80-100 °C and so test data at that temperature has been checked to establish the safe environmental limits. Cracking is also dependent on the pH and on the chloride content. The pH value is taken at room temperature since this is the value reported for the laboratory test data on which the limits are based. The limits of H2S as a function of pH and chloride content are given by the following graph (Figure 7.6).
Considering the 25Cr superduplex stainless steel, this is more resistant to hydrogen sulphide in general, but, as with the 22Cr grade, the limits of H2S all converge together at high chloride content and low pH (Figure 7.7).
22.214.171.124 O2 – containing conditions
The 22Cr duplex stainless steels are not highly pitting resistant and would not generally be selected for conditions containing oxygen if chloride ions were present at temperatures above 10 - 20 °C. The super-duplex (25Cr) grades are used up to 30 °C in aerated seawater and so would provide pitting resistance in some combinations of chloride ions and oxygen. As with the other stainless steels, the combination of CO2 / H2S and O2 could be dangerous if elemental sulphur was formed and adhered to the steel surface as it will initiate pitting in the duplex grades, with or without chloride ions present.
7.1.7 Nickel Alloys and Titanium
The nickel alloys (generally considered to include the 40%Ni-containing Alloy 825 and more highly alloyed grades) are effectively immune to CO2-corrosion and highly resistant to the presence of H2S as they have a strong passive layer which is relatively pitting-resistant.
However, in the presence of oxygen and chlorides, even these materials have limits as regards pitting and stress-corrosion cracking. In fully aerated warm brines, Alloy 625 is resistant to about 60 °C and Alloy C276 up to about 80 °C. Above 90 °C it is generally necessary to consider pure titanium or its alloys for handling hot aerated brines.
In the combined presence of H2S and oxygen, there is a significant pitting risk, because of the potential formation of elemental sulphur, a potent pitting agent. Where this arises, the most highly pitting resistant grades, such as Alloy C22, Alloy C276 and Alloy 59 have to be considered.
ii Intetech internal review of cracking data published in the period 1998-2008
iii ISO 15156-3 wording is “any combination of chloride concentration and in situ pH occurring in normal production environments are acceptable”: CCS environments with high CO2 pressures are arguably more severe than “production environments”.