Typical corrosion resistant nickel based super alloys are a solid solution Nickel whilst few are precipitation hardened materials uch as In 718, 625 and X750. However Ni alloys significantly consist of other alloying elements, still nickel retains its FCC structure from the nickel base element content. Therefore many of these alloys offer outstanding ductility, malleability and formability. They are also easily weldable.

Nickel based alloys are categorized as high temperature corrosion resistant alloys and lower temperature materials. The alloys produced for applications at the elevated temperatures are known as high resistant alloys (HRAs) while those for low temperature aqueous or condensed systems are known as corrosion resistant alloys (CRA).

Chemistry and corrosion resistance of Nickel based alloys

While inhibiting corrosion in the several conditions, nickel based alloys offer superior performance as compare to stainless steel grades. It is because nickel can be combined more widely than iron. Various elements can be added in the large percentage depending on the application requirement. Normally, the industrial corrosive conditions can be categorized as reducing and oxidizing media. The reducing conditions are controlled by hydrogen discharge from acids for example hydrochloric or sulfuric acid. The oxidizing potential may be developed by cathode reactions for example reduction of dissolved oxygen from atmosphere, chlorine gas, hydrogen peroxide, chromate, nitric acid and metallic ions solutions etc.

As every material suffers from corrosion, nickel alloys are not an exception. These metallic substances are attacked by uniform corrosion and localized corrosion. Uniform corrosion occurs in the reducing and oxidizing media. On the other hand localized corrosion for example pitting and crevice corrosion normally occurs in the oxidizing conditions. Stress corrosion cracking (SCC) or called as environmentally assisted cracking (EAC) may take place at the electrochemical potential range.

From the chemical composition point of view, corrosion resistant Nickel based alloys are categorized as – pure nickel, nickel-copper alloys, nickel-molybdenum alloys, Ni-Cr-Mo alloys and Ni-Cr-Fe-Mo alloys.

Commercially pure nickel is used to deal with highly concentrated caustic media. Nickel is mildly corroded in the warm caustic solutions as compare to nickel alloys as composition agents chromium and molybdenum dissolve in these alloys. Nickel can withstand cold reducing acids as slow discharge of hydrogen on its surface. Warm reducing acids and oxidizing acids rapidly attack pure nickel.

The key use of Nickel-Copper alloys or Monel grades is to deal with pure hydrofluoric acid.

Although, in the oxidants like oxygen in hydrofluoric acid, nickel-copper alloys are resistant to general corrosion as compare to Nickel 200 in hot reducing and oxidizing acids for example sulfuric acid and nitric acid.

Nickel-Molybdenum alloys normally known as Hastelloy B grades were developed to withstand reducing HCl at all concentrations and temperatures. Additionally, more costly materials for example tantalum and Nickel-Molybdenum alloys are recommended for warm hydrochloric acid. Ni-Mo alloys are also employed in handling other corrosive reducing media for example dilute sulfuric, acetic, formic and hydrofluoric acids. Alloy B2 has the minimum corrosion rate in boiling 10% sulfuric acid. But, Nickel-Molybdenum alloys do not perform good in the oxidizing acids such as in hydrochloric acid mixed with ferric ions.

There are several commercial NiCrMo alloys that were originally formed from Hastelloy C (N10002) that was introduced into the market in 1932. Advanced grade is Hastelloy C2000 however more commonly used industrial alloy is grade C276. Hastelloy grades are the most flexible nickel alloys as they comprise of molybdenum for security against corrosion in the reducing conditions and chromium that secures from corrosion in the oxidizing conditions. Hastelloy C276 has minor corrosion rates in reducing and oxidizing conditions. The chief applications of Hastelloy alloys are in the presence of hot chloride solutions. In these media, many stainless steel grades suffer from crevice corrosion, pitting corrosion and stress corrosion cracking. Although, Ni-Cr-Mo alloys are extremely resistant to chloride induced attack in the several industrial applications.

At last, Ni-Cr-Fe alloys such as Incoloy 825 that is less resistant to corrosion as compare to Ni-Cr-Mo alloys. Although, these are less superior in preventing  corrosive attack as compare to Ni-Cr-Mo alloys, they are more economical and hence have extensive uses in the several industrial applications where applications of stainless steels are limited. Alloy 825 is better resistant to sulfuric acid than Inconel 600 because the foremost consists of molybdenum and copper which significantly prevents the attack of sulfuric acid. Incoloy 825 is mildly corroded in nitric acid. Alloy G30 is used in the production of industrial phosphoric acid and nitric acid.

Cracking behavior of nickel alloys

The chief drawback in the use of nickel alloys is not environmental assisted cracking as these materials are less sensitive to EAC as compare to austenitic steel grades. Mill annealed nickel CRAs possess tensile strengths below 1000 MPa and wide elongation to damage such that they are not sensitive to failure mechanisms related to hydrogen uptake. The nickel alloy if processed by cold working or elevated temperature aging they become less ductile and their sensitivity to hydrogen embrittlement may increase. The specific conditions causing EAC in nickel are hot caustic and wet hydrochloric acid.

Following table shows various embrittling conditions for different nickel based alloys that cause embrittlement:

 Pure Nickel:

Generally, pure metals are less prone to EAC as compare to alloys because metals have extensive ductility and smaller mechanical strength. Commercially pure nickel is not sensitive to stress corrosion cracking excluding heavily cold processed conditions in the availability of elevated temperatures above 250oC concentrated caustic solutions and liquid metal.

Monel Grades:

Nickel-Copper alloys such as Monel 400 is not sensitive to stress corrosion cracking due to its small mechanical strength and good ductility. This alloy is attacked by SCC in acidic solutions that include mercury salts , liquid mercury, hydrofluoric acid and fluosilicic acid. The age hardenable Monel K500 is used in marine water applications particularly as shafts and bolting for its high strength, although K-500 may be sensitive to hydrogen embrittlement (HE). Monel K500 is also widely used in sour service in oil plants. Cracking in K-500 is found in many oil units particularly due to hydrogen embrittlement caused by cathodic security or if the alloy is combined to less corrosion resistant carbon steel.

Nickel-Molybdenum alloys

Nickel-Molybdenum alloys are used in the high temperature reducing acids. These alloys comprise of about 30% molybdenum that makes them extremely resistant to general corrosion in the reducing acids. Also as they contain about 70% Nickel they become resistant to SCC in hot concentrated chloride solutions. Hastelloy B and B-2 were free from cracking in the test for 7 days in 25% sodium chloride and in chloride concentration of CaCl2 and MgCl2 at 121oC, 149oC, 177oC, 204oC and 232oC. Cracking was not noticed for 50% cold processed alloy B2 the hardness is high. Although B2 and alloy B3 are subjected to temperatures in range 550oC to 850oC, their ductility is reduced by solid phase transformation that produces intermetallic phases.

Hastelloy B3 was produced as an enhancement in the production of intermetallic and short ordering phases while welding and hence they offered improved resistance to SCC. Alloy B2 is damaged by intergranular stress corrosion cracking in the heat affected region when subjected to organic solvents comprising of sulfuric acid at 120oC. In the hydroiodic acid at temperatures exceeding 177oC, alloy B2 was sensitive to transgranular stress corrosion cracking.

Nickel-Chromium-Molybdenum Alloys

The main drawbacks of stainless steels is that these alloys are sensitive to chloride induced localized attack like pitting and crevice attack and stress corrosion cracking. These alloys for example Hastelloy C276 are the extremely resistant materials to the chloride induced localized attack that limits the application of austenitic stainless steels. In few environments, SCC occurred in high strength NiCrMo alloys, although cracking only noticed in the severe conditions such as up to 200oC, pH below 4 and in presence of H2S. U-bend samples of C2000, C22 and C276 materials were free from cracking in boiling 45% MgCl2 solution in the test of 1008 hours. Grades C4 and C276 are resistant to cracking in 25% sodium chloride solution at temperature of 232oC although these materials were sensitive to cracking in MgCl2 solution of the equal chloride concentration at the same temperature limit. Grade C22 was resistant to SCC in 20.4% MgCl2 solution up to 232oC and also in the 50% cold processed form and in 50% cold processed and aged form at 500oC for 100 hours. Alloy C4 and Inconel 625 are immune to cracking in humidity content of 72% at temperature 18.5oC in hot gases.

When Ni-Cr-Mo alloys are aged at temperatures up to or above 600oC for prolong periods such as up to 1000oC, prolong ordering reactions and precipitation of tetrahedrally close packed (TCP) phases may be formed. These phases are formed by thermal aging that may widely decrease the ductility of Hastelloy grades. Annealed C276 alloy’s yield stress at room temperature is 360 MPa, ultimate tensile stress is 807 MPa and elongation rupture is 63%, although for aged Hastelloy C276 at 760oC for 16000 hours, the yield strength improved to 476 MPa and tensile strength enhances to 894 MPa and extension to rupture reduces to 10%.

Heat aged Hastelloy C276 was prone to hydrogen induced cracking in conditions of hydrogen sulfide (H2S). NiCrMo alloys were attacked by induced cracking in media of super critical water oxidation. It has been observed that Hastelloy C276 and Inconel 625 are attacked by intergranular cracking when subjected to different aqueous solutions in the region of critical point of water at 374oC.

Nickel-Chromium-Iron-Molybdenum alloys

It is one of the biggest groups in the family of nickel alloys that include alloy 600, alloy 825, alloy 800 and Hastelloy G30. Alloy 600 is employed in the fabrication of steam generator tubes in the nuclear power plants. It is a broadly studied material for its immunity in stress corrosion cracking in warm water and caustic media. It is attacked by SCC in the elevated temperature pure water above 300oC in service and in lab. Considering its significance in the nuclear power sector, the SCC of alloy 600 in pure water and in caustic environments has been widely analyzed in the past 30 years. The cracking of this material is widely based on factors including temperature, extent of tensile stress, deformation rate, hydrogen gas, pH of solution and electrochemical potential. The metallurgical aspects include the presence of nominally alloying elements called as contaminants, extent of cold and heat processing. The alloy is commonly attacked by intergranular type of cracking in the nuclear applications. In few conditions, cracking may be of transgranular type.

Incoloy 800 is also employed in the production of nuclear plants. In the steam generators, this alloy is superiorly resistant to cracking as compare to Inconel 600 due to moderate Nickel composition in Incoloy 800. Alloy X750 is also used in the internal components of nuclear reactors such as fasteners. But it is also prone to SCC in the elevated temperature water in nuclear reactors. Incoloy X750 is also sensitive to hydrogen embattlement at temperatures below 150oC. Equivalently, low temperature hydrogen induced cracking was also noticed in Inconel 718.

Incoloy 825 is superiorly immune to stainless steel grade 316 in the chloride media due to larger content of nickel. it was sensitive to transgranular type of stress corrosion cracking in 45% MgCl2 solutions at high temperatures exceeding 146oC. It is widely employed in oil and gas production in sour wells but the service of other nickel alloys for example C276 is better than 825. The former alloy is used in the cold processed form for enhanced strength. Environmental factors may influence the stress cracking functionality of alloy 825 in oil and gas wells such as temperature, magnitude of chloride and hydrogen sulfide gas.

The components of alloy G30 used in the industrial formation of hydrofluoric acid are attacked by cracking but they were not attacked after subjected to 45% MgCl2 solution at 154oC for 500 hours.

Application resistance to stress corrosion cracking (SCC)

Nickel based corrosion resistant alloys are extensively used in the chemical processing plants in the severe applications. Hastelloy B2 is used to handle warm reducing acids that are extremely aggressive. The commercial nickel 200 is used to fabricate system components for handling hot caustic solutions. Hastelloy C grades such as C276 is a versatile material that can be used in each condition although their functionality in the hot reducing acids is lower than Nickel-Molybdenum alloys and in hot caustic it would receive higher corrosion rate than Nickel 200. Contrast to austenitic steel grades, Nickel alloys are resistant to SCC in warm chloride conditions and also prevent SCC in conditions such as hot caustic and wet hydrofluoric acid conditions.

Caustic conditions

Caustic media is an intensely concentrated solution of sodium hydroxide or caustic soda, potassium hydroxide or caustic potash and calcium hydroxide or caustic lime that occurs in CPI and other industries for example oil refineries and pulp and paper. It is likely that the cracking sensitivity of Ni alloys is related to deallloying mechanism. Nickel offers the best service in the caustic media. Considerable magnitudes of Molybdenum in nickel alloys are damaging and chromium is an advantageous element in the high concentrations. In the slight strain rate conditions, alloy C276 is found to be sensitive to transgranular cracking in 50% NaOH at 147oC. The mill annealed and aged for 24 hours at 677oC samples of alloy C22 did not attain cracking subsequent to immersion in 50% NaOH solution at 147oC for 720 hours.

Inconel 600 is attacked by stress corrosion cracking in hot caustic solutions. Inconel 600 and Incoloy 800 were attacked by SCC in deaerated 10% NaOH solution at 550oF or 288oC. Inconel 600 contains higher content of Nickel therefore it is superiorly resistant to SCC as compare to Incoloy 800. Alloy 600 also offers better performance than Inconel 690 due to the same fact because nickel significantly resists attack in the caustic conditions.

Wet hydrofluoric acid

However hydrofluoric acid is considered as a weak acid, it is widely attacking in nature and only a few materials handle it adequately. Monel alloy 400 and Hastelloy C276 are tested for resistance to SCC considering the acid content and temperature limit. Alloy 400 was corroded in HF media in the vapor phase however it was secured in liquid phase. Contrast, Hastelloy C276 was secured in the both states. Internal crack penetrations from flat unstressed samples subjected to HF were noticed for Monel 400, Inconel 600 and Incoloy 825. Alloy 400 prevented cracking in liquid region of 20% HF. It was an extensively stressed material suffering from SCC in ammonia vapors at 300oC. Heat treatments that prevent residual stresses and cold processed microstructures significantly decrease the sensitivity of Monel 400 to variety of environmentally induced cracking (EIC).

The U bend samples had described that Ni-Cr-Mo alloy grades for example C22, C2000 and C276 were sensitive to SCC in wet HF in liquid and vapor phases. The degradation appearance in Hastelloy C22 was observed subsequent to 20% HF at 93oC for 240 hours. The corrosion in the liquid phase was more intense than that in the vapor form. Few attacks were occurred by stresses however it is evident that HF also increases internal corrosion on the plain relieved samples. C2000 is a resistant alloy to the cracking in wet HF as it consists of 1.6% copper concentration. Alloy 400 is more suitable to use in the vapor phase rather the liquid state referring the availability of chromium is advantageous for HF vapor phase applications. Inconel 600 was sensitive to cracking and internal penetration in HF conditions.

Materials for Oil and gas

A variety of materials are utilized in oil and gas industry for oilfield conditions on the base of severity of environment. Earlier carbon and low alloy steel were used in this sector due to convenience of operations and cheap production of oil. But in the present time the crude refining has become tougher and stronger materials are demanded.

The reliable and high performance materials are austenitic grades such as stainless steels and nickel based alloys.

The natural factors that accelerate the corrosion rate of alloys are temperature, chloride content, partial pressure of hydrogen sulfide and carbon dioxide, sulfur and pH level of solution. High temperatures, large chloride content and partial pressure of H2S and small pH make the condition extremely severe. Generally the factors that have a crucial role in the selection of materials are – mechanical characteristics and corrosion resistance. For oil plant tubular and tubing products, lowest crucial yield stress are 550 MPa for shallow wells and above 1100 MPa for deep wells.

Nickel alloys with large content % of chromium and molybdenum are superiorly resistant to corrosion. Annealed and cold processed solid solution Nickel based alloys and precipitation hardened alloys are suitable for use in the oil wells. For example cold processed and solid solution alloy C276 (UNS 10276) can be used up to 218oC or 425oF in the conditions of 700 kPA or 100 psi partial pressure of hydrogen sulfide and chloride content and corresponding in situ pH offered the highest hardness below 40 HRC and yield strength is below 1034 MPa.

Alloy C276 is suitable for use at any chloride concentration and partial pressure of H2S and CO2 about 260oC. Although the availability of elemental sulfur may result in catastrophic cracking in this alloy. Various nickel based alloys that are recommended for use in Oil and gas plant to prevent SCC attack for sour well applications are Incoloy 825, Hastelloy C22, age hardenable Inconel 625, Inconel 718 etc.

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