Corrosion resistance behavior of nickel super alloys in caustic solutions

Nickel based super alloys are widely used in the chemical plants in the targeted applications. For instance, applications of Hastelloywire grade B2 is in handling hot reducing acids as it offers nominal corrosion rates in this media. Commercial nickel grade Ni 200 is used to handle hot caustic solutions. Other grades of Hastelloy family such as C276 containing Ni, Cr and Mo are versatile and can be used in almost each condition, although their function in hot reducing acids would be lower than Nickel-Molybdenum alloys and in hot caustic it would have a higher corrosion rate as compare to Nickel 200. Unlike austenitic stainless steels, nickel alloys prevent stress corrosion cracking in hot chloride conditions. Although nickel alloys may experience stress corrosion cracking in conditions of hot caustic and dilute hydrofluoric acid conditions.

Caustic Conditions

Caustic conditions involve highly concentrated solutions of sodium hydroxide or caustic soda, potassium hydroxide or caustic potash and calcium hydroxide or caustic lime that may be seen in the industries of oil refineries, pulp and paper. It is likely that the cracking sensitivity of Nickel alloys is related to a dealloying phenomenon.

Cross section of 0.6mm thick sheet of C276 that was in use for ten months in a heat exchanger between water and 50% NaOH and traces of perchlorate at temperatures about 100oC. Cracking occurred in the dealloyed layer subjected to the caustic solution. The supreme performing material in caustic conditions is commercially pure Nickel. Large magnitudes of molybdenum in nickel alloys are detrimental and chromium seems to be an advantageous element in high concentrations. During dealloying chromium and molybdenum dissolve leaving behind a porous pure nickel layer even also the alloy is plated on the surface with pure nickel. In slow strain rate conditions, Hastelloy C276 was prone to transgranular cracking in 50% NaOH at 147oC. Mill annealed and aged for 24 hour at 677oC, Hastelloy C22 resist cracking when kept in 50% NaOH solution at 147oC for 720 hours.

Inconel 600 experiences stress corrosion cracking in hot caustic solutions. Lab SCC test was conducted by using cylindrical slow strain rate samples and spring loaded bend beam samples of grades 600 and 800 in deaerated 10% sodium hydroxide solution at 550oF. Stress corrosion racking was noticed in both alloys, although alloy 600 offered better resistant to cracking as compare to alloy 800, feasibly due to higher nickel concentration. It is found that resistance to stress corrosion cracking increases with magnitude of Nickel in tan alloy, however there was an extensive variation in results depending on the hydroxide concentration and temperature. 

Machining of Nickel base super alloys

Machining of Nickel alloys should be done carefully by using sharp tools with positive rake angles. Adequate feed rate and depth of cut are essential and tools should be controlled to prevent rubbing. Even in the supreme conditions, stress can occur that may cause distortion of the work. For the highest dimensional stability, it is recommended to rough the part to size, stress relieve it and then finish it to size. Stress relieving has nominal influence on shapes, however may influence mechanical properties.

Category of alloys

Group A: Alloys comprise of 95% or more nickel. They have average mechanical strength and high hardness. They are hardened by cold processing. The alloys are gummy in the annealed and hot processed condition and cold processed material is preferred for the supreme machinability and smooth finish.

Group B: Comprises of standard nickel-copper alloys. They have higher strength and nominally lower hardness than those in group A. They are only hardened by cold processing. Cold drawn and stress relieved material provides the supreme machinability and smooth finish.

Group C: Comprises of solid solution nickel-chromium-iron alloys that are similar to the austenitic stainless steels. They are only hardened by cold processing and are machined readily in the cold-drawn or cold-drawn and stress relieved condition. These alloys are Inconel 600 wire, 601, Incoloy 800, 825 and Monel K500.

Group D: Comprises mainly of the age hardenable alloys. Group D1 comprises of alloys in the unaged condition. Group D2 comprises of alloys of group D1 in the aged condition, and many other alloys in aged and unaged conditions.

Cutting Fluid

Any cutting fluid can be used in machining nickel alloys. They respond well to general sulfurized mineral oil, sulfur offer enhanced lubricity and anti-weld properties. If the temperature of oil and work material is sufficient during machining to result in brown sulfur staining of the material. The stain can be removed with a cleaning solution of the sodium cyanide. It should be performed before heat processing including welding due to exposure to high temperature the staining may cause intergranular surface corrosion. To prevent intergranular corrosion, the components should be immersed in cleaning solution for sufficiently long time to prevent the stain. High speed machining operations create high temperatures that preclude the use of a sulfurized oil because of sulfur embrittlement of carbide tools. 

Materials for Solid fuel oxide cell for performance at high temperatures

Solid fuel oxide cell serves at high temperature with fuel like hydrogen gas or reformed natural gas on the anode side and air on the cathode zone. Moisture could occur on both electrodes hence in contact with metallic interconnects. Sulfur contaminants present in the fuel gas contact with the metallic interconnects. Sulfur contaminants present in the fuel gas stream are also anticipated to occur, however upstream desulfurization has been applied to reduce the sulfur contamination level to sub ppm or ppb levels. Hence, besides oxidation, interconnect could also experience sulfidation, hot corrosion and carburization. 

Thermal stresses produced in the SOFC stack because of large temperature gradients across the current collector could also speed up the corrosion process due to premature cracking and spallation of the oxide layer. The availability of complex gaseous species in the fuel condition also cause establishment of grain boundary corrosion, internal oxidation and localized metal loss causing overall reduction of component service life. Sulfidation refers to vigorous corrosion resulting into combined effects of oxidation and reactions with sulfur that may present in the fuel gas streams.

The metallic interconnect is also needed to have sufficient strength to help maintain the structural integrity of the stack during Solid oxide fuel cell service at high temperatures and under thermal cycling. The high temperature alloys for interconnect should have thermal fatigue resistance against feasible structure fracture during thermal cycling, creep resistance to maintain the size stability at high service temperature and rupture resistance to withstand peak thermal stresses produced during SOFC operation. The above stated strengths can be more or less correlated to the yield strength. For stainless steels, the compositions with higher yield strength often possess high creep and fatigue strengths.

Many alloys except annealed low carbon steels do not have standard yield strains, the stress is referred as yield strength. When feasible, the yield strength from bar analyses at room temperature and high temperature was gathered.

Nickel based superalloys

Depending on the ratio of chromium and aluminum, nickel based super alloys are classified containing Cr and Al into these categories:

A NiO scale with Cr2O3 and Al2O3 internal oxides for low chromium and aluminum concentrations.

An Cr2O3 scale with Al2O3 internal oxides for high Cr above 15% however low Al below 3%. An exclusive alpha- Al2O3 scale for considerably high chromium above 15% and high aluminum above 3%. The presence of an inner layer drastically decreases the local oxygen activity at the metal –interface so that an enrichment of alumina particles occurs. Alumina layer develops below the Cr3O3 inner layer that significantly improves the oxidation resistance and also acts as an electrical insulating layer. So, an aluminum concentration of 3% was established to be critical maximum. But none of used nickel base alloys contained chromium content higher than 18% and aluminum higher than 3%. Recommended nickel superalloys Inconel bars for use are Inconel 625.

Stainless steels

Stainless steels are popular for their oxidation resistance. They contain the compositions to withstand all temperatures and hence do not strengthening by heat processing although a few grades need. Austenitic steels are featured by larger linear thermal expansion coefficients. 

Development of super alloys for use in challenging industries

Superalloys have the properties that make them special among other materials. Often comprising of various elements they act as a complex entity as compared to traditional materials here often a single element dominates.

Considering the power production, the industrialization that mark the transformation of agrarian society to industrial society has brought social and economic development. It introduces technological innovation that is basically focused on large scale metal extraction and energy production. The power production sector faces new challenges with increasing demand of energy generation.

High temperature materials are developed to sustain their properties at high temperature even also retain their integrity with nominal environmental impacts. Gas turbines such as land based or aero based demanded the development of high temperature materials. These materials should possess the following properties:

a.       Potential to withstand stress at a service temperature near their melting temperature

b.      A significant resistance to mechanical degradation over increased period of time

c.       Withstand extreme service conditions

These high temperature materials are designed to meet the demands of energy efficiency and also reduce the emission of greenhouse gases. Superalloys are widely used in the gas turbine sector thus, it is crucial to understand the function of the different components in a gas turbine. Applications of Inconel bars in different components of gas turbines are volume flow machines. Gas turbines are mainly of the following types: land based and aero/jet engine. Major components of gas turbines are:

Compressor: It includes alternating set of rotating airfoils- rotors and stationary airfoils. It sucks air from outside and compresses it into the combustion chamber to increase in pressure ratio as well as enthalpy.

Combustion chamber: Compressed air enters into combustion chamber to get mixed with fuel and ignition occurs.

Turbine: Hot gas enters the turbine and enlarges resulting into extraction of mechanical work needed to drive the compressor that is performed by a shaft that connects the turbine to compressor.

Exhaust: Gases after combustion find their way through the exhaust. Usually gas turbine emissions are carbon dioxide, carbon monoxide and NOx.

There are different types of gas turbines following their use. The design of a turbine is based on its type like land based gas turbine or jet engine. The basic difference is the variation in thermodynamic cycles and turbine entry temperature that is often stable for land based gas turbine due to fewer start-up or shut down cycles as compared to jet engines where the TET can vary during the full flight cycle.

The hot components in the combustor and turbine are critical considering the thermal and mechanical loads and material used. While designing a gas turbine, main focus is on the TET because of steep increase in TET due to direct association of efficiency of gas turbines and TET and pressure ratio that increases the demands of high temperature functional alloys.

There are different super alloys used for applications in the aircraft industry, chemical plants, power production and nuclear power plants. They are fabricated in cast and wrought forms. 

Mesh applications in Electrowinning and fuel cells

Wire mesh is a porous media that is used in electrowinning cells. It can be installed in new or existing electrowinning cells. They can be fabricated to fit the cathodes. They can be reused by nominally washing off the precious metal while on the cathode. Specialists in the high performance metals describe the benefits of stainless steel mesh:

a.       Extensive reduction in slag: Decreased slag loss, enhanced metal recovery and smaller melting furnace needed.

b.      Durable mesh offers long service life

c.       Flux costs decreased

d.      Furnace liner life increased

e.      Extensive labor savings: The cathode is easy to install and installed once over the life of the mesh and does not need drying or meltdown

f.        Size stability: Prevents sag even after prolong use

g.       Stable wire size: Offers outstanding solution flow over large surface areas

h.      Prevents wire in bottom of cell

i.       

      Clean handling – user performance – No steel wool type and doesn’t harm the environment

Saves on labor and longer lasting. Durable mesh cathode wire size permits better wash results and prevents loose wire in the bottom of the cell. Cleaner handling with operator performance. More stable dimension.

The recycling of used fuel is a major problem for the sustainable use of nuclear energy. Pyroprocessing technology has attracted wide interest in terms of recycling of used oxide fuels from nuclear power plants and using them as fuels for fast reactors. By the integrated unit processes of pyroprocessing, the used oxide fuel is electrolytic reduction and electrorefining.

In the oxide reduction process as a front end process of pyrprocessing, the used oxide fuel contained in a permeable cathode basket is electrochemically reduced to metal in molten Li2O-LiCl salt at 650oC.

Usually stainless steel mesh is used as permeable basket containing fuel in oxide reduction and electro-refining process. The reduction product received after the oxide reduction is used as an anode in the ER process by loading it in a mesh basket. The separation of the reduction product from the oxide reduction cathode basket is needed. Although, it is difficult to separate the reduction product received after the oxide reduction from the stainless steel wire mesh cathode basket at ambient temperature without damaging or deforming the basket as the residual salt in the basket is frozen. It should be noted that the surfaces of the cathode basket including the walls of the side and the bottom made of permeable stainless steel wire meshes, except the welds for suitable salt draining after the oxide reduction. It was developed by sintering multiple layers into a single piece.

At the center of the cathode basket, the electrical conductor was brought into contact with the simfuel at the center of the basket. The anode assembly comprises of platinum connected to electrical conductor and the metal shroud. An anode shroud comprised of a lower porous shroud made of the same material. 

Function of Nickel based super alloys in powerful acids media

However wide corrosion data is available for prolong functional stainless steels, although data still lacks for the high performance alloys. It includes Nickel based super alloys with relatively high molybdenum concentration of about 30% for the chemical industry and super alloys for the gas turbine plant.

The corrosion rates in different acids are noticed. As anticipated, the highest corrosion rate occurred in boiling 10% HCl acid, the most powerful reducing acid. Highest corrosion resistance by Hastelloy wiregrade B2 is seen in HCl acid. Unlikely, it showed minimum resistance in boiling 10% nitric acid. As already seen in other study, the concentration of molybdenum is a major factor in deciding the corrosion function in hydrochloric acid, whilst the concentration of chromium is decisive in nitric acid.

The extent of corrosion resistance of Hastelloy B2 is not severely affected by cold deformation and welding. For cold forming of about 50%, cold forming with after welding and cold forming subsequent to welding, the stable corrosion rate was below 0.25 mm/a.

Behavior of alloys with regards to types of local corrosion called pitting, crevice and stress corrosion cracking is investigated. The concentration of chromium and molybdenum concentration is crucial for the resistance to pitting and crevice corrosion. These types of corrosion occurred by chloride ions and the availability of oxidizing agents increases the effect. 10% FeCl3 solution is normally used to evaluate the pitting and crevice corrosion of stainless and chemically resistant steels. 

Although this test solution is not sufficiently extreme for nickel based alloys that usually contain large magnitudes of molybdenum. To evaluate the pitting corrosion of nickel based alloys, solution comprising of 7% sulfuric acid + 3% HCl + 1% FeCl3 + 1% CuCl2 was used to stimulate the conditions in scrubbers. In these test solutions and at the higher temperatures of 323K and 375K, Hastelloy C276 did not receive pitting or crevice corrosion due to high pitting index of about 70. It should be considered that at this stage in presence of crevices, crevice corrosion occurs before pitting. The materials active in crevice and pitting corrosion on the surface is prevented by cathodic protection by the anodic crevice region.

Incoloy 825 was also evaluated. The main objective to correctly determine the critical pitting corrosion potential using electrochemical methods because this potential allows a reliable comparison of the pitting corrosion behaviour of different material in a medium. The problem related with the early appearance of crevice corrosion on surface should be understood as it prevents the correct determination of the critical pitting corrosion potential. Besides, it is observed that the potentiodynamic quick test with a ramping rate of 1000mV/min that is usually used, does nt permit differentiation of the pitting corrosion specifically of the nickel based alloy.

The study aimed on potentiostatic holding analyses with the holding periods of 24 hours for every potential step of 100mV, measured against a saturated calomel electrode. The sample was evaluated for local corrosion after each holding period. For practical applications, there is a damage limit, irrespective of different types of corrosion. Hastelloy G is evaluated for 24 hours tests in five different solutions at 343K or 70oC. Nickel based alloys Inconel 625, Hastelloy C22 and Hastelloy C276 showed suitable performance.