Inconel 625’s performance in wet corrosive conditions

Inconel alloy 625 is known for its high strength, outstanding fabricabiity and weldability and supreme corrosion resistance. The great and versatile corrosion resistance of alloy 625 in a wide range of temperatures and corrosive conditions is the major reason for its wide applications in chemical processing and other processes. This post shows the performance of alloy 625 in various wet conditions.

Hydrogen Embrittlement

The degree of sensitivity to hydrogen embrittlement is measured as ductility loss. Inconel 625 holds moderate ranking as compare to Incooloy 800, SS 316, SS 304, In 825, In X750, In 718 and Monel 400.

Intergranular corrosion

Inconel alloy 625 is stabilized against intergranular corrosion by precipitation of niobium carbides at 927oC to 1038oC annealing temperature. Niobium carbides interact with carbon to make it less available to precipitate as chromium carbides in the grain boundaries. Chromium depletion near the grain boundaries is an effect of intergranular chromium carbide precipitation. This process occurs at specific temperatures and is called as sensitization. Sensitization of an alloy makes it sensitive to intergranular corrosion. Inconel 625 can become sensitive to this corrosion by an inadequate annealing that would prevent the development of niobium carbides by subsequent sensitizing treatment. Corrosion occurs by intergranular attack of Inconel 625 due to sensitization of the grain boundaries.

Annealing samples at different temperatures followed by subjecting them to sensitizing temperatures in the range of 704oC to 871oC for one hour. The samples are tested in boiling 65% nitric acid allowing the precipitation of chromium carbides at the grain boundaries when an alloy is exposed to sensitizing temperatures of 704oC to 871oC. At lower annealing temperatures, stabilizing niobium carbides are precipitated, hence a subsequent sensitizing heat processing of 704oC to 871oC will result into nominal or no precipitation of chromium carbides at the grain boundaries.

Corrosion in bleach plant condition

Pulp and paper bleach plant conditions are studies. Exposures in solutions varying from pH 1.4 to 9.5 with up to 5500 ppm chlorides and 80oC with strong chlorine base oxidizers present were made on 38 samples of 26 different materials. The test included 8 chlorination stage, nine chloride dioxide stage and three hypochlorite stage bleach plant conditions. Inconel 625 ranks high in offering corrosion resistance in bleach plant conditions. High chromium and molybdenum concentration is responsible for Inconel 625’s pitting resistance in this condition.

Marine water corrosion

Inconel alloy 625 stands high in sea water conditions owing again  as it contains high chromium and molybdenum concentration.

The most corrosion resistant alloys are – Hastelloy C, super alloy Hastelloy C276 wire, Inconel 625 and Hastelloy X

Very resistant alloys are – Inconel 718

Alloys resistant while receiving some pitting are – Inconel 600, Inconel X750, Incoloy 800, Incoloy 825, Monel 400 and K500.

Inconel alloy 625 often acts as a cathode when placed in contact with other materials in sea water. Alloy 625 offers good corrosion fatigue strength in sea water. 

How nickel alloyed with other elements improves the application experience

Nickel alloyed with molybdenum offers remarkable increase in resistance to reducing conditions for example HCl. The corrosion potentials of these alloys in acidic media both aerated and deaerated end to be more active than their Flade potentials, thus the alloys are not passive. For instance, the corrosion potentials of nickel based alloys containing 3 to 22.8% Molybdenum in 5% sulfuric acid concentration, hydrogen saturated, all lie within 2mV of a platinized platinum electrode in the same solution. Failed against active corrosion potential, the alloy with 15% molybdenum for instance is corroded at 1/12^th rate of nickel in deaerated 10% HCl at 70oC and the rate reduces further with increasing molybdenum concentration.

Molybdenum alloyed with nickel has nominal effect on hydrogen overpotential however increases anodic polarization thus the corrosion rate of alloy is anodically controlled. The method of increased anodic polarization is related most likely to a sluggish hydration of metal ions released by molybdenum or to a porous diffusion barrier layer of molybdenum oxide instead the development of a passive layer typically of chromium or the passive chromium-nickel alloys.

As an alloying element in nickel, tungsten acts similarly to molybdenum however it is less effective. Because Ni-Mo alloys have inadequate physical characteristics such as low ductility and low workability, other elements such as iron is added to develop multi element alloys. These are hard to work however they offer a significant improvement. These alloys offer better corrosion resistance against HCl and H2SO4 acids than nickel however it is not enhanced with respect to oxidizing media for example HNO3. As the NiMoFe alloys have active corrosion potentials and hence they do not create passive-active cells and they resist pitting in strong acid media.

Alloying nickel with molybdenum and chromium, an alloy is received that is resistant to oxidizing conditions offered by chromium element and reducing conditions with the contribution of molybdenum. An example of such alloy that also comprises of iron and tungsten is Hastelloy C276 bar resistant to pitting and crevice corrosion in sea water that even doesn’t tarnish noticeably when exposed to seawater conditions.

Some commercial Chromium – Nickel – Iron – Molybdenum alloys corresponding to composition to high nickel stainless steels contain some content of copper. They are made to prevent corrosion against sulfuric acid to its any concentration level. The performance of alloyed copper is identical to alloyed palladium in titanium to accelerate the cathodic reaction to the level where the anodic current density reaches or exceeds the critical value for anodic passivation.

Nickel-Copper alloy system

As nickel and copper in all concentrations develop solid solutions, production of various nickel-copper and copper-nickel alloys is feasible. A common and highly popular nickel-copper alloy is Monel alloy 400 that is widely used in the modern industries. Containing 31% copper, it offers similar corrosion resistance as of nickel in various manners however better than nickel in some ways. As alloy 400 prevents corrosion in high speed seawater, it is commonly used for valve trim and pump shafts. 

Gradual development stages of Hastelloy C alloys for better corrosion resistance

To overcome the problems of solution annealing of Hastelloy C after welding, the chemical composition of alloy C was modified that reduced carbon and silicon content. This change was made possible by AOD process – an advanced melting technique. Low carbon and low silicon alloy was called as Hastelloy C276 that offered similar corrosion resistance as of alloy C however without negative effects of continuous grain boundary precipitates in the weld HAZ of alloy C276. This material could be used in many applications in the as-welded condition without severe intergranular corrosion. The corrosion behavior of alloy C and alloy C276 has been adequately covered. The grain boundary precipitation mechanism and time temperature transformation for these Hastelloy grades are also documented.

Applications of Hastelloy C276 wire in the process industries are wide, different and versatile for its supreme resistance in both oxidizing and reducing conditions, in fact also with halogen ion contamination. Although there are specific process conditions where alloy C276 with its low carbon and silicon content is sensitive to corrosion because it is not perfectly heat stabilized in correspondence to precipitation of carbides and intermetallic phases. Within the broad scope of chemical processing, examples are where extreme intergranular corrosion of a sensitized microstructure has occurred. To overcome this sensitivity, an enhancement was made in Hastelloy C276 that was called Hastelloy C4. In highly oxidizing media, both alloy C276 and C4 containing 16% chromium offer significant resistance. This limitation has lead to the development of other alloys for example Hastelloy C22.

Hastelloy C4

Wide reduction in carbon and silicon content in alloy C, removal of tungsten from its composition and reduction in iron and addition of titanium, have offered dramatic improvement in the precipitation mechanism of intermetallic phases when subjected to the sensitizing range of 550oC to 1090oC for longer periods virtually preventing the intermetallic and grain boundary precipitation of the mu phase and other phases. These phases affect ductility, toughness and corrosion resistance. The general corrosion resistance offered by Hastelloy C276 and Hastelloy C4 are basically same in various corrosive conditions except in highly reducing media such as HCl, Hastelloy C276 offers better performance however in strong oxidizing conditions, Hastelloy C4 is superior.

Hastelloy C4 offers supreme corrosion resistance to a wide range of media including organic acids and acid chloride solution. In contrast to alloy C276 that is widely used in the world, alloy C4 is gradually replaced by alloy 59.

Hastelloy C22

 Hastelloy C22 was developed to hold superior oxidizing properties of Inconel alloy 625 while retaining the localized corrosion resistance properties of Hastelloy C276. It was developed by eliminating tungsten, decreasing iron content at the risk of decreased corrosion resistance in oxidizing chloride solutions where tungsten works good. Additionally, both Hastelloy C276 and C4 are attacked quickly in oxidizing, nonhalide solutions due to their low chromium concentrations of about 16%. This alloy composition with about 21% chromium, 13% molybdenum, 3% tungsten, 3% iron with balance nickel offered better corrosion resistance than alloy C276 and C4.

Which Alloys prevent environmental degradation?

Nickel based alloys containing high nickel content have a wide range of applications. The electrochemical properties of nickel and its crystallographic characteristics enable it to accommodate large magnitudes of alloying elements. The alloys utilized for corrosion resistant purposes are primarily solid solution strengthened alloys supplied in the fully annealed form.

Nickel is nobler than iron however more active than copper in electromotive series. In the reducing conditions for example dilute sulfuric acid, nickel offers superior resistance than iron however is less resistant than copper or NiCu alloys. The nickel-molybdenum alloys offer superior resistance in reducing acid media as compare to nickel or nickel-copper alloys.

Nickel can develop a security passive layer in some conditions. Although this passive layer is not specifically stable and hence nickel cannot be usually in oxidizing conditions like nitric acid. When alloyed with chromium it develops a more stable layer and corrosion resistance is attained to a variety of oxidizing conditions. Although these alloys can corrode in environments comprising of significant magnitudes of chlorides or other halides, particularly if oxidizing species are present. Combination with molybdenum or tungsten is essential to enhance its corrosion resistance by oxidizing chloride solutions.

Monel alloy 400

Alloys containing 33% copper with nickel produces Monel 400, offers an alloy with several characteristics of pure nickel with enhancements. A major region of application is in water handling, including brackish water and marine. As similar to nickel 200, alloy 400 pits in stagnant seawater however the corrosion rate is significantly decreased. The unavailability of chloride stress corrosion cracking is also a factor in choosing this alloy.

The general corrosion resistance offeredby Monel 400 wire in nonoxidizing acids like sulfuric, hydrochloric and phosphoric is better than pure nickel. This alloy cannot withstand oxidizing media for example nitric acid, ferric chloride, chromic acid, wet chlorine, sulfur dioxide or ammonia.

Alloy 400 attains outstanding resistance to hydrofluoric acid solutions at different concentrations and temperatures. It is exposed to SCC in moist, aerated hydrofluoric or hydrofluorosilicic acids and vapor. Monel corrosion is nominal in all kinds of environments. Indoor exposure develops an extremely light tarnish that is easily eradicated by occasional wiping. Outer surfaces subjected to rain develop a thin gray green patina. In sulfurous conditions, a smooth brown adherent film develops.

As high nickel concentration of Monel 400 is resistant as nickel 200 to caustic soda throughout the concentration range. It also resists anhydrous ammonia and to ammonium hydroxide solutions of about 3% concentration. It is subjected to stress corrosion cracking at high temperatures, in concentrated caustic and in mercury.

Incoloy alloy 800

Alloy 800 contains about 20% chromium, 32% nickel and 46% iron as balance. It is primarily used for its oxidation resistance at high temperatures. At average temperatures the general corrosion resistance of Incoloy 800 is similar to that of other austenitic nickel-iron-chromium alloys. Although with increase in temperature, alloy 800 continues to attain supreme corrosion resistance whereas other austenitic alloys are unsatisfactory for this service. It prevents corrosion in nitric acid at concentrations up to 70%. It prevents corrosion in variety of oxidizing salts, however not halide salts.  

Performance of Inconel alloy 625 in industrial corrosive factors

Carburization

Development of stable oxide layer on the surface is a key to the carburization resistance of an alloy. A test was designed to determine the relative high temperature carburization resistance of complex  alloys used in ethylene furnaces. A test included exposing alloy to a mixture of hydrogen and methane. Specimens are regularly eliminated for weighing by pulling the samples to the cold end of the test container and flushed with argon to reduce oxidation. A mass change in H2 – 1% methane at 1000oC for around 1000 hours for different commercial alloys. Alloy 625 offers supreme carburization similar to alloys that are directionally solidified and SS 330 and higher than Incoloy 800 and SS 309. Alloy 625 offers better performance than Incoloy 800 and Inconel 600.

Chloride based conditions

Chlorine is a major factor in various industrial process conditions for example mineral chlorination, ethylene dichloride and vinyl chloride monomer development, aluminum smelting and refining, fuel element reprocessing and heat recovery units. High temperature halide corrosion is noticed in fossil fuel boilers, coal gasification unit, gas turbines and in municipal and chemical waste incinerators.

Municipal waste contains about 0.5% halides on dry basis. Many lab studies imitating waste incineration conditions have been limited to temperatures varying from 120oC to 650oC or 248oF to 1202oF. As nickel and nickel based alloys are widely popular for offering resistance to halogen corrosion, they are successfully used in the lab temperature in simulated HCl conditions. 

The temperature limit chosen was about 593oC – 927oC or 1100oF  - 1700oF. This temperature level is common for flue gas studies. Boiler internals and flue stacks could specifically be expected to attain heat exposure in this temperature limit. The simulated condition chosen for the evaluation was nitrogen, carbon dioxide, oxygen, hydrochloric acid, Hydrogen bromide and sulfur dioxide.

It is found most alloys received negative mass variations resulted by spalling, vaporization losses or both. The rate of metal loss increased with increase in service temperature. High iron alloys showed higher mass changes at 704oC as compare to at 927oC. Alloys containing high iron content received wide internal oxidation. The morphology of corrosion scale at high temperature different from that at lower limits. At high temperature, voids could be commonly seen that were more visible in high iron alloys. Inconel alloy 600 and Inconel 625 wire offered outstanding resistance to corrosion at the more higher temperature.

Alloy 625 containing high nickel, chromium, niobium and molybdenum offers supreme aqueous corrosion resistance in a wide range of vigorous conditions. In various environments, its resistance is similar to Hastelloy G3, C276 and Hastelloy C22. It prevents general corrosion and intergranular corrosion in various environments of chemical processing plants. It also resists chloride and sulfide stress corrosion cracking, hydrogen embrittlement in oil containing tubular items. It also prevents general and localized corrosion in the pulp and paper plants. In addition of it, the alloy’s various other characteristics made it fit for use in heat exchangers. 

How Inconel 600 is influenced by dynamic strain aging

Dynamic strain aging (DSA) of Nickel based alloy Inconel 600 is analyzed. Dynamic strain aging is applied on alloys that are used in nuclear power plants.

In austenitic stainless steels carbon and nitrogen at high temperatures can interact with dislocations and result in to occurrence of DSA serrations. In nickel base alloys interstitial and substitutional solute atoms often cause a jerky flow.

DSA occurs in alloy Inconel 600 wires over a wide range of temperature and strain. Interstitials have a special role in DSA of pure nickel that has been found for Nickel-Carbon and Nickel-Hydrogen interstitial alloys. In Nickel-Carbon alloys the lower critical temperature of DSA had an activation enthalpy of half of wide diffusion of carbon and it was related with carbon diffusion in the dislocation core. Moreover it was noticed that serrations were not influenced by cooled vacancies. DSA temperature in Inconel 600 was close to austenitic stainless steel and much higher than Nickel-Carbon binary alloy.

Inconel 600 is commonly used in vessel head penetrations in pressurized water reactors. Primary water stress corrosion cracking and intergranular stress corrosion cracking in parts in addition of steam generators are issues of nuclear power plants.

Various uniaxial tensile tests were performed in lab air in axial displacement control with constant cross head speeds on Inconel alloy 600. A split design furnace with temperature control was used for tests at high temperatures. An elevated temperature. Inconel 600 was used in mill annealed condition and was cut by using water jet cutting and spark erosion methods to develop the samples.

Base material in pre-strained conditions and some samples cooled after the tensile test were evaluated by using internal friction method. The samples utilized for this measurement were strained to fracture and then water cooled from the test temperature to room temperature instantly.

The samples after tensile testing were cut from the gauge length parts of tensile test samples through an abrasive disc-saw. The samples for analyzing the as-supplied condition were cut in the tensile test direction. All IF samples were polished with 1200 grit emery paper to prevent surface effects.

Temperature dependencies of internal friction and pendulum frequency were measured at temperatures from room level to 600oC.The varying natural frequency of pendulum was noticed.

Stable elongation rate tensile tests were performed to receive the map of DSA serration appearance on the basis of strain rate and temperature for commercial grade Inconel alloy 600. The influence of DSA on mechanical characteristics of evaluated alloy is also checked.

It is found that yield stress and ultimate tensile stress reduces with increasing temperature about 200oC and remain stable after it. The strain hardening coefficient increases with increasing temperature limit up to 200oC. The elongation fracture increases for Inconel 600 however its ductility decreases at 400oC. Amplitude of internal friction increases with cold defomation and increases more subsequent to tensile tests at higher temperatures and particularly when the DSA serrations were available. The internal friction in deformed alloy 600 is not stable and it decreases gradually with annealing.

Significance of Differential Pressure variations in filter element

Contaminant loading in a filter element is eventually the process of blocking the pores throughout the element. The filter element becomes blocked with contaminant particles, there are few pores of fluid flow and pressure needed to keep flow through media improves. Basically the differential pressure across the element improves nominally as there is an abundance of media pores for the fluid to pass through and pore blocking process has nominal effect on the overall pressure loss.  Although, point is reached at which successive blocking of media pores nominally reduces the count of available pores for flow through the element.

At this level the differential pressure across the element rise exponentially, the magnitude, size, shape and arrangement of pores throughout the element for why some elements last longer than others. For a specific filter media thickness and filtration rating, there are fewer pores with cellulose media than fiberglass media. The contaminant loading process would block the pores of filter media element quicker then identical mesh media.

The mesh element is relatively uninfluenced by contaminant loading for a longer time. The element selectively captures the different size particles, as the fluid flows through the element. The small pores in the media are not blocked by large particles. These downstream small pores available for the large magnitude of small particles present in the fluid.

The element loads with contamination, the differential pressure will improve over time, nominally initially, then soon as the element it’s maximum life.

Every sintered mesh filter element follows a characteristic pressure differential versus contaminant loading relationship that is referred as filter element life profile. The practical life profile is definitely affected by the system application conditions. Variations in the system flow rate and fluid viscosity affect the clean pressure differential throughout the filter element and have a specific effect on the element life.

The filter element life profile is hard to evaluate in service systems. The system operation against idle time, the duty cycle and the changing ambient contaminant conditions all influence the life profile of the filter element. Additionally, the precise instrumentation for recording the variations in the pressure loss in the filter element is seldom available. Many machinery users and designers describe filter housings with differential pressure indicators to describe when the filter element should be replaced.

With Multipass test data, pressure differential versus contaminant loading relationship, stated as the filter element life profile. These service conditions  like flow rate and fluid viscosity affect the life profile of a filter element. In order to compare the life profiles of different mesh element these service conditions should be same and filter elements should be of same size. Then magnitude, size, shape and arrangement of pores in the filter element describe the characteristic life profile.

Pressure Ratings

Position of the filter in the circuit is the basic determinant of pressure rating. The housings are normally designed for three areas in a circuit- suction, pressure or return lines. A feature of these locations is their higher operating pressures. Suction and return line filters are normally designed for minimum pressure about 34 bar.

Which are the recommended aircraft metals?

Understanding the uses, strengths and limitations of structural metals is essential for proper construction and maintenance of equipments, particularly air components. For maintenance and repair of aircraft components, a nominal difference from required design or using lower quality materials may cause loss of lives and equipments. Using unsuitable materials can affect the craftsmanship. Choosing right material for a required repair job needs the knowledge of physical properties of the different metals.

Crucial properties of aircraft materials

a.       Hardness- It describes an ability of a material to prevent abrasion, penetration, cutting and physical distortion. Hardness can be increased by cold processing the material, for steel, heat processing is required. Structural components are usually made from materials in the soft form and are heat treated to harden to retain a finished shape. Hardness and strength are closely related to each other.

b.      Strength- The essential characteristics of a material is strength. Strength is the potential of a material to prevent deformation. It is also a potential of a material to prevent stress without cracking. The load or stress type affects the strength it attains.

c.       Density- It describes the weight of a material per unit of volume. In aerospace, the specific weight of a material per unit volume is recommended as this property is used in measuring the weight of a component before its production. Density is a crucial property while choosing a material for use in the design of a component to maintain the required weight and aircraft balance.

d.      Malleability- A metal that can be hammered, rolled or pressed in different shapes without cracking, breaking and leaving any other rigorous effect is called malleable. Malleability is an essential property of a sheet metal that is formed in curved shapes like cowlings, fairings etc. Copper is a common malleable metal.

e.      Ductility- It is a characteristic of a metal that allows it to be fully drawn, bent or twisted in different shapes without cracking. It is an important property of a metal that is used in producing wire and tubes. Ductile metals are widely used in the production of aircrafts for their easy forming and resistance to damage under shock loads. Therefore aluminum alloys are used for cowl rings, fuselage and wing skin and developed or extruded components like ribs, spars and bulkheads. Chome based steel is easily produced in required shapes. Ductility and malleability are similar properties of a metal.

f.        Conductivity- It is a property that allows a metal to conduct heat or electricity. The heat conductivity of a metal is essential in welding because it allows the magnitude of heat that will be needed for complete fusion. Metal conductivity to a specific limit shows the jig type to be used to limit expansion and contraction. In aircraft, electrical conductivity is considered in addition of bonding to prevent radio interference.

g.       Thermal expansion- It descries contraction and expansion occurred in metals due to heating and cooling. Heat given to a metal results into its expansion. Cooling and heating create a negative effect on the design of welding.

Aircraft materials

Commonly used metals in the production of aircraft components are Titanium, aluminum, alloy Inconel bar, ferrous metals etc. 

Improvement technologies of gas turbine super alloys

Identification of the material’s creep strength is essential for gas turbine engines, evaluation of age hardening, creep and gamma prime volume fraction and constantly increasing service temperature needs for aircraft engines resulted in the production of wrought alloys with increasing magnitudes f aluminum and titanium. Component forgeability issues resulted to this direction of development not crossing the specific level. The chemistry of wrought alloys is limited by hot working factors. This condition resulted into the development of cast nickel-base alloys. Casting compositions can be adjusted for sufficient high temperature strength because of no need of forgeability. Additionally the cast components are intrinsically stronger than forging at the elevated temperatures because of coarse grain size of castings.

Rotating airfoils should withstand vigorous combination of temperature, stress and environment. The bucket is generally loaded and is the limiting factor of gas turbine. Performance of nozzles is to direct the hot gases towards the bucket. So, they should be able to withstand high temperatures. Although they are subjected to lower mechanical stresses as compare to buckets. An essential design need for the nozzle materials is that they should have supreme high temperature oxidation and corrosion resistance.

Nickel based alloys Inconel bar are the materials chosen for the demanding gas turbine applications.  These alloys offer excellent combination of high temperature strength and hot corrosion resistance which makes them perfect for heavy duty gas turbine applications. Enhancements in processing technology have enabled development of the alloy in large ingot sizes. The alloys are used in the whole heavy duty gas turbine industry.

Air craft engines

A wide failure mechanism for gas turbine airfoils included nucleation and growth of cavities around transverse grain boundaries. Prevention of transverse grain boundaries through direction solidification of turbine blades and vanes made an essential step in temperature potential of the castings. Using directional solidified nickel based super alloys can enhance the turbine blade metal temperature potential by around 14oC relative to the conventionally cast super alloys.

Using directionally solidified super alloys can enhance the turbine blade temperature potential by 14oC relative to the conventionally cast super alloys. They offer long term creep rupture strength and hot corrosion resistance. The alloys can be directionally solidified in the form of large hollow blades.

Single crystals

In single  crystal castings, whole grain boundaries are discarded from the microstructure and a SC with a controlled orientation is developed in an airfoil shape. The castings needed no grain boundary strengtheners like Carbon, Boron, Zirconium and Hafnium. Discarding thee elements while designing the single crystal compositions improve the melting point and overall the high temperature strength. The single crystal alloys offer around 20oC metal temperature benefit over the standard directional solidified alloys. Chances to further enhance the metal temperature of these materials by increasing the magnitude of refractory elements resulted into the development of SC super alloys that offered improvement by 30oC metal temperature as compare to traditional SC alloys.