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Corrosion behavior of Inconel 625

Inconel 625 is known for its strength, outstanding fabrication, welding and excellent corrosion resistance properties. Prevention of corrosion of Inconel alloy 625 at the diverse temperatures and corrosive conditions is the basic need for its extensive acceptance in the chemical processing and other applications. This post shows the lab and field data about the resistance of Inconel 625 to the diverse aqueous media causing localized attack, stress corrosion cracking and other kinds of corrosion. The flexibility of alloy for use in oxidizing, carburizing, chlorinated solvents and simulated waste incinerator conditions comprise of halide species are shown.


Inconel 625, UNS N06625 is fit for use in the various intense attacking conditions. In the nominal conditions, like ambient conditions, fresh and marine water, neutral salts and alkaline media, there is no corrosion. In the variety of aggressive conditions, chromium offers resistance to oxidizing chemicals however high nickel and molybdenum concentration make alloy resistance to nonoxidizing conditions. The concentration of molybdenum makes Inco 625 resistant to pitting and crevice attack. The included niobium content stabilizes alloy against sensitization while welding, hence avoiding immediate intergranular attack. Moreover, high concentration of nickel offers resistance to chloride based stress attack. This pair of corrosion resistance properties makesInconel alloy 625 a commonly used material  in the chemical and aerospace sector. This post describes the performance of alloy in two sections as resistance to aqueous and high temperature attack.

Aqueous corrosion

Uniform corrosion:

Uniform corrosion, hence called general corrosion is a type of attack that is often distributed and proceeds at about the same rate over the complete metal surface. The analyses are made to determine the resistance to alloys to uniform corrosion primarily to help in the choice of materials. The tests are normally performed in the different acids to consider various situations. Following table describes the general corrosion resistance of the different alloys including In 625 in the various non-oxidizing and oxidizing acids. Few alloys that are suitably resistant to reducing acids such as Hastelloy B2 may not be fit for use with oxidizing chemicals. Although, In 625 is one of the resistant alloys to uniform attack and may be used in reducing and oxidizing acids.

Alloy 40% HCOOH 10% HCl 10% H2SO4 55% H3PO4 85% H3PO4 10% HNO3 50% H2SO4 + 2.5 % Fe2(SO4)3
SS 304 180 mpy Above 12,000 2200 mpy 400 mpy 9500 mpy 1 mpy 46 mpy
SS 316 30 mpy Above 12,000 mpy 400 mpy 18 mpy 450 mpy 1 mpy 36 mpy
SS 317 9000 mpy 360 mpy 0.5 mpy 28 mpy
Ni 200 10 mpy 8000 mpy 130 mpy 500 mpy Above 12,000 mpy 8,000 mpy
In 600 10 mpy 4400 mpy 360 mpy 1800 mpy
Incoloy 825 8 mpy 1000 mpy 18 mpy 6 mpy 35 mpy 0.6 mpy 11 mpy
Hastelloy G-3 2 mpy 1000 mpy 24 mpy 5 mpy 24 mpy 0.8 mpy 11 mpy
In 625 7 mpy 620 mpy 44 mpy 10 mpy 110 mpy 1 mpy 23 mpy
Hastelloy C276 3 mpy 220 mpy 16 mpy 7 mpy 18 mpy 18 mpy 260 mpy
Hastelloy B2 0.4 mpy 7 mpy 2 mpy 3.5 mpy 3.5 mpy Above 12,000 mpy

 Localized corrosion attack

Pitting is a type of localized attack that is limited to a small region instead the entire surface. Pitting and crevice attack or localized attack as related to ppm chloride content and pH of alloys are shown in the following figure:

pitting corrosion resistance Inconel 625

Chromium and molybdenum have been discovered to be advantageous in enhancing pitting resistance. Inconel 625 has large chromium and molybdenum concentration that accounts for its superior pitting attack. The empirical connection between the critical temperature for pitting to start and chromium and molybdenum and hence rank as first for pitting resistance as shown in above figure.

Chloride stress corrosion cracking

Nickel and nickel base alloys are normally resistant to chloride stress corrosion cracking as compared to stainless steels. The advantageous effect of nickel on limiting the stress corrosion cracking in boiling 42% MgCl2 is observed. The alloys comprising of 42 – 45% nickel will not be attacked in this condition. Inconel 625 consists of 58% nickel, hence it will not be attacked. In the conditions of stress corrosion cracking, Inconel 625 may be attacked by hot water and chlorides, shown in the following table with other conditions that are damaging.

Heat processing Conditions in which SCC occurred
Annealed High temperature water, degassed
Annealed Polythionic acid
Annealed Oxygenated water plus lead
Annealed Water, 21 ppm O2 at 316oC
Annealed Steam, cycled between 318 and 385oC ( 605 to 725oF), 10 ppm Cl-, 8 ppm Oxygen
Cold processed 288oC water, 100 ppm O2
Cold processed and aged at 204oC  or 400oF Water, saturated with hydrogen sulfide + 5% NaCl + 0.5% acetic acid + NACE solution at room temperature
Cold processed and aged at 500oC or 932oF Water with 5% NaCl and 0.5% acetic acid at room temperature

 Inconel 625 and different alloys were analyzed in accelerated wet oxidation condition that comprised of a synthetic sewage with chlorides and fluorides, pH 3.2 and a temperature 292oC or 562oF as shown in the following table:

Alloy Stress corrosion cracking in liquid phase
Hastelloy C-276 No signs of cracking
Inconel 625 No signs of cracking
Hastelloy G No signs of cracking
26 – 1 3/10 in 281 hours
20 – Cb3 No signs of cracking
20 – Cb3 tube 1/10 in 440 hours
Incoloy 825 1/9 in 295 hours
Nitronic 50 9/10 in  30 hours
Nitronic 10 8/10 in 16 hours
Nitronic 60 9/10 in 11 hours
Nitronic 33 9/10 in 11 hours
SS 316 – L 10/10 in 12 hours
SS 304- L 10/10 in 12 hours
SS 316 tube machined and heat treated 9/9 in 12 hours
SS 316 tube machined 9/9 in 12 hours
SS 316 tube 10/10 in 12 hours

In 625 and other alloys mentioned containing nickel above 34% are extremely resistant to SCC in this media.

Sulfide stress cracking

Sulfide stress cracking or SSC is an issue in the oil and gas units. It often causes brittle failure of a metal in stress in water mixed with hydrogen sulfide. SSC attack is shown in the following table for various alloys.

Alloy Condition Yield stress Tensile stress Hardness, HRc Threshold stress
MPa Ksi MPa Ksi MPa Ksi
Titanium Annealed 527 MPa 76.4 ksi 659 MPa 95.6 ksi 20 HRc 516 MPa 74.9 ksi
MP35N Cold rolled and annealed 2005 MPa 290 ksi 2046 MPa 296.8 ksi 51 HRc 1965 MPa 285 ksi
Hastelloy C-276 Cold rolled 1503 MPa 218 ksi 1631 MPa 236.5 ksi 41 HRc 1473 MPa 213.6 ksi
Hastelloy G Cold rolled 1496 MPa 217 ksi 1617 MPa 234.5 ksi 35 HRc 1470 MPa 213.2
Monel K-500 Cold rolled and annealed 1124 MPa 163 ksi 1317 MPa 191 ksi 36 HRc 1101 MPa 159.7 ksi
In 625 Annealed 558 MPa 81 ksi 979 MPa 142 ksi 20 HRc 546 MPa 79.2 ksi
In 625 Cold rolled 1327 MPa 163 ksi 1355 MPa 196.5 ksi 39 HRc 1301 MPa 188.7 ksi
In 718 Solution annealed and aged 986 MPa 143 ksi 1172 MPa 170 ksi 34 HRc 960 MPa 139.3 ksi
In X750 Solution annealed and aged 1304 MPa 150 ksi 1318 MPa 191.2 ksi 37 HRc 1013 MPa 147 ksi
Nitronic 50 Cold rolled 880 MPa 127.6 ksi 1076 MPa 156.1 ksi 26 HRc 862 MPa 125 ksi
3RE60 Annealed 591 MPa 85.7 ksi 756 MPa 109.7 ksi 20 HRc 532 MPa 77.1 ksi
17-4 PH Solution annealed and aged 858 MPa 124.5 ksi 974 MPa 141.2 ksi 30 HRc 86 MPa 12.5 ksi
Custom 450 Solution annealed and aged 579 MPa 84 ksi 945 MPa 137 ksi 25 HRc 290 MPa 42 ksi

 The analyses were made at room temperature under constant load in a mixture of oxygen free water comprising of 3000 ppm dissolved H2S, 5% NaCl and 0.5% acetic acid. The threshold stress of Inconel 625 is in close to yield strength in annealed and cold rolled conditions. The threshold stress, although is larger for few alloys in this condition.

Hydrogen embrittlement

An extent of inclination to hydrogen embrittlement was determined as a function of ductility loss. Inconel 625 possesses a moderate ranking as compared to the other alloys. Data made in pressurized gaseous hydrogen is described in the following table:

Alloy Hydrogen pressure, ksi Strength ratio
903 5 ksi 1
AISI 316 10 ksi 1
OFHC Copper 10 ksi 1
6061 T6 10 ksi 1
802 7 ksi 0.99
7075 – T73 10 ksi 0.98
Incoloy A 286 10 ksi 0.97
RA 330 7 ksi 0.95
Be-Cu 10 ksi 0.93
AISI 310 10 ksi 0.93
AISI 347 5 ksi 0.91
Astroloy 5 ksi 0.90
Hastelloy X 5 ksi 0.87
FM 718 7 ksi 0.86
AISI 1020 10 ksi 0.79
Ti-6Al-4V 10 ksi 0.79
Inconel 625 5 ksi 0.76
AISI 1042 10 ksi 0.75
HY 100 10 ksi 0.73
Monel 400 7 ksi 0.65
MP35N 10 ksi 0.50
Inconel 718 10 ksi 0.46
AISI 4140 10 ksi 0.40
Rene 41 10 ksi 0.27
Inconel X-750 7 ksi 0.26
17-7 PH 10 ksi 0.23
AISI 410 10 ksi 0.22
250 Maraging 10 ksi 0.12

 Intergranular attack

Inconel 625 is stabilized against intergranular corrosion by precipitation of niobium carbides at 927 – 1038oC or 1700 – 1900oF annealing limit. Niobium carbides tie up carbon, making it less available to precipitate as chromium carbides in the grain boundaries. Chromium reduction near the grain boundaries is an effect of intergranular chromium carbide precipitation. This process takes place at specific temperatures and is called as sensitization. Due to sensitization, an alloy becomes vulnerable to integranular corrosion. It can be made inclined to such kind of attack by insufficient annealing processing that would prevent the development of niobium carbides hence making carbon more accessible to precipitate intergranular attack as chromium carbides by subsequent sensitization treatment. At lower annealing temperatures, stabilized niobium carbides are precipitated hence a subsequent sensitizing heat treatment of 704oC- 871oC or 1300oF to 1600oF will cause nominal or no precipitation of chromium carbides at the grain boundaries.

Corrosion in Bleach plant conditions

Pulp and paper bleach plant condition attack analyses have been studied in following table:

Corrosion data in Pulp and paper bleach plant conditions

Alloy Total depth pitting (mills) Alloy Total depth pitting (mills)
Hastelloy G AL-6X 229 mills
Hastelloy G-3 Alloy 28 190 mills
Inconel 625 T-317X X S/S 229 mills
Hastelloy C276 JS-700 247 mills
Titanium GR-2 Incoloy 825 311 mills
Titanium 12 254 SLX 414 mills
20Mo-6 5 mills T-317LM S/S 647 mills
AL 29-4 7 mills Nitronic 50 658 mills
SC-1 29 mills T-317L S/S 690 mills
Schomac 30-2 41 mills 26 – 1 770 mills
AL-29-4-2 102 mills T 316l S/S 1158 mills

 Tests in solutions of pH 1.4 – 9.5 with about 5500 pp, chlorides and 80oC or 176oF with power chlorine base oxidizers were made on 38 samples of different materials. It involved 8 chlorination stages,9 chloride dioxide stages and 3 hypochloride stage bleach unit conditions. From the above table, it can be seen that Inconel 625 and other extremely corrosion resistant alloys offer great prevention in bleach plant conditions. High chromium and molybdenum contents of Inconel 625 are responsible for its pitting resistance in this media.

Behavior in Marine water

Different alloys have been ranked by corrosion resistance in marine conditions in the following table. Inconel 625 offers the best resistance in marine media by its high concentration in chromium and molybdenum.

Alloy Remarks
Most resistant
Hastelloy C Fully resistant excluding the welds
Hastelloy C-276 Nominal carbon can be welded
Inconel 625 Close to Hastelloy C in overall resistance
MP35N Outstanding performance in preliminary tests
Chlorimet – 3 Cast alloy, outstanding for pumps
Rene 41 Excellent pitting resistance
Hastelloy X Excellent pitting resistance
Extremely resistant
F Often adequate, concentration of molybdenum offers pitting resistance
Illium R
Inconel 718
Resistant, slight pitting
Inconel 600 Nominal pitting at regions of stagnant marine water
Inconel X-750
Incoloy 800
Incoloy 825
Monel 400
Monel K-500

 Inconel 625 is very noble as compare to other alloys. It often acts as cathode when kept in contact with other materials in marine water. Marine water corrosion – fatigue strength for alloy 625 is high. The outcomes of extra corrosion – fatigue analyses on Inconel 625 and other alloys in marine water are described in the following table:

Alloy Ultimate tensile strength Corrosion- fatigue strength at 100 megacycles in marine water
Ksi Kgf/mm2 Ksi Kgf/mm2
Inconel – 120 205 Ksi 144 70 Ksi 49.2
Inconel 718 189 Ksi 133 60 Ksi 49.2
Inconel 625 149 Ksi 105 50 Ksi 35.1
Inconel 718 grain size 0.068 189 Ksi 133 40 Ksi 28.2
Inconel 625 129 Ksi 91 40 Ksi 28.2
Inconel 718 grain size 0.152 mm 32 Ksi 22.5
Hastelloy C 108 Ksi 76 32 Ksi 22.5
Monel K-500 176 Ksi 124 26 Ksi 18.3
Incoloy 800 89 Ksi 63 24 Ksi 16.9
18% Ni Maraging steel 250 Ksi 176 23 Ksi 16.2
18% Ni Maraging steel 250 Ksi 176 15 Ksi 10.6
Ni-Al Bronze CA 955HT 115 Ksi 81 15 Ksi 10.6
AISI SS 304 79 Ksi 56 15 Ksi 10.6
AISI SS 306 85 Ksi 60 14 Ksi 9.8
AISI SS 304L 75 Ksi 53 14 Ksi 9.8
AISI SS 316L 79 Ksi 56 13 Ksi 9.1
Ni-Al Bronze CA 955 87 Ksi 61 12.5 Ksi 8.8
Mn-Ni-Al Bronze CA 957 100 Ksi 70 9 Ksi 6.3
Mn Bronze CA 865 73 Ksi 51 8 Ksi 5.6
18% Ni Maraging steel (unsecured) 250 Ksi 176 5 Ksi 3.5
Mild steel 60 Ksi 42 2 Ksi 1.4

 Elevated temperature attack

Oxidation test

However alloy 625 is popular for resistance to aqueous attack, it also offers suitable resistance to oxidation and scaling at the elevated temperatures. An oxidation analyses was performed through air plus 5% added water vapor moving at the speed of 500 cc/min at 1000oC or 1832oF for 1008 hours. Water addition through bubbling air by water controlled at 33oC or 91oF to develop 5% water vapor. Regularly the samples were pulled out of the furnace and weighed subsequent to quenching to room temperature to receive a mass variation as a function of time.

A cyclic oxidation analysis was performed on the elevated temperature alloys for example Inconel 601, 617 to determine the scale reliability. In this condition, cyclic oxidation analyses was performed at 982oC or 1800oF in which the samples were heated to temperature for 15 minutes and quenched in air for five minutes. The specimens were weighed subsequent to 100 cycles to receive the mass variation. The outstanding resistance of Inconel 625 to cyclic oxidation describes the potential of alloy to maintain the security oxide coating in the vigorous cyclic condition. Incoloy 800 must be stopped subsequent to 1200 cycles.

Carburization resistance

 Development of stable oxide scale on the surface is a base of carburization resistance of a material. An analysis was made to find the relative high temperature carburization resistance of complex alloys utilized in the ethylene furnaces. The analysis involves subjecting alloys to a mixture of hydrogen and 1 – 3% methane at required temperatures. The samples are regularly eradicated for weighing by eradicating them to the cold end of the test chamber and flushed with argon to decrease oxidation. Inconel 625 offers outstanding resistance to carburization at 1000oC  in hydrogen gas – 1%Methane at 1000oC for 1000 hours. Mass variation in nominally oxidizing – carburizing condition, H2 – 5.5%CH4-4.5%CO2 at 1000oC or 1832oF for around 1000 hours. Inconel 625 offers comparable resistance to Inconel 600 and superior to alloys 800 and 600 in this condition.

Service of alloy 625 in chlorine based media

Chlorine is a main component in the various industrial process conditions for example mineral chlorination, ethylene dichloride and vinyl chloride monomer development, aluminum smelting and refining, fuel element treatment and heat recovery units. Elevated temperature halide attack has been noticed in fossil fuel boilers, coal gasification unit, gas turbines and in municipal and chemical waste incinerators. Municipal waste consists of 0.5% halides in dry form. Many lab analysis simulating waste incineration conditions have been limited to temperatures varying from 120 – 650oC or 248 – 1202oF. As nickel and nickel base alloys have a prolong reputation for their resistance to halogen attack. It refers to their significance in extending the temperature limit of lab data in simulated HCl conditions. The temperature limit chosen in this analysis is 593 – 927oC or 1100 – 1700oF. This limit is a standard for the flue gas temperature. The boiler interiors and flue stacks can specifically be considered to experience thermal exposure in this temperature limit. The considered condition for this analysis was Nitrogen-10% carbon dioxide-9% oxygen-4%HCl – 130 ppmHBr-100ppm SO2.

The tests were made in a 100mm dia mulllite tube in a horizontal electrical resistance furnace. The test samples were in the form of pins, 7.6 mm dia by 19 mm length. The samples of the whole alloys were subjected to 4% HCl conditions at 593, 704, 816 and 927oC or 1100, 1300, 1500 and 1700oF. The gas composition was developed by combining a gas mixture of nitrogen, sulfur dioxide, carbon dioxide and oxygen with hydrochloric acid and hydrogen bromide gases. The eventual gas chemistry was metered in the mullite tube with electronic flow controllers at an overall rate of 500 cc/min.

The metal loss for alloys analyzed in N2-10%CO2-9%O2-4% HCl- 130 ppm HBr – 100ppm sulfur chlorides at 593, 704, 816 and 927oC or 1100oF, 1300oF, 1500oF and 1700oF for 300 hours is described in following table:

Alloy 593oC or 1100oF 704oC, 1300oF 816oC, 1500oF 927oC or 1700oF
SS 309 -2.62 mg/cm2 -117.89 mg/cm2 -77.70 mg/cm2
SS 316 -5.48 mg/cm2 -152.08 mg/cm2 -32.75 mg/cm2 45.74 mg/cm2
SS 347 -1.61 mg/cm2 327.80 mg/cm2 -221.65 mg/cm2 -68.36 mg/cm2
In 600 -0.32 mg/cm2 -0.98 mg/cm2 -6.93 mg/cm2 -10.18 mg/cm2
In 601 -0.05 mg/cm2 -2.19 mg/cm2 -7.24 mg/cm2 -29.53 mg/cm2
In 617 -0.51 mg/cm2 -8.38 mg/cm2 -10.81 mg/cm2 -44.64 mg/cm2
In 625 -0.52 mg/cm2 -2.06 mg/cm2 -5.86 mg/cm2 -5.91 mg/cm2
In 690 -0.13 mg/cm2 -1.12 mg/cm2 -14.27 mg/cm2 -24.21 mg/cm2
In 800 -0.53 mg/cm2 -245.22 mg/cm2 -6.63 mg/cm2 -18.98 mg/cm2
In 825 -1.53 mg/cm2 -127.34 mg/cm2 -8.52 mg/cm2 -25.89 mg/cm2


With high nickel, chromium, niobium and molybdenum concentrations, Inconel 625 offers good aqueous corrosion resistance in the variety of severe conditions. It is resistant to aqueous attack in the various media similar to alloys such as grade C-276.