Role of Alloying Elements and Carbides in the Chlorine-Induced Corrosion of Steels and Alloys

The high temperature corrosion of steels and Ni-base alloys in oxidizing and chloridizing environments is of practical interest in relation to problems in waste incineration plants and power plants using Cl containing fuels. The behaviour of the most important alloying elements Fe, Cr, Ni, Mo, Mn, Si, Al upon corrosion in an oxidizing and chloridizing atmosphere was elucidated: the reactions and kinetics can be largely understood on the base of thermodynamic data, i.e. free energy of chloride formation, vapor pressure of the chlorides and oxygen pressure p O2 needed for the conversion chlorides → oxides. The mechanism is described by ‘active oxidation’, comprising inward penetration of chlorine into the scale, formation of chlorides at the oxide/metal interface, evaporation of the chlorides and conversion of the evaporating chlorides into oxides, which occurs in more or less distance from the surface (depending on p O2 ). This process leads to loose, fragile, multilayered oxides which are unprotective (therefore: active oxidation). Fe and Cr are rapidly transferred into such scale, Ni and Mo are relatively resistant. In many cases, the grain boundaries of the materials are strongly attacked, this is due to a susceptibility of chromium carbides to chloridation. In contrast the carbides Mo 2 C, TiC and NbC are less attacked than the matrix. Alloys on the basis Fe-Cr-Si proved to be rather resistant, and the alloying elements Ni and Mo clearly retard the attack in an oxidizing and chloridizing environment.


Introduction: Reactions and mechanisms
Oxidation can be considerably accelerated in presence of chlorine, hydrogen chloride and chlorides, this is a problem in waste incineration plants and power plants fired with chloride containing coal.Chlorine formation occurs from HCl through catalysis by oxide scales 2HCl + ½ O 2 = H 2 O + Cl 2 (1)   or from chlorides by reaction with oxides such as Fe Chlorine can somehow diffuse through oxide scales rapidly and reacts at the interface oxide/metal to chlorides of Fe, Ni and the other alloying elements, e.g.Cl 2 + Fe → FeCl 2 (s) → FeCl 2 (g) (3) FeCl 2 (s) has a vapor pressure at 500 °C of 5 × 10 -4 bar, and will evaporate steadily.The FeCl 2 (g) passes through cracks and pores of the scale outwards, and when reaching regions with sufficient oxygen pressure it is oxidized according to FeCl 2 (g) + 3/2 O 2 = Fe 2 O 3 + Cl 2 (4)   The oxides growing from the vapor phase near or at the scale surface are very fragile and not protective.The chlorine from reaction (4) may enter the atmosphere but mostly will reenter the scale and continue its catalytic action in the reaction sequence (2), (3) and (4).This mechanism is also called 'active oxidation' (Fig. 1) since it does not lead to a dense protective scale (passivation) but to an active, accelerated corrosion [1][2] .
The chlorine plays a catalysing role and often is not easily found in failure case studies 3 .In a detailed study of effects of chlorides on the oxidation of a low alloy steel it was shown that the outward diffusion of the gaseous FeCl 2 (g) is rate determining for this corrosion process [4][5] .'Active oxidation' also takes place on high alloy steels [5][6] under formation of voluminous non protective scales.These steels also show a typical grain boundary attack under oxidizing and chloridizing conditions, i.e. holes and internal oxides are formed at grain boundaries where chromium carbides had been before 3,7 .
After these studies on steels, it was of fundamental interest how the other alloying elements in steels behave in the 'active oxidation' process.Furthermore the reaction of carbides was of interest and both reactions of alloying elements and of carbides in model alloys were studied fundamentally in an oxidizing and chloridizing atmosphere [7][8][9][10] .

Experimental
The experiments were carried out in the temperature range 400 -700 °C, exposure tests in flowing N 2 -5% O 2 -500-1500 vppm HCl and thermogravimetric tests in He-5% O 2 -500-1500 vppm HCl.The pure elements Fe, Cr and Ni were tested and model alloys Fe-15% Cr -with either 10% Mn, 10% Mo, 5% Ti, 5% Al or 5% Si and Fe-35% Cr.In some of these materials carbides were generated by addition of 0.3% carbon and an appropriate heat treatment.Furthermore some commercial alloys were studied: Alloys 800, 825, 625 and 625 Si 9-10 .

Pure metals: Fe, Cr and Ni
Thermogravimetric studies on the corrosion of Fe in He-5% O 2 -500 vppm HCl showed the most severe corrosion of the three metals.At 400 °C an outer 2-3 µm thick oxide layer and a 6 -10 µm thick FeCl 2 (s) layer was observed after 168 h.At 500 °C the outer oxide scale is 10 -15 µm thick and its crystalline, filigrane morphology indicates its formation from the gas phase according to reaction (4).Beneath this layer are grown oxides and chloride at the oxide/metal interface.Most striking is the appearance of the corrosion products after 168 h at 600 °Cthe oxide is formed like a dome over the sample.Its shape corresponds to the stagnant diffusion boundary layer through which FeCl 2 (g) diffuses into the region where pO 2 is high enough for the conversion FeCl 2 (g) → Fe 3 O 4 and Fe 2 O 3 .Fe 3 O 4 was found at the interior and Fe 2 O 3 at the outer surface of the oxide bubble, FeCl 2 (s) on the surface of the metal phase.
On chromium at 400 and 500 °C thin Cr 2 O 3 layers were formed and chlorides were detected only locally.At 600 and 700 °C the scales frequently cracked and spalled as seen in the kinetics.This leads to a multilayered oxide scale see Fig. 2, beneath this scale a layer was detected consisting of a mixture of oxide and chloride.
On the nickel sample at 400 °C simultaneous formation of oxides and chlorides occurred.At 500 °C considerable evaporation of NiCl 2 leads to paralinear kinetics.Also at 600 and 700 °C NiCl 2 evaporation prevails and the evaporating chloride is not oxidized near the sample but carried away by the gas flow.

Binary alloys Fe-15% Cr and Fe-35% Cr
The corrosion of the binary Fe-Cr alloys is strongly dependent on the temperature and less on the HCl content (500 or 1500 vppm) of the atmosphere.
At 400 and 500 °C after 168 h, the mass gains by corrosion are low, protective Cr 2 O 3 -scales are formed and only locally destroyed by outgrowth of (Fe, Cr)Cl 2 crystals.At 500 °C the Fe-15% Cr alloy already shows indications of active oxidation and enhanced attack, compared to Fe-35% Cr.
At 600 °C the attack on both alloys is very severe, mass gains are higher than for all other alloys studied, even higher than on iron.The adverse features of both, Fe and Cr, in the 'active oxidation' are combined: poorly adherent multilayers of oxides are formed due to repeated cracking and spalling.The layers are Fe-rich at their outside (Fe 2 O 3 , (Fe,Cr) 3 O 4 ) and Cr-rich at their inside surface (FeCr 2 O 4 , Cr 2 O 3 ).At the oxide/metal interface the mixed chloride (Fe,Cr)Cl 2 was found, and the grain boundaries of the alloys were attacked.It was shown that the evaporating chlorides were nearly completely converted to oxides, FeCl 2 in the outer part of each layer and CrCl 2 in the inner part.The repeated fracture and spalling of the outermost growing layer leads to formation of the special structure, with oxide bubbles and layers, see Fig. 2.
Fe-15% Cr-5% Ti corrodes as severely as the binary Fe-Cr alloys.The morphology of the scale indicates that the mechanism of the scale growth is the same: formation of a multilayer by active oxidation, repeated cracking and spalling of the layers and formation of oxide bubbles over cracks.The metal phase contains intermetallic compounds Cr 6 Fe 18 Ti 5 as precipitates and they are preferentially attacked, due to the highly negative free energies of Ti-chlorides formation.
Fe-15% Cr-10% Mn is less corroded than the Fe-Cr alloys.A voluminous scale is formed consisting of Fe 2 O 3 and Mn 2 O 3 at its outside and Cr 2 O 3 at the inside, the morphology similar as in the case of Fe, like large bubbles.In the cavity beneath, the oxygen pressure obviously is low, since its bottom is covered with chlorides, (Mn, Fe, Cr)Cl 2 crystals for the condition 500 vppm HCl and a CrCl 2 -layer at 1500 vppm HCl.The latter layer obviously hems the corrosion, which is less at the higher HCl content.
Fe-15% Cr-10% Mo shows improved corrosion resistance compared to the binary Fe-Cr alloys.Beneath the Feand Cr-oxides in the scale the metal phase is depleted of Fe and Cr and enriched in Mo.Obviously selective corrosion of Fe and Cr has occurred and the Mo reacts much slower, because the free energy of Mo-chlorides formation is less than the values for FeCl 2 and CrCl 2 formation.This zone of about 5µm thickness is porous and allows the access of the corrosion gases to the uncorroded metal phase, but its presence obviously retards the attack.
Fe-15% Cr-5% Al proved to be very corrosion-resistant in the exposure tests with 500 and 1500 vppm HCl.At the relatively low gas flow velocities a protective scale of γ-Al 2 O 3 was formed and only locally outgrowth of Fe and Cr-oxides occurred.This observation already indicates the corrosion resistance may not be reliable.In fact, the thermogravimetric experiments at higher flow velocities showed strong active oxidation under formation of a multilayered porous scale.Probably at the higher flow velocities the evaporating AlCl 3 is transported away before conversion to a protective oxide scale.Also differences were observed after different surface finish, a ground specimen corroded much faster than a polished one.
Most corrosion resistant was the Fe-15% Cr-5% Si alloy, also at high gas flow velocities.The protection is effected by a thin Cr 2 O 3 -rich oxide scale, only locally some iron oxides have grown by active oxidation.An internal layer of SiO 2 was not detected, but probably initial formation of such layer and enhanced Cr-diffusion in the alloy have led to formation of the protective scale.Fe-Cr-Si alloys have been tested thoroughly under simulated waste incineration conditions, and they may be used as coatings for effectively improving corrosion resistance [11][12] .

Commercial alloys containing nickel
To study the role of nickel, the commercial alloys Alloy 800, i.e.Fe-30.3Ni-20.3Cr and Alloy 600 Ni -16 Cr-8.3 Fe were tested at 600 °C in the oxidizing-chloridizing atmosphere.These alloys behave relatively inert compared to the Cr-steels.The mass increase after 168 h is about 1.75 mg/cm 2 for Alloy 800 and 0.5 mg/cm 2 for Alloy 600 in both atmospheres with 500 or 1500 vppm HCl (compared values between 5-50 mg/cm 2 , see Fig. 3).As in the case of Mo the attack on Ni is less than on Fe and Cr, because the free energy of NiCl 2 -formation is much less negative than for FeCl 2 and CrCl 2 .In the alloys 800 and 600 the selective corrosion of

Alloys with carbide precipitates
Well-defined carbides were obtained in ternary Fe-Cr-C and quaternary Fe-Cr-M-C alloys by appropriate heat treatments 7 .These alloys were tested as the others in the oxidizing-chloridizing environment at 600 °C for 168 h.
M 23 C 6 carbides in Fe-15% Cr-0.8% C, Fe-35% Cr-0.3% C and Fe-20% Cr-12% Ni-0.3% C were strongly attacked beginning from the sample surface.The preferential attack of the carbides leads to holes in the metal matrix, see Fig. 4a-4c, thus the components must have left the speci-  Also M 7 C 3 obtained by heat treatment of Fe-20% Cr-33% Ni-0.5% C is attacked preferentially and the corrosion leaves cavities along the grain boundaries (Fig. 4d).So the chromium carbides M 23 C 6 and M 7 C 3 are weak components in the corrosion of alloys in oxidizing-chloridizing conditions.Often failure cases have been reported where the grain boundary attack in such environment is due to the grain boundary carbides in high temperature steels and alloys.
In contrast other carbides such as Mo 6 C, TiC and NbC prepared in Fe-15% Cr-10% Mo-0.3% C, Fe-15% Cr-5% Ti-0.3% C and Fe-18% Cr-10% Ni-0,8% Nb-0.1% C, behaved better corrosion resistant than the matrix.These carbides remain nearly unattacked even after the surrounding metal matrix has corroded, see Fig. 4e-g.This observation was surprising since also for these carbides the reactions leading to volatile chlorides and CO have negative free energies.So the reason for the stability of TiC, NbC and Mo 6 in the oxidizing-chloridizing environment is not clear as yet.

Discussion
The corrosion behaviour of the alloying elements in oxidizing-chloridizing environments can be discussed and explained largely by thermodynamic data (see Fig. 5): • The Gibbs' free energy of the reaction metal + chlorine ® metal chloride.The more negative this value the faster is the reaction with this element in the metal matrix and the corrosion attack occurs according to the sequence Mn > Cr > Fe > Ni > Mo.• The oxygen pressure for the conversion of the evaporating chlorides into oxides, e.g.reaction (4).The lower this oxygen pressure, the smaller is the distance from the metal surface at which the oxide is formed.Therefore the oxide layers are Fe-rich at their outside and Cr-rich at their inside, and the Ni-and Mo-chloride are transported away to a great part unoxidized by the gas flow.• The vapor pressures of the chlorides, they also play a role for the destiny of the corrosion products, evaporation or oxidation.

Conclusions
The corrosion of metals and alloys in oxidizingchloridizing environments occurs by 'active oxidation' [4][5][6] , i.e. the reaction steps: i) formation of Cl 2 from HCl or chlorides in and on the oxide scale, ii) penetration of Cl 2 into the scale and reaction to metal chlorides at the metal phase, (iii) outward diffusion of metal chlorides through the scale and (iv) oxidation of the chlorides to crystalline, porous oxides, where a region with a sufficient p O2 is reached in or over the scale.The Cl 2 reenters the scale, so the active oxidation is a circuit catalysed by chlorine, leading to an unprotective oxide scale.
The reaction of Cl 2 with the alloying elements [8][9][10] is the faster the more negative the free energy of chloride formations is, Cr, Fe and Mn are attacked strongly, whereas Ni and Mo react slowly and behave rather inert.From Ni and/or Mo rich alloys the other alloying elements are reacted selectively and a porous Ni and Mo rich metal layer remains.Thus Ni and Mo are favourable for the corrosion resistance of alloys in oxidizing-chloridizing environments.
On Cr, Fe-Cr and Fe-Cr-M alloys (M = Ti, Mn) friable unprotective scales are formed.Layers of Cr 2 O 3 , spinels and Fe 2 O 3 result from the evaporation and oxidation of FeCl 2 and CrCl 2 , oxide bubbles arise where such layers are cracking and repeated spalling and cracking leads to a multilayered scale.Si and Al as alloying elements M can significantly retard this corrosion process.Fe-Cr-Si alloys are recommended as protective coatings in oxidizing-chloridizing environments [11][12] .
Chromium carbides M 7 C 3 and M 23 C 6 are strongly attacked in oxidizing-chloridizing environments, this leads to grain boundary corrosion of alloys with such grain boundary carbides, starting from the surface [7][8][9][10] .Since gaseous reaction products, CrCl 2 and CO, are formed, holes are left in the metal matrix and internal oxidation follows.Other carbides such as Mo 6 C, TiC and NbC are more corrosion resistant than the alloy matrix and stay uncorroded.Accordingly, presence or formation of chromium carbides should be avoided in alloys applied in oxidizing-chloridizing environments.

Figure 1 .
Figure 1.Schematics of the reaction circuit in active oxidation of iron and steels.
Fe and Cr leads to a porous, sponge like structure of the outer zone of the metal phase.The evaporating chlorides FeCl 2 and CrCl 2 are oxidized above the oxide/metal interface and form a multilayered scale, as in the case of the Fe-Cr alloys, consisting of Fe 2 O 3 and (Fe, Ni) Cr 2 O 4 .The morphology of the scale on Alloy 800 is characterized by voluminous oxide bubbles and on Alloy 600 by many thin layers.The attack by the oxidizing-chloridizing is even more suppressed by additional alloying with Mo, this was shown for Alloy 625, i.e.Ni-22.2Cr-9.2 Mo-4.6 Fe-3.4 Nb, and for Alloy 625Si, i.e.Ni-20.8Cr-9.5 Mo-3.2 Fe-1.5 Si, for which the corrosion resistance was best.

Figure 4 .
Figure 4. Metallographic cross sections of model alloys with welldefined carbide precipitates after 168 corrosion in N 2 -O 2 -HCl at 600 °C.a-d) the chromium carbides M 23 Cand M 7 C 3 are attacked selectively; e-g) Mo-rich carbide M 6 C, TiC and NbC are clearly more corrosion resistant than the metal matrix.

Figure 5 .
Figure 5. Schematic of thermodynamics and reactions in the corrosion of the alloying elements Mn, Cr, Fe, Ni and Mo in a flowing oxidizing and chloridizing atmosphere.