UNDERSTANDING THE CORROSION BEHAVIOR OF 35Ni19Cr ALLOY
USING X-RAY MICROANALYSIS

A. Wong-Moreno ¹, D. López-López ², L. Martínez ³

1, 2  Instituto Mexicano del Petróleo, Blvd. Ruiz Cortines 1517-12, Fracc. Costa de Oro, 94299, Boca del Río, Veracruz, México, acwong@imp.mx; dlopez@imp.mx

3 Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México, Av. Universidad S/N, Col. Chamilpa, C.P. 62210, Cuernavaca, Morelos, México, lorenzo@ce.fis.unam.mx

ABSTRACT

X-ray microanalysis of corroded specimens of 35Ni19Cr austenitic steel was performed in order to understand its oil-ash corrosion behavior. Corrosion testing involved the exposure of the alloy at temperatures in the range of 600°C - 900°C, to a sulfate-rich oil ash, which is also constituted by low melting point sodium vanadates. The curve describing the corrosion behavior as a function of temperature exhibits two relative maximums at 715°C and around 800°C, suggesting that there is an evolution of corrosion mechanisms as temperature is increased. X-ray microanalysis of the corrosion product scale and of the metal subjacent to the interface metal/scale let characterize three corrosion mechanisms prevailing along the temperature range: metallic dissolution caused by molten vanadium compounds, accelerated oxidation and sulfidation. Microanalysis also provided evidence of internal degradation at temperatures above 675°C consisting in internal oxidation, sulfidation or both. It was concluded that the resultant corrosion behavior depends on both: the oxidation, oil-ash corrosion and sulfidation resistance of the alloy, and the stability of the oil ash, which determines the chemical compounds responsible for the corrosion process observed.

Key words: X-ray microanalysis; oil ash corrosion; 35Ni19Cr austenitic steel; sulfidation; high temperature oxidation

RESUMEN

   Se llevó a cabo un estudio por microanálisis de especímenes corroídos de acero austenítico 35Ni19Cr con el fin de entender su comportamiento de corrosión por cenizas de combustóleo. Los ensayos de corrosión involucraron la exposición de la aleación a temperaturas en el intervalo de 600°C - 900°C, a un depósito de ceniza con alto contenido de sulfatos alcalinos y que también está constituido por vanadatos de sodio de bajo punto de fusión. La curva que describe el comportamiento de corrosión en función de la temperatura exhibe dos máximos relativos a 715°C y alrededor de 800°C, lo cual sugiere que hay una evolución de los mecanismos de corrosión operantes a medida que la temperatura se incrementa. Los resultados del estudio por microanálisis de la costra de productos de corrosión y del metal debajo de la interfaz metal/costra, permitió caracterizar los tres mecanismos de corrosión operantes a lo largo del intervalo de temperatura considerado: disolución metálica causada por compuestos de vanadio de bajo punto de fusión, oxidación acelerada y sulfidación. El microanálisis de las probetas corroídas proporcionó evidencia de la ocurrencia de degradación interna de la aleación a temperaturas mayores a 675°C, la cual consiste de oxidación interna, sulfidación o ambas. Se concluye que el comportamiento de corrosión resultante depende tanto de la resistencia de la aleación a oxidación, sulfidación y corrosión por cenizas, como de la estabilidad de la ceniza de combustible, ya que ésta determina el tipo de compuestos químicos responsables del proceso de corrosión.

Palabras clave: Microanálisis, corrosión por cenizas del combustóleo, acero austenítico 35Ni19Cr, sulfidación, oxidación a alta temperatura.

1. Introduction

   High temperature corrosion behavior of heat resistant alloys is a very important aspect in the selection of materials for very aggressive conditions. The 35Ni19Cr alloy is a heat resistant austenitic alloy with a good combination of mechanical properties and resistance to carburization, oxidation and thermal cycling. It exhibits good metallurgical stability and it does not show embrittlement caused by sigma phase. Carburization is a material deterioration process that can affect the performance of fossil-fired boiler components that operate at elevated temperatures, causing inclusive their failure in some cases [1-8]. For that kind of components (hangers, supports, flame stabilizers) alloys such as 304, 310, 309 austenitic steels are typically used, but it is evident the need of an alloy with better resistance to carburization besides to oil-ash corrosion. Given the good resistance to carburization of 35Ni19Cr alloy, it was considered to determine its corrosion behavior by oil-ashes in order to know if it could be an alternative alloy for these components. This paper shows the results of corrosion testing of 35Cr19Ni alloy exposed to a boiler sulfated oil-ash at several temperatures in the range of 600°C-900°C, and the results of a microanalysis study carried out in order to understand the corrosion behavior of this alloy.

Table 1. Chemical Composition of 35Ni19Cr Alloy

Table 2. Chemical analysis of the residual oil ash (weight %) and its main compounds.

 

2. Experimental Procedure 

Test specimens: Chemical composition of the alloy studied is shown in Table 1. 15x12x1.5 mm test specimens were cut from sheets. The samples were ground to a 600 grit SiC finish, degreased, weighed and cleaned in acetone prior to testing.
Oil ash deposit: Samples were exposed to a residual oil ash with high sodium sulfate content, whose characterization is shown in Table 2. This oil ash is a representative sample of fuel oil-ash deposits collected directly from the primary superheater banks of a 84 Mw utility boiler. The boiler burns heavy high sulfur fuel-oil with high contents of sodium and vanadium. Besides the oil-ash chemical analysis, Table 2 also lists the V/(Na+S) ash molar ratio, which is considered as a corrosivity index [9,10], and the major compounds identified by X-ray diffraction analysis: Sodium sulfate, sodium vanadil vanadate of V/Na=6 (m.p. 625°C), lower melting point vanadates with high sodium content and lower V/Na ratio (m.p. »535°C), and the corrosion product FeVO4 (m.p. 816°C). The deposits were ground to 100-mesh before they were contacted with alloys.
Test Conditions: Corrosion crucible tests were conducted for 250 hours under isothermal conditions in electric furnaces. The specimens were totally packed in oil-ash deposit powder contained in silica crucibles at nine temperatures in the range of 600°C - 900°C. The amount of deposit added was 500 mg per cm² of initial area of the specimen. The atmosphere used was static air. At least four specimens of each alloy were exposed at each test temperature. It has been shown that crucible tests, one of the most simple laboratory procedures, are very useful for observing the effect of some of the variables involved in high temperature corrosion processes [11]. The comparison of materials performance in a qualitative or semiquantitative way under different conditions (temperature, composition of the corrosive agent, atmosphere) by this technique is totally reliable[12]. Besides the corrosion crucible tests oxidation testing in static air was carried out at temperatures in the range of 560°C-950°C, in order to compare with oil ash corrosion. The oxidation results confirmed the good oxidation resistance of this alloy at elevated temperatures.
Post Corrosion Examination: After testing, three corroded specimens per temperature were descaled according the ASTM G1 standard and the weight change and the thickness loss of three specimens per temperature were determined. The fourth corroded sample from each test was cross-sectioned, mounted in conductive bakelite and examined as-polished (polished without water) by scanning electron microscopy and microanalysis to analyze the chemical composition, morphology and distribution of reaction products, and to determine the characteristics and depth of any subsurface corrosive attack. Elemental X-ray mapping was performed using a Microspec WDX-3PC system connected to a Zeiss DSM960 scanning electron microscope. Furthermore, EDX line profiles were performed using an EDAX DX Prime 60 system, to characterize the metallic degradation suffered by the alloy and understand its corrosion behavior.

3. Corrosion Results 

Corrosion as a function of temperature is shown in Figure 1. It can be compared the oxidation resistance of 35Ni19Cr alloy with its performance under exposure to oil ashes. Corrosion caused by oil ashes is 2-4 orders of magnitude higher than that resulting from high temperature oxidation. It was found two relative maxima, at 715°C and around 800°C, as the specimens exposed to 785°C and 805°C were completely consumed. This fact is indicated by the arrows on top of their respective data in the graph (the data included in the graph for these temperatures were estimated from their initial weight, but obviously the real corrosion amount should have been higher than those values). The presence of the peaks at 715°C and around 800°C suggested the occurrence of additional corrosion processes to that of metallic dissolution caused by molten vanadium compounds. It is well known that the curve describing corrosion by sodium vanadates or vanadium pentoxide is exponential as temperature is increased [13, 14]. Evidence of the processes involved in the corrosion by sulfate-rich oil ashes will be shown in the next section.

Figure 1. Oxidation and oil-ash corrosion of 35Ni19Cr austenitic steel as a function of temperature.

 4. Microanalysis Results 

Figure 2 shows the magnitude of the subsurface attack and if it consisted in internal oxidation (O), sulfidation (S) or both (O, S). The depth of internal attack has been added to the thickness loss in order to estimate the total metal corroded. The arrows on top of the bars corresponding to 785°C y 805°C mean that all the specimens exposed at these temperatures were completely consumed. As it was mentioned in the last paragraph the metal loss at these temperatures was higher than 1.5 mm and this is the meaning of the arrows on top of their respective bars. The scale of the vertical axis was limited to 0.6 mm in order to show the magnitude of the internal attack of the results obtained at the other temperatures. As it can be observed from this Figure, the sulfate-rich oil ash causes sulfidation from 675°C, and it is also developed an internal oxidation process at temperatures above 785°C.
Figure 3 shows the zone near to the interface metal/scale of specimens exposed to 675°C, 715°C, 750°C and 900°C. Chromium sulfides are the black phases at the subjacent zone to the metal/scale interface of the specimens exposed to 675°C, 715°C and 750°C. They precipitated at the chromium-depleted zone.

Figure 2. Thickness loss and internal attack depth of 35Ni19Cr exposed to an oil ash with V/(Na+S)=0.55. O: internal oxidation; S: sulfidation. The arrows on top of the bars corresponding to 785°C and 805°C indicate that the specimens were totally consumed at these temperatures. Their height should be higher than 1.5 mm, but the scale is limited to 0.6 mm in order to show the internal degradation depth data.

The corrosion front is almost uniform at 675°C, but evidence of metallic dissolution is observed at 715°C and 750°C. Figures 4 and 5 shows in more detail the zone near to the interface of specimens exposed to 900°C and 785°C, respectively.
As it was mentioned before, at 785°C and 805°C the corrosion rate was catastrophic and evidence of simultaneous vanadium corrosion and intergranular sulfidation caused by hot corrosion was found in additional specimens exposed during only 125 hours. Figure 3 shows a characteristic feature of these specimens in the group of three micrographs corresponding to 785°C. The backscattered electron image was taken from the bottom of the Cr-depleted zone in the metallic matrix and its Cr and S maps shows that chromium sulfides have been oxidised releasing sulfur to continue the sulfidation process. Therefore they act as nucleation sites for the selective oxidation of chromium at grain boundaries, confirming that grain boundaries are the fastest diffusion paths for chromium [15, 16]. At 900°C there was also observed internal oxidation and precipitation of some Mn sulfides particularly at the bottom of the Cr-depleted zone as can be seen in Figure 4. This figure shows the combined effect of two corrosion mechanisms by oil ashes: sulfidation caused by hot corrosion and corrosion induced by vanadium compounds, which at this temperature causes internal oxidation and also intergranular corrosion. 

Figure 3. Internal attack of the subjacent zone to metal/scale interface in specimens of 35Ni19Cr alloy exposed at temperatures below and above 785°C, temperature at which corrosion was catastrophic. Microanalysis of this zone revealed remarkable matrix Cr-depletion and the presence of chromium sulfides. At 785°C and 900°C it was also identified internal metallic oxides. The micrograph corresponding to 785°C was taken from the bottom of the Cr-depleted zone. The network of internal chromium oxides in this zone revealed chromium sulfides were oxidized releasing sulfur to continue the sulfidation process into the alloy. (Note the scale at the micrographs as they were taken at different magnifications).

Figure 4. Backscattered electron image of the interface scale/metal (a and b) and x-ray mappings that show the role of vanadium and sulfur in the oil-ash corrosion of 35Ni19Cr alloy exposed at 900°C. The specimen was mounted in cross section and observed as-polished in order to see the neighborhood of the metal/scale interface (on the top of the image is found the corrosion product scale). It can be observed internal oxidation (from Cr and Mn maps) and intergranular corrosion (see V map inside the alloy) as well as the presence of some Mn sulfides, particularly at the bottom of the Cr-depleted zone. (a) SEI image, (b) Cr-Ka map, (c) Mn-Ka map, (d) S-Ka map, (e) V-Ka map, (f) Fe-Ka map, (g) Ni-Ka map.

  The X-ray maps of V, Cr, S and Mn suggest that most of the internal corrosion products are at grain boundaries (Cr and Mn sulfides, Cr and Mn vanadates and chromium oxide).
    Figure 5 shows some features of the cross section of 35Ni19Cr alloy exposed 125 hours at 785C. At this temperature, the alloy exhibited catastrophic corrosion rates in the 250-hour tests. Microanalysis and XRD results showed that metallic oxides and metallic vanadates mainly constitute the external scale. Scale fissuring is related to the formation of a non- protective scale, responsible of the catastrophic corrosion exhibited by this alloy. A vanadium-rich layer at the metal/scale interface, and the presence of metallic oxides (Cr, Fe, Ni) that have re-precipitated above it, is the result of the corrosion mechanism involves an accelerated oxidation process. Cr and S X-ray maps obtained from the Cr-depleted zone are evidences of the occurrence of a sulfidation process followed by oxidation. It can be observed inside the alloy zones (see the arrow) containing chromium oxide that are a consequence of the massive precipitation of sulfides that are oxidized releasing the sulfur for continuing the sulfidation process into the alloy. 

Figure 5. 35Ni19Cr alloy exposed to oil ash with V/(Na+S)=0.55 at 785°C by 125 hours. The scale and the zone exhibiting internal attack (BSE image) were typical of the developed under the most corrosive conditions (785°C and 805°C). It can be observed inside the alloy zones (see the arrow) containing chromium oxide that are a consequence of the massive precipitation of sulfides that are oxidized releasing the sulfur for continuing the sulfidation process into the alloy. The high sulfur concentration in the bottom of the Cr-depleted zone is evidence of the auto-sustaining nature of the sulfidation process. V has been incorporated to the oxide scale near to the metal/scale interface, making it even less protective. The region marked with a B (right corner) is the mounting material.

The high sulfur concentration in the bottom of the sulfidized zone is evidence of the auto-sustaining nature of the sulfidation process. Therefore, the high corrosion rates experimented by the alloy at 785°C and 805°C resulted from the simultaneous occurrence of sulfidation and oxidation, being the first one, favored by the high Ni content of the alloy.
EDX line profiles of Fe, Cr, Ni, Si, S, V, O were obtained from the cross- sectioned samples exposed to each test condition. Only for purposes of illustrating the zone where the linescans were taken, Figure 6 shows the cross section of the specimens exposed to 600°C (a), 715°C (b), 750°C (c) and 900°C (d), and the respective line along the elemental line profiles were measured. This line was drew on the backscattered electron image using the plus marks available, however these marks do not represent the exact sites where the analysis were made. The distribution of points for analysis was regular in the zone far from the metal/scale interface, both in the alloy and in the scale.
However, in both the depleted zone and the scale nearest to the metal/scale interface, the analysis was made more meticulously. In those zones, the number of points analyzed was higher, in order to get a more detailed profile because of the relevance of these zones for the understanding of the global corrosion process. It is worth to mention that the analysis was not made using the spot mode, but using a 2 microns width rectangle. In this way, it was obtained an average concentration of each element in the zone covered by the rectangle, which was parallel to the interface.
Figures 7 and 8 show two of the profiles acquired, along a zone near to the metal/scale interface of the specimens exposed at 600°C and 715°C. The vertical axis indicates the concentration of the element. Note that the axe scales are different in order to make visible the variations registered. By comparing both profiles, it is pretty clear that a different corrosion process is occurring at 715°C, as is suggested by the Cr (and Fe) depleted zone and the precipitation of sulfur inside the alloy. The Cr and S profiles clearly show the precipitation of chromium sulfides below the metal/scale interface. It can also be observed the incorporation of  5% V to the oxide scale lattice near to the interface, as well as a high concentration of sulfur in it, making it more defective.

Figure 6. BSE Images of the areas where the linescans were carried out at four of the test temperatures. The center of the 2 microns width rectangles used for the analysis was located on the line drawn using plus symbols.

Figure 7. Line profiles obtained from the specimen exposed at 600°C. (weight %).

Figure 8. Line profiles obtained from the specimen exposed at 715°C. (weight %).

Figure 9. Effect of temperature on Cr line profiles of 35Ni19Cr steel exposed to an oil ash with V/(Na+S)=0.55. (weight %).

Microanalysis also showed matrix Cr-depletion around sulfides due to the sulfidation process.
In Figure 9, the Cr line profiles obtained from the specimens exposed to 600°C, 675°C, 715°C, 750°C and 900°C are compared, in order to show, as temperature is increased, the evolution of the depth of the zone where oxides and sulfides precipitated inside the metal. The Cr levels reached as a result of matrix depletion can also be compared. It is clear that if the matrix is becoming depleted in chromium as a result of the oil ash corrosion process, which intrinsically involves accelerated oxidation besides sulfidation at several temperatures, it will develop a less protective scale. Furthermore, the presence of V and S in the scale also reduces its protectiveness.

 
5. Discussion

It was concluded that, besides high temperature oxidation and depending on temperature, in the range of temperature of 600°C - 900°C, vanadium corrosion (molten salt corrosion), sulfidation or both took place. Sulfidation involves the development of a scale that contains sulfides. The kinetics of this process is faster than the kinetics of oxidation, due to the scales containing sulfides are less protective because they are more defective and sulfides have melting points lower and are less stable than oxides [17]. As the time elapses, sulfur ions may diffuse to the metal/oxide interface, increasing the sulfur potential there and, when the activity is high enough, internal sulfidation occurs.
Matrix chromium depletion depends on the prevailing corrosion mechanism, and microanalysis showed that it could reach figures as high as 1.5% and 2% at 715°C and 900°C, respectively (Figure 9). Cr-depletion is a common process in austenitic alloys due to their chromium diffusion coefficient is low [15].
At temperatures lower than 675°C, corrosion by sodium vanadates rules the process and no Cr-depletion occurs, because metallic dissolution (caused by the molten vanadates) is taking place. However, it is abrupt at the sulfidized zone at the range of temperature where both sulfidation and vanadium corrosion take place. At 715°C the scale contains metallic sulfides in the Cr₂O₃ layer, which explains its non-protective nature, as the ionic transport through sulfide-containing scales is faster than in Cr₂O₃ scales [17]. At 900°C, high temperature oxidation has a preponderant role, and matrix Cr-depletion is gradual at the internal oxidation zone, only increased in the neighborhood of oxidized grain boundaries.
EDX line profiles showed that sulfidation extent is deeper than internal oxidation at 785°C and 805°C, while at 825°C and 900°C oxidation was more relevant than sulfidation.
Mn as alloying element promotes the formation of spinel oxides, which enhances the incorporation of sulfur to the oxide scale and its subsequent diffusion to the metal[17]. Under sulfidizing conditions, the results confirmed that Mn can precipitate as sulfides in the metallic matrix, enhancing the diffusion of sulfur into the alloy.

 
6. Conclusions

From X-ray microanalysis results, it can be concluded that the prevailing corrosion mechanisms, as a function of temperature, are as following:
1. T < 715°C:
Corrosion is induced basically by molten vanadates; high temperature oxidation is also involved as the vanadates enhance the oxygen transport to the metal.
2. T ≈ 715°C:
A sulfidation process caused by the formation of eutectics such as Me-MeS, which melt at temperatures around 700°C is added to the corrosion process induced by molten vanadates
3. 750°C < T < 825°C:
This is the range of temperature the corrosion rate reach the maximum. In it there is a synergistic action of sodium vanadates and sodium sulfate, which besides vanadium corrosion causes a simultaneous sulfidation/oxidation process that involves Cr-depletion of the metallic matrix. It could be related with the classical concept for low temperature hot corrosion.
4. 825°C < T< 884°C (Na₂SO₄ decomp. Temp.) :
In this temperature range it takes place the synergistic action of sodium vanadates and sodium sulfate, but the attack is moderated by high temperature oxidation of the alloy. Therefore, the oxide scale developed by the alloy protects it in certain grade from the attack of molten compounds, resulting in a decrease of corrosion rate.
5. T > 884°C:
At this temperature, the decomposition of Na₂SO₄ occurs, producing Na₂O, which acts as an inhibitor, and hence decreasing sulfidation attack. Therefore the corrosion above this temperature is basically vanadium corrosion strongly moderated by high temperature oxidation

Furthermore, regarding the corrosion resistance of 35Ni19Cr alloy, from these results is concluded that in spite of its good carburization and oxidation resistance, it seems not to be a suitable material when the oil ashes are sulfate-rich, particularly around 800°C.

ACKNOWLEDGMENTS
The authors acknowledge the economic support given by CONACYT and the PADEP-UNAM (México) for the development of this research.

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