Influence of multi-layered thermal diffusion coatings on high-temperature sulfidation resistance of steels

Highlights

• High temperature sulfidation of the coatings vs. bare stainless steel is studied.

• Coatings based on aluminides and borides are obtained via thermal diffusion process.

• No structural degradation and integrity loss of the coatings are observed.

• Factors affecting the coatings' sulfidation resistance are discussed in detail.

• Multi-layered dense aluminide coating structure provides high sulfidation resistance.

Abstract

Corrosion resistance of protective coatings on steels against high-temperature sulfidation conditions was studied. The coatings made via thermal diffusion process had multi-layered architectures and consist of aluminides, iron borides or iron boride - titanium dioxide layers. These coatings applied onto carbon steel and austenitic stainless steel 316 were exposed in the H₂S-Ar gas flows at 500 °C, and their performance was compared to bare stainless steel 316L. While stainless steel experienced sulfidation corrosion of the surface and intensive sulfide scale peeling, the considered coatings remained their integrities with no peeling. The structural examination revealed no new phases' formation within the coatings, no coating thickness reduction, no delamination and crack occurrence. This type of coatings has much potential due to their increased stability in the considered sulfidation conditions and since they are suitable for protection of inner or inner and outer surfaces of long tubing.

Introduction

Performance, reliability and environmental safety in the fossil fuel-fired power plants depend, among different factors, on the resistance of components made of steels and alloys against high temperature corrosion occurring during combustion processes. One of the important factors of the gaseous corrosion is related to the presence of different elements in fuels, which form corrosive ingredients at combustion. Thus, a presence of sulfur in fuels leads to formation of H₂S-containing gases at the operation in coal fired plants, including modern integrated gasification combined cycle (IGCC) plants, as well as at combustion of bitumen, biomass and some other fuel materials. These flowing gases create serious corrosion problems for tubing and other components in coal fired boilers, water-wall panels, super-heaters and re-heaters when operation temperatures reach 400–600 °C. Furthermore, the failure of these components leads to necessity of costly maintenance, unpredicted operation shutdowns and environmental problems. Depending on specific operational features in different power generation plants and refinery sulfur recovery plants, e.g. combustion system design, contents of air at combustion, flow rates, amount of sulfur in the fuel, etc., the corrosive sulfidation atmosphere may vary from high reducing to oxidizing, and, in these conditions, the corrosive action of H₂S on metallic components may differentiate [1,2]. However, regardless the sulfidation conditions, the interaction of H₂S with metals leads to the formation of metal sulfides on the surface of tubing and other production components [[3], [4], [5]]. Although it is desirable that, for reliable performance, the forming scales should be dense, well adhered to the metallic product and should withstand corrosive conditions, in fact, the forming sulfide scales are not protective. Furthermore, the sulfide scales on steels are significantly less protective than oxide scales, and the metal sulfidation rates are usually much higher than the oxidation rates [[5], [6], [7]].

High corrosion rates in H₂S-containing gaseous environments are related to dissociation of H₂S to hydrogen and sulfur at rather low temperatures and consequent interaction of metals with S resulting in the metal sulfide scale formation:

$${\mathrm{H}}_2\mathrm{S}➔{\mathrm{H}}_2+\mathrm{½}{\mathrm{S}}_2$$

$$\mathrm{Me}+\mathrm{S}➔{\mathrm{Me}}_{\mathrm{x}}\mathrm{S}\text{and}\mathrm{Me}+{\mathrm{H}}_2\mathrm{S}➔{\mathrm{Me}}_{\mathrm{x}}\mathrm{S}+{\mathrm{H}}_2$$

The values of metal sulfide formation free energies are rather low [3,6], i.e. these “easily formed” sulfides are not very stable. It is interesting that the H₂S dissociation occurs preferentially on the scale surface, and it is especially pronounced in the case of iron sulfide-based scales [3,8]. Hydrogen ingresses into the metals and scales interstitially, generates the lattice defects and hydrogen clusters, and finally leads to a significant increase of corrosion and embrittlement. Low melting points eutectics are formed in many Me-Me_xS systems, and usually metal sulfides contain large concentrations of mobile defects. These features promote rapid diffusion of sulfur and hydrogen through the porous scales with their consequent interaction with the base metals and disintegration of the components.

In many power generation plants, the tubing for water-wall panels, super-heaters and re-heaters and other components are often made of carbon steels or low alloyed steels; these materials are selected due to their high thermal conductivity combined with good weld properties and low cost. However, in harsh service conditions, these steels do not provide adequate protection at temperatures above 300–400 °C and elevated pressures, which are required by the Super-critical and, moreover, Ultra Super-critical and Advanced Ultra Super-critical coal fired technologies and IGCC [9]. Their poor performance is especially observed in the conditions where H₂S is present [10]. Thus, the recent studies clearly demonstrated that low-alloy steels (K18, T/P1, T/P12 and T/P22 with contents of Cr 0.2–2.5%, Mo up to 1.1% and Mn up to 0.8%) experienced severe degradation under the testing in Ar-H₂S (1 vol.-%) at 450–550 °C, which is attributed to accelerated formation of iron sulfide (FeS) scale and its peeling-off, as well as surface cracking [11]. Although the alloying with Cr promotes the resistance to elevated temperatures and H₂S [8,12,13], stainless steels and even more expensive Ni-based and Ti-based alloys also experienced corrosion issues related to peeling of the sulfide scales and surface micro-cracking, albeit to a lesser extent [6,[14], [15], [16], [17]]. Stainless steels and alloys containing Al usually exhibit higher sulfidation resistance [17,18]. According to Datta et al. [19], conventionally produced stainless steels and alloys do not have high sulfidation resistance, and the addition of the semi-refractory and refractory metals from the IV-VI groups, such as Ti, Zr, Hf, V, Nb, Ta, W, Mo, can improve their performance. However, this route leads to a significant cost increase of expensive steels and alloys. The presence of oxygen and/or H₂O, i.e. the combined action of oxidation and sulfidation, may significantly change the corrosion rate of steels because of formation of sulfides and oxides with different adhesions to the steel surface and different coefficients of thermal expansion; although the influence of the combined action of these gases is not well understood. To resolve these high-temperature H₂S corrosion issues, surface engineering of steel components, e.g. tubing, can be considered as a high potential route.

Surface engineering, in particular, different coating materials and technologies may be considered for the integrity improvement of the steel surface in high temperature corrosion environments. They may include options of applying the chemically inert compounds by dipping, chemical electrolytic or electroless deposition, cold and thermal spraying and cladding, physical vapor deposition (PVD) and chemical vapor deposition (CVD) and other options [[20], [21], [22], [23], [24], [25]]. However, the majority of these routes are not well suited for the considered application. Many coating options either have poor performance under high temperatures, or their thicknesses are insignificant with a possible presence of structural defects comparable with the coating thickness. These defects usually become the corrosion initiators. Porosity in the coatings, which occur at some processing routes (e.g. thermal spray and similar technologies), also promotes corrosion by being the corrosion initiator; the corrosion products accumulated inside the pores create the wedging effect with further corrosion evolution. Some processes cannot provide good adhesion of the coating to steel substrates resulting in the detachment issues, especially pronouncing at elevated temperatures and pressures. Although some processing routes, like PVD and thermal spraying, can provide good adhesion of the coatings to metallic substrates and may be of high potential when appropriate coating compositions are selected, they are hardly applicable when the inner surface of long tubing or complex shape components need to be protected. In this regard, CVD technological routes could be more suitable, especially for components with various shapes. However, some of them have limitation related to complexity of production equipment, productivity and necessity to use hazardous gases for certain processes.

Despite these shortcomings, thermal diffusion surface engineering route, which is based on the CVD principles, may be considered. This route is versatile, applicable for protection of surfaces of components with various configurations and dimensions and of inner or inner and outer surfaces of long tubing, it does not require complicated equipment and dedicated controlling devices with no necessity to introduce reactive, often hazardous, gases to the process chamber [[26], [27], [28], [29], [30]]. Applying this technology, the coatings based on the compounds with high energies of crystalline lattice and with nano- or submicron crystalline structures can be formed on the steel surfaces. These materials' features (e.g. certain thermodynamic characteristics and structure) are favorable for corrosion protection [[31], [32], [33], [34]].

Considering the thermal diffusion technology, which also named as thermo-chemical processing or pack cementation, boronizing and aluminizing processes may be selected as the most promising for high temperature sulfidation protection of steels. These processes provide the formation of protective coatings consisting of either iron borides or aluminides. These coatings have multi-layered architectures with two or more protective layers formed simultaneously during a single process cycle due to the features of diffusion processes [[26], [27], [28], [29],35]. These crystalline compounds (borides and aluminides) possess high crystalline lattice energies, strong and short covalent bonds (e.g. FeB, FeAl, CrAl, etc.), which define high chemical inertness of materials [28,33], and the related coatings have diffusion-induced bonding between the protective layers and the steel substrates. The recent studies demonstrated high corrosion resistance and applicability of these materials in different aggressive environments [[26], [27], [28], [29], [30],[36], [37], [38], [39], [40]]. An additional positive feature of these coatings, especially based on aluminides, is the formation of protective oxide scale at high temperatures, which does not have a spallation issue [30,36,38,40]. Although these coatings have not been explored for the high temperature sulfidation applications before, it could be logical to consider them for these applications.

The use of oxide materials with a high crystalline lattice energy and a high degree of covalence in the Me-O bonds, such as SnO₂, TiO₂, ZrO₂, ThO₂, Al₂O₃, Y₂O₃ (i.e. oxides of metals of III–IV groups of the Periodic Table), may promote corrosion resistance of protective coatings [32,34]. These oxides may be selected for the top layer applied over the thermal diffusion coatings. In this regard, the top layer based on TiO₂, as an example from the mentioned group of materials, can be considered as a route to create a multi-layered coating architecture for additional potential protection of boronized steel, particularly to promote its oxidation resistance.

This paper presents the results obtained from the testing of thermal diffusion coatings based on iron borides and aluminides in high temperature (500 °C) sulfidation conditions, when these coatings, as well as bare stainless steel 316L, were exposed in the gas flows contained H₂S. This testing was conducted employing a special testing rig, when different materials can be exposed simultaneously. As mentioned above, while low-alloy steels had poor performance in sulfidation conditions, there is limited information related to the performance of significantly more expensive austenitic stainless steels in these conditions. Moreover, the thermal diffusion coatings have never been systematically studied in high temperature H₂S-containing gaseous flows. Structural examination (e.g. surface and cross-section structures and compositions) clearly displayed intensive scaling, peeling and surface cracking of stainless steel. In contrast, the thermal diffusion coatings revealed their minimal or practically no degradation (depending on the type of the coating) even when they were applied on inexpensive carbon steel.