WO2021052704A1

WO2021052704A1 - High temperature protective coatings, especially for use in petrochemical processes

Info

Publication number: WO2021052704A1
Application number: PCT/EP2020/073354
Authority: WO
Inventors: Alexander Stenzel; Michael Schuetze; Jeffrey B Hoke
Original assignee: Basf Se
Priority date: 2019-09-19
Filing date: 2020-08-20
Publication date: 2021-03-25

Abstract

The present invention relates to an interdiffusion barrier-free coated substrate wherein the substrate is formed by a first alloy comprising a first chemical element El₁ in an amount of a first concentration c₁(El₁) and a relative chemical activity a₁(El₁), wherein the coating is formed by a second alloy comprising the first chemical element El₁ in an amount of a second concentration c₂(El₁), which is different from the first concentration, and a relative chemical activity a₂(El₁), wherein │C₁(El₁) - c₂(El₁) │ / (c₁(El₁) + c₂(El₁)) ≥ 0.3, wherein the first alloy further comprises at least a second chemical element El₂ in an amount of a first concentration C₁(El₂), wherein the second alloy comprises the second chemical element in an amount of a second concentration C₂(El₂), which is different from the first concentration, wherein │ C₁(El₂) - C₂(El₂) │> 0.1 wt.-%, wherein the higher concentration of C₁(El₁) and C₂(El₁) and the lower concentration of C₁(El₂) and C₂(El₂) are in the same alloy, and wherein │ a₁(El₁) - a₂(El₁) | at a temperature ϑ is below 0.3 and ϑ is in the range from 500°C to 1500°C. The invention further relates to a method for the preparation of said coated substrate.

Description

High Temperature Protective Coatings, especially for use in petrochemical processes

Description

The present invention relates to an interdiffusion barrier-free coated substrate and a method for the preparation of said coated substrate.

Coatings are used to optimize the chemical or physical properties of surfaces and to protect the underlying material from environmental attack. Typical examples are coatings that prevent cor- rosive attack by forming a passivating slow-growing oxide layer and thermal barrier coatings which protect the substrate material from direct exposure to hot gases in turbine engines. Thereby, the composition of coating and substrate differ widely which leads to concentration gradients and interdiffusion effects especially at high temperatures.

This phenomenon particularly affects the sub-surface zone of the substrate material and the inner part of the coating due to outward and inward diffusion of alloying/coating elements.

In the past diffusion barriers in form of intermediate layers were developed to suppress or at least reduce these interdiffusion processes between substrate and coating. Mainly ceramic or intermetallic diffusion barriers were developed that ideally need to exhibit the following charac- teristics: First, inertness between the adjacent materials; second, optimally adjusted thermal expansion coefficients between substrate and the layers; and third, sufficient adhesion to pre- vent delamination.

However, virtually all diffusion barriers developed so far suffer from their brittleness leading to crack formation and delamination from the substrate limiting their durability. Thus, none of these barriers has found its way to a wider technological application.

For technical processes, where a high resistance of the coating to spalling is required, diffusion coatings are preferentially applied as protective coatings (corrosion protection as main function). In diffusion coatings, the substrate surface is enriched with an element or an alloy composition that provides resistance against high-temperature corrosion. For example, patent EP 2 781 561 B1 discloses a pack cementation process with aluminum to form a protective layer of alumina on the top of the coating. Typically, aluminum, chromium and/or silicon are used in pack cemen- tation, which has been a well-established method in industry for many years. More advanced coatings also include further functional properties going beyond mere surface protection. In this context the most important function appears to be the catalytic activity of the surface influencing the chemical reaction situation at the component surface in a beneficial way. Such concepts are of significant interest in petrochemical industry and have been described e.g. in WO 2010/009 718 A2 and Patent WO 2013/181 606 A1 . The latter patent publication discloses a catalytically- active multicomponent coating for the chemical conversion of petrochemicals in steam crackers. Multi-component coatings are commonly consolidated by a thermal treatment with an interaction of the pre-coating and the base alloy.

As already mentioned, during operation interdiffusion effects can take place at high tempera- tures resulting in a change of the coating composition and, thus, in the coating/surface proper- ties potentially impairing their functions. Therefore, as a further function in addition to those mentioned in the last section, compositional stability of the coating is desired maintaining the original optimized properties. Since the existing interdiffusion barrier layer concepts have so far not provided a clearly satisfactory solution, an objective of the present invention is to provide a different approach.

In this approach the complete coating and/or the base alloy in its total thickness or a significant part of the base alloy subsurface zone is stabilized against interdiffusional effects and can, thus, eliminate the necessity for an additional diffusion barrier.

Furthermore, this approach can be combined with other functions, e.g. catalytic functions.

The object is achieved by an interdiffusion barrier layer-free coated substrate having a coating on the substrate, wherein the substrate is formed by a first alloy comprising a first chemical element El₁ in an amount of a first concentration c₁(El₁) and a relative chemical activity a₁(El₁), wherein the coating is formed by a second alloy comprising the first chemical element El₁ in an amount of a second concentration c₂(El₁), which is different from the first concentration, and a relative chemical activity a₂(El₁), wherein | c₁(El₁) - c₂(El₁) | / (c₁(El₁) + c₂(El₁)) ³ 0.3 (preferably ³ 0.4), wherein the first alloy further comprises at least a second chemical element El₂ in an amount of a first concentration c₁(El₂), wherein the second alloy comprises the second chemical element in an amount of a second concentration c₂(El₂), which is different from the first concentration, wherein | c₁(El₂) - c₂(El₂) | > 0.1 wt.-%, wherein the higher concentration of c₁El and c₂(El and the lower concentration of c₁(El₂) and c₂(EI₂) are in the same alloy, and wherein the chemical activity difference | a₁(El - a₂(El | at a temperature q is below 0.3 and q is a temperature selected from the temperature range from 500°C to 1500°C, preferably from the range from 750°C to 1250°C. Within the meaning of the present invention the term “concentration” of a chemical element in a phase (substrate or coating) means the weight amount of the element in weight-% (wt.-%) based on the total weight of the phase. A chemical element which is absent in a phase is to be considered as having a concentration of 0 wt.-% in that phase according to the invention.

The term “relative activity” describes how strong a real solution deviates from an ideal solution of identical composition. Relative activities of chemical elements in alloys can be quantified by thermodynamic calculations used in commercial programs, like JMatPro, ThermoCalc, or HSC Chemistry. In these commercial programs, thermodynamic databases are implemented, which contain the enthalpy H, the entropy S and the heat capacity C_p of elements and compounds along with a variety of additional information for thermodynamic calculations.

Surprisingly, a protective coating/base alloy couple that provides high temperature corrosion protection, high temperature creep resistance, and a deliberate catalytic surface activity with at the same time being stable against interdiffusional effects can be obtained by an interdiffusion layer-free coated substrate of the present invention.

In general, all high temperature protective coatings have in common that beside a compositional change of the coating itself a diffusion zone develops in the substrate beneath the coating due to the interdiffusional effects.

As mentioned above, interdiffusion would result in an intermixing of the coating and the base alloy (substrate) components during exposure to elevated temperatures.

In this context, two specific categories may be distinguished: First, the diffusion of elements from the coating, for example aluminum, chromium, and silicon, into the substrate, which leads to a loss of expedient elements in the coating. Second, the diffusion of elements from the sub- strate, for example iron, nickel, cobalt, and chromium, into the coating, which results in a degra- dation of the coating functionalities and of the subsurface zone properties. Either case may oc- cur individually, but in most of the cases, it is a superposition of both.

The driving force for interdiffusion is the chemical potential gradient between coating and sub- strate alloy. Directional solid state high temperature diffusion occurs along a chemical potential gradient. The chemical potential gradient or activity gradient follows the alloy concentration gra- dient in a material system. Critical for interdiffusion effects are those elements within the base alloy-coating system that exhibit a high diffusion rate and a high concentration gradient between coating and base alloy. The chemical potential can be calculated at a given composition for the coating as well as for the substrate. Interdiffusion between these two diffusion partners can be prevented or at least reduced by equalizing or approaching the respective chemical potentials to each other between the coating and the substrate. Surprisingly, this does not require that the concentration gradient of the same element has to be adjusted to zero in the alloy subsurface and the coating but other alloying elements can take over this role, in that they affect the chemical activity of the respective element whose activity gradient has to be adjusted.

Since the activities of the components, a,, are directly related to the chemical potential by the equation m_i ^*- m_i ⁰=RTInai, the coating composition can be modified in such a way, that the activity gradients are reduced for the interesting elements. Here m,° is the chemical potential of the component in the standard state, i.e. under atmospheric pressure, at 25°C, and with its most stable crystal structure. The activity a, is hereby a dimensionless quantity, which is related to a measured mole fraction x_i, mass fraction w_i, or vapor pressure p, by the activity coefficient y_i, which is likewise a dimensionless quantity [B. Predel, M. Hoch, M. Pool, Phase Diagrams and Heterogeneous Equilibria, Springer Berlin, 2004,]. The equation is given exemplarily for the former: a_i = y x_i. Thereby, the mole fraction x, is defined as: x_i = n_i/n_tot. Thus, n, is the amount of a constituent (stated in the unit “moles”) divided by the total amount of all constituents in a mixture (also stated in the unit “moles”). Therefore, the mole fraction x, is also dimensionless, and the sum of all mole fractions is equal to 1.

The relative activities of the components can be calculated, among others, by using recent thermodynamic simulation software, like JMatPro or ThermoCalc. The calculations are based on the CALPHAD method, which makes use of all available experimental and theoretical data on phase equilibria, thermophysical as well as thermochemical information in a system. In this work, the Fe alloys module and the Ni alloys module of JMatPro version 10.2 were used. The Fe alloys module in JMatPro deals with the following elements: Al, B, C, Co, Cr, Cu, Fe, Mg,

Mn, Mo, N, Nb, Ni, O, P, S, Si, Ta, Ti, V, and W. The Ni alloys module in JMatPro deals with the following elements: Al, B, C, Co, Cr, Cu, Fe, Hf, Mn, Mo, N, Nb, Ni, O, Pt, Re, Ru, Si, Ta, Ti, V, W, and Zr. Data not already available from the databases can be determined and added to the databases for the calculations.

All the thermodynamic properties available are described with mathematical models that reliably display or predict the course of the Gibb’s free energy or different thermodynamic quantities for each system.

This interdiffusion effect for a chemical element El₁ is thus driven by different relative activities for said element El₁ in the substrate compared to the coating. The interdiffusion effect results in diffusion of said element El₁ from the El₁-rich phase (substrate or coating) into the El₁-poor phase (coating or substrate), i.e. along the concentration gradient even though the driving force is the activity rather than the concentration. As mentioned above this interdiffusion has to be prevented or at least suppressed.

Surprisingly it was found that a concentration gradient for a chemical element El₁ can be upheld even at high temperature when the difference in the activity of said element El₁ is sufficiently low, which is possible by an inverse concentration gradient of a second chemical El₂, wherein said chemical element El₂ is an activity influencing element for the chemical element El₁.

The influencing effect of a chemical element to the activity of another chemical element is known in the art.

Accordingly, it was found that a substrate can formed by a first alloy comprising a first chemical element El₁ in an amount of a first concentration c₁(El₁) and a relative chemical activity a₁(El₁), wherein the coating is formed by a second alloy comprising the first chemical element El₁ in an amount of a second concentration c₂(El₁), which is different from the first concentration, and a relative chemical activity a₂(E_l1).

The difference between the concentrations is expressed by the following in equation: | c₁(El₁) - c₂(El₁) | / (c₁(El₁) + c₂(El₁)) ³ 0.3, preferably ³ 0.4, even more preferably > 0.5.

The activity influencing element El₂ is also present in different concentrations so that the first alloy further comprises at least a second chemical element El₂ in an amount of a first concentra- tion c₁(El₂) and the second alloy comprises the second chemical element in an amount of a second concentration c₂(El₂), which is different from the first concentration with a difference of

I c₁(El₂) - c₂(El₂) I > 0.1 wt.-%.

It is important that the higher concentration of c₁(El₁) and c₂(El₁) and the lower concentration of c₁(El₂) and c₂(El₂) are in the same alloy in order to allow the second element El₂ to influence the activity of El₁.

The inventors have found that in the temperature range from 500°C to 1500°C the difference in relative activity | a₁(El - a₂(E_l1) | can be achieved resulting in a value of below 0.3, which is caused by the difference of | c₁(El₂) - c₂(El₂) | even though a concentration gradient | c₁(El₁) - c₂(El₁) I / (c₁(El₁) + c₂(El₁)) is equal to or higher than 0.3.

In one preferred embodiment of the present invention El₁ is Cr and El₂ is Mo or Ta.

Preferably, | c₁(El₁) - c₂(El₁) | / (c₁(El₁) + c₂(El₁)) ³ 00.6.3 and preferably £ 0.7. Preferably, | c₁(El₂) - c₂(El₂) I > 8 wt.-% and preferably £ 11 wt.-%. Preferably the higher concentration of c₁(El₁) and c₂(El₁) and the lower concentration of c₁(El₂) and c₂(El₂) are in the first alloy, i.e. the substrate.

For the example of high carbon and high chromium-containing steels as base alloys (substrate, especially as substrate for catalysts, where the active catalytic layer can be applied), the me- chanical properties can change by chromium (Cr) subsurface zone depletion resulting from dif- fusion of Cr into the coating. When the Cr content decreases by diffusion to a certain critical value, the Cr carbide precipitations, providing creep resistance of the substrate material, start to dissolve. Thus, the high temperature strength of the alloy can be reduced, or Kirkendall porosity can be a result of the interdiffusion processes.

Similarly the Cr activity influencing chemical element can be represented by Nb. Accordingly in another preferred embodiment El₁ is Cr and El₂ is Nb. Preferably, | c₁(El₁) - c₂(El₁) | l (c₁(El₁) + c₂(EI_I)) ³ 0.6 and preferably £ 0.7. Preferably, | c₁(El₂) - c₂(El₂) | > 10 wt.-% and preferably £ 13 wt.-%. Preferably the higher concentration of c₁(El₁) and c₂(El₁) and the lower concentra- tion of c₁(El₂) and c₂(El₂) are in the first alloy, i.e. the substrate.

In another preferred embodiment of the present invention El₁ is Al and El₂ is Fe or Nb. Prefera- bly, | c₁(El₁) - c₂(El₁) | / (c₁(El₁) + c₂(El₁)) ³ 0.4 and preferably £ 0.6. Preferably,

I c₁(El₂) - c₂(El₂) I > 14 wt.-% and preferably £ 18 wt.-%. Preferably, the higher concentration of c₁(El₁) and c_c(El₁) and the lower concentration of c₁(El₂) and c₂(El₂) are in the second alloy, i.e. the coating.

Here, the reverse case occurs in high temperature applications. The coating can continuously loose expedient elements, which are extremely important to form a dense, adherent, and pro- tective oxide scale. Especially MCrAIY coatings (where M=Ni, Fe or Co) applied on nickel-based super alloys show this type of degradation.

Within the coating, the b-phase is formed, which particularly acts as an aluminum reservoir for the formation of the highly protective alumina scale. In the long term, the aluminum reservoir in the coating depletes, whereby the capability of the coating to form the protective alumina scale diminishes. Additionally, the base alloy is enriched in aluminum, which leads to a deterioration of the mechanical properties of the base alloy. However, depending on coating type and func- tion as well as substrate material other degradation processes resulting from interdiffusion can prevail as well.

In a further preferred embodiment El₁ is Al and El₂ is Hf. Preferably, | c₁(El₁) - c₂(El₁) | / (c₁(El₁) + c₂(El₁)) ³ 0.4 and preferably £ 0.6. Preferably, | c₁(EI₂) - c₂(EI₂) | ³ 17 wt.-% and preferably £

20 wt.-%. Preferably the higher concentration of c₁(El₂) and c₂(El₂) and the lower concentration of c₁(El₂) and c₂(El₂) are in the second alloy.

The chemical element Al can also act as activity influencing element to the element Si. Accord- ingly, in a preferred embodiment of the present invention El₁ is Si and El₂ is Al. Preferably, | | c₁(El₁) - c₂(El₁) | / (c₁(El₁) + c₂(El₁)) ³ 0.5 and preferably £ 0.7. Preferably, I c₁(El₂) - c₂(El₂) I ³ 14 wt.-% and preferably £ 18 wt.-%. Preferably, the higher concentration of c₁(El₁) and c₂(El₁) and the lower concentration of c₁(El₂) and c₂(El₂) are in the second alloy.

In a preferred embodiment of the present invention El₁ is Si and El₂ is Cu. Preferably,

I c₁(El₁) - c₂(El₁) I / (ci(El₁) + c₂(El₁)) ³ 0.5 and preferably £ 0.7. Preferably, I c₁(El₂) - c₂(El₂) I ³

15 wt.-% and preferably £ 19 wt.-%. Preferably, the higher concentration of c₁(El₁) and c₂(El₁) and the lower concentration of c₁(El₂) and c₂(El₂) are in the second alloy. In a further preferred embodiment of the present invention EI₁ is Si and El₂ is Ru. Preferably, | c₁(EI₁) - c₂(EI₁) I / (c₁(EI₁) + c₂(EI₁)) ³ 0.5 and preferably £ 0.7. Preferably, | c_I(EI₂) - c₂(EI₂) | > 13 wt.-% and preferably £ 16 wt.-%. Preferably the higher concentration of c₁(El₁) and c₂(El₁) and the lower concentration of c_I(EI₂) and c₂(EI₂) are in the second alloy.

Another aspect of the present invention is the role of the coating as a catalyst, preferably a steam cracking catalyst, comprising the coated substrate of the present invention.

Addressing these issues, the present invention provides an adaptation process, which results in stabilization against interdiffusion inherent to the coating or to the base alloy (substrate).

In contrast to conventional diffusion barriers, which are implemented between the coating and the base alloy as a separate entity, the envisaged concept allows the coating (or the base alloy) to act as a diffusion sluggish phase in its entirety.

The adaptation process can be applied to both, the high temperature protective coating and, the high temperature base alloy (substrate) or base alloy subsurface zone, and prevents the migra- tion of elements from the base alloy into the coating and vice versa. Any alloy can serve as base alloy (substrate), for instance, nickel-based or iron-based alloys that form either chromium oxide or aluminum oxide at the outer scale.

The composition of the coating is adjusted by minimizing the activity gradient of the elements diffusing between the base alloy and the coating or vice versa. Thus, the sub-surface zone of the coating and the base-alloy can be stabilized.

Even though the diffusion occurs in a boundary zone of substrate and coating the driving force of interdiffusion is still given as long activity gradients are present within the whole phase, i.e. the coating or substrate.

Accordingly, another aspect of the present invention is a method for the preparation of an inter- diffusion barrier layer-free coated substrate. The method comprises the steps of

(a) providing a substrate formed by a first alloy and a coating formed by a second alloy;

(b) identifying a chemical element El₁, which is interdiffusion active at a temperature q is in the range from 500°C to 1500°C;

(c) determining a chemical element El₂, which is influencing the relative activity of EI₁;

(d) adjusting the concentrations of EI₁ and El₂ in the substrate and/or the coating and deter- mine the relative activity of EI₁ in substrate and coating; (e) repeating step (d) until the difference of the relative activities in substrate and coating is reduced.

(f) preparing the coated substrate having the concentrations of EI₁ and El₂ in the substrate and/or the coating resulting from steps (d) and (e).

Preferably, the coated substrate of step (f), wherein the first alloy comprises a first chemical element El₁ in an amount of a first concentration c₁(El₁) and at least a second chemical element El₂ in an amount of a first concentration c_I(EI₂) and wherein the second alloy comprises the first chemical element EI₁ in an amount of a second concentration c₂(EI₁), which is different from the first concentration and wherein the coating is formed by a second alloy comprising the first chemical element EI₁ in an amount of a second concentration c₂(EI₁), which is different from the first concentration, and a relative chemical activity a₂(EI₁), and wherein the first alloy further comprises at least a second chemical element El₂ in an amount of a first concentration c₁(EI₂) and wherein the second alloy comprises the second chemical element in an amount of a sec- ond concentration c₂(EI₂), which is different from the first concentration and wherein the higher concentration of c₁(El₁) and c₂(EI₁) and the lower concentration of c_I(EI₂) and c₂(EI₂) are in the same alloy.

Preferably, at least one of the inequations applies: a) c₁(EI₁) - c₂(EI₁) / (c₁(EI₁) + c₂(EI₁)) ³ 0.3, preferably ³ 0.4, preferably, ³ 0.5; b) c₁(EI₂) - c₂(EI₂) > 0.1 wt.-%; c) a₁(EI₁) - a₂(EI₁) at a temperature q is below 0.3 and q is in the range from 500°C to 1500

C.

The interdiffusion barrier layer-free coated substrate in step (f) can be prepared by conventional processes for preparing alloys and coatings.

Another aspect is an interdiffusion barrier layer-free coated substrate obtainable by a method according to the present invention.

Preferred chemical elements EI₁ / El₂ are Cr/Mo, Cr/Ta, Cr/Nb, Al/Fe, Al/Nb, Al/Hf, Si/AI, Si/Cu, Si/Ru. More preferred is Cr/Mo or Cr/Nb, even more preferred chemical elements EI₁ / El₂ are Cr/Nb.

Accordingly, an embodiment of the present invention is an interdiffusion barrier layer-free coat- ed substrate having a coating on the substrate, wherein the first alloy comprises a first chemical element EI₁ in an amount of a first concentration c₁(EI₁) and at least a second chemical element El₂ in an amount of a first concentration c_I(EI₂) and wherein the second alloy comprises the first chemical element EI₁ in an amount of a second concentration c₂(EI₁), which is different from the first concentration and wherein the coating is formed by a second alloy comprising the first chemical element El₁ in an amount of a second concentration c₂(El₁), which is different from the first concentration, and a relative chemical activity a₂(El₁), and wherein the first alloy further comprises at least a second chemical element El₂ in an amount of a first concentration c₁(El₂) and wherein the second alloy comprises the second chemical element in an amount of a sec- ond concentration c₂(El₂), which is different from the first concentration and wherein the higher concentration of c₁(El₁) and c₂(El₁) and the lower concentration of c₁(El₂) and c₂(El₂) are in the same alloy and wherein chemical elements El₁ / El₂ are Cr/Mo or Cr/Ta or Cr/Nb and wherein the first alloy comprises 30 wt.-% to 40 wt.-% Cr and the second alloy comprises 7 wt.-% to 11 wt.-% Mo or 10 wt.-% to 14 wt.-% Nb or 7 wt.-% to 11 wt.-% Ta. Optionally the first alloy fur- ther comprises 40 wt.-% to 50 wt.-% Ni. Optionally the first alloy further comprises 10 wt.-% to 20 wt.-% Fe. Optionally the first alloy further comprises Mn, C and Nb in an amount of > 0 wt.-% and £5 wt.-%. Especially the first alloy further comprises 40 wt.-% to 50 wt.-% Ni and 10 wt.-% to 20 wt.-% Fe and Mn, C and Nb in an amount of > 0 wt.-% and £ 5 wt.-%. Preferably, the sec- ond alloy further comprises 25 wt.-% to 35 wt.-% Ni. Preferably, the second alloy further com- prises 5 wt.-%- to 8 wt.-% Si. Preferably the second alloy further comprises 5 wt.-% to 15 wt.-% Fe. Preferably, the second alloy further comprises 35 wt.-% to 45 wt.-% W. Preferably, the sec- ond alloy further comprises 7 wt.-% to 11 wt.-% Mn. Especially, the second alloy further com- prises 0.5 wt.-% to 2.5 wt.-% C. Preferably, the second alloy further comprises 25 wt.-% to 35 wt.-% Ni and 5 wt.-%- to 8 wt.-% Si and 5 wt.-% to 15 wt.-% Fe and 35 wt.-% to 45 wt.-% W and 7 wt.-% to 11 wt.-% Mn and 0.5 wt.-% to 2.5 wt.-% C.

Accordingly, a further embodiment of the present invention is an interdiffusion barrier layer-free coated substrate having a coating on the substrate, wherein the first alloy comprises a first chemical element El₁ in an amount of a first concentration c₁(El₁) and at least a second chemi- cal element El₂ in an amount of a first concentration c₁(El₂) and wherein the second alloy com- prises the first chemical element El₁ in an amount of a second concentration c₂(El₁), which is different from the first concentration and wherein the coating is formed by a second alloy com- prising the first chemical element El₁ in an amount of a second concentration c₂(El₁), which is different from the first concentration, and a relative chemical activity a₂(El₁), and wherein the first alloy further comprises at least a second chemical element El₂ in an amount of a first concentra- tion c₁(El₂) and wherein the second alloy comprises the second chemical element in an amount of a second concentration c₂(El₂), which is different from the first concentration and wherein the higher concentration of c₁(El₁) and c₂(El₁) and the lower concentration of c₁(El₂) and c₂(El₂) are in the same alloy and wherein chemical elements El₁ / El₂ are Al/Fe or Al/Nb or Al/Hf and where- in the first alloy comprises 2 wt.-% to 5 wt.-% Al and the second alloy comprises 5 wt.-% to

15 wt.-% Al and 12 wt.-% to 20 wt.-% Fe or 17 wt.-% to 21 wt.-% Hf or 12 wt.-% to 20 wt.-% Nb. Optionally the first alloy further comprises 50 wt.-% to 60 wt.-% Ni. Optionally the first alloy fur- ther comprises 7 wt.-% to 11 wt.-% Co. Optionally the first alloy further comprises 8 wt.-% to

16 wt.-% Cr. Optionally the first alloy further comprises Mo, W, Ta and Ti in an amount of > 0 wt.-% and £7 wt.-%. Especially the first alloy further comprises 50 wt.-% to 60 wt.-% Ni and 7 wt.-% to 11 wt.-% Co and 8 wt.-% to 16 wt.-% Cr and Mo, W, Ta and Ti in an amount of

> 0 wt.-% and £7 wt.-%. Preferably, the second alloy further comprises 25 wt.-% to 35 wt.-% Ni. Preferably, the second alloy further comprises 25 wt.-%- to 35 wt.-% Cr. Preferably the second alloy further comprises C, Mn, and Si in an amount of > 0 wt.-% and £1 wt.-%. Especially, the second alloy further comprises 25 wt.-% to 35 wt.-% Ni and 25 wt.-%- to 35 wt.-% Cr and C, Mn, and Si in an amount of > 0 wt.-% and £1 wt.-%.

Accordingly, a further embodiment of the present invention is an interdiffusion barrier layer-free coated substrate having a coating on the substrate, wherein the first alloy comprises a first chemical element El₁ in an amount of a first concentration c₁(El₁) and at least a second chemi- cal element El₂ in an amount of a first concentration c₁(El₂) and wherein the second alloy com- prises the first chemical element El₁ in an amount of a second concentration c₂(El₁), which is different from the first concentration and wherein the coating is formed by a second alloy com- prising the first chemical element El₁ in an amount of a second concentration c₂(El₁), which is different from the first concentration, and a relative chemical activity a₂(El₁), and wherein the first alloy further comprises at least a second chemical element El₂ in an amount of a first concentra- tion c₁(El₂) and wherein the second alloy comprises the second chemical element in an amount of a second concentration c₂(El₂), which is different from the first concentration and wherein the higher concentration of c₁(El₁) and c₂(El₁) and the lower concentration of c₁(El₂) and c₂(El₂) are in the same alloy and wherein chemical elements El₁ / El₂ are Si/AI or Si/Cu or Si/Ru and where- in the first alloy comprises 0.5 wt.-% to 2.5 wt.-% Si and the second alloy comprises 5 wt.-% to 9 wt.-% Si and 12 wt.-% to 20 wt.-% Al or 13 wt.-% to 21 wt.-% Cu or 11 wt.-% to 19 wt.-% Ru. Optionally, the first alloy further comprises 15 wt.-% to 25 wt.-% Ni. Optionally the first alloy fur- ther comprises 50 wt.-% to 60 wt.-% Fe. Optionally the first alloy further comprises 20 wt.-% to 30 wt.-% Cr. Optionally the first alloy further comprises Mn, and C in an amount of > 0 wt.-% and £ 1 wt.-%. Especially the first alloy further comprises 15 wt.-% to 25 wt.-% Ni and 50 wt.-% to 60 wt.-% Fe and 20 wt.-% to 30 wt.-% Cr and Mn and C in an amount of > 0 wt.-% and £ 1 wt.-%. Preferably, the second alloy further comprises 30 wt.-% to 40 wt.-% Ni. Preferably, the second alloy further comprises 20 wt.-%- to 30 wt.-% Fe. Preferably, the second alloy further comprises 20 wt.-%- to 30 wt.-% Cr. Preferably, the second alloy further comprises 4 wt.-%- to 10 wt.-% Si. Preferably, the second alloy further comprises 5 wt.-%- to 15 wt.-% Al. Especially, the second alloy further comprises 30 wt.-% to 40 wt.-% Ni and 20 wt.-%- to 30 wt.-% Fe and 20 wt.-%- to 30 wt.-% Cr and 4 wt.-%- to 10 wt.-% Si and 5 wt.-%- to 15 wt.-% Al.

The present invention provides a new method for producing diffusion coatings that on the one hand protect in their entirety metallic alloys including additional functions, e.g. a catalytic effect, and on the other hand possess a high stability against interdiffusion effects with the substrate. Thus, conventional interdiffusion barriers would no longer be needed.

The composition of the desired coating strongly depends on the composition of the base alloy, which is chosen for a certain field of application. The respective base alloy (substrate) or coat- ing composition has to be designed in a way that the respective chemical potential gradients are minimized which does, however, not require minimization of concentration gradients since the chemical potential of one element can be aligned by other alloying elements affecting the chem- ical potential or activity of the former. The results of the following examples illustrate the present invention, wherein the scope of the invention is not limited to the examples. The coating in the examples is based on a multi- component coating system that is of particular interest for petrochemical applications and pri- marily on preventing the interdiffusion of Cr from the substrate into the coating. Moreover, the invention is conceptually demonstrated in regard to two further base alloy (substrate)/coating couples by means of thermodynamic calculations. However, as already mentioned any other type of coating/substrate system can be enhanced in the same sense.

A catalytic coating is applied on two base alloys, a pure chromium and a nickel-based alloy. In order to reduce the diffusion of elements from the substrate into the coating, the activities of the diffusing elements are aligned within the coating to the activity in the substrate. The converse process, namely, the adjustment of activities within the substrate to the activity within the coat- ing may lead to a similar result. This is conceptually demonstrated in regard to two further base alloy/coating couples by means of thermodynamic calculations.

Brief Description of the Drawings

A more complete description of the invention and of its advantages can be obtained by refer- ence to the following detailed description and the accompanying drawings, wherein:

Fig. 1 displays the function of the Cr activity vs. Mo content in the coating alloy at 1012°C.

Fig. 2 displays the function of the Cr activity vs. Nb content in the coating alloy at 1012°C.

Fig. 3 displays the function of the Cr activity vs. Ta content in the coating alloy at 1012°C.

Fig. 4 displays a technical drawing of a fixture for diffusion couple tests.

Fig. 5a shows in a diagram the concentration profile for Cr after a diffusion couple test per- formed at 1062°C for 150 h with the base alloy BA0 and coating composition CA0.

Fig. 5b shows in a diagram the concentration profile for Cr after a diffusion couple test per- formed at 1062°C for 150h with the base alloy BA0 and coating composition PA2.

Fig. 6 shows in a diagram the amount of diffused Cr (which was calculated by integration of the Cr concentration profile) vs. the temperature (1012°C & 1062°C) at which the diffusion couple tests were performed with the chromium-rich nickel-base alloy BA0 for 150 h.

Fig. 7 shows in a diagram the amount of diffused Cr (which was calculated by integration of the Cr concentration profile) vs. the temperature (1012°C & 1062°C) at which the diffusion couple tests were performed with the pure chromium for 24h. Fig. 8 displays the function of the Al activity vs. Fe content in the coating alloy at 1100°C. Fig. 9 displays the function of the Si activity vs. Al content in the coating alloy at 1000°C.

Detailed description of the drawings

The interdiffusion within the exemplary base alloy-coating systems has been found to be deter- mined mainly by a high Cr activity gradient. The activity of Cr is strongly influenced by the pres- ence of other elements in the alloy and by the alloy phase structure which determines Cr-diffusion. Thermodynamic calculations on the influence of Nb, Ta and Mo on Cr activity are presented in Figs. 1-3. As the amount of these elements increases, the Cr-activity within the coating rises without an increase of the actual Cr content and thereby aligns to the Cr-activity in the substrate (0.79 at 1012°C). The most pronounced effect can be observed for tantalum, which increases the Cr-activity up to approximately 0.80 for contents higher than 9 wt.-%, whereas Mo exhibits the slightest increase in Cr-activity up to around 0.58 for contents higher than 8 wt.-%. The addition of Nb increases the Cr-activity to values higher than 0.65 for con- tents between 9-16 wt.-%.

Diffusion couple tests were performed to measure the concentration profiles after interdiffusion in an electron probe microanalyzer (EPMA). Therefore, coating alloys CA0, PA1, PA2 and PA3 of the following examples were manufactured separately as bulk material and not by the slurry method on the base alloy, which is mentioned here for clarification. These bulk materials were produced in a way that they did not contain any chromium and were subsequently machined with wire erosion to receive cubical samples with an edge length of 6 mm. An identical sample geometry was chosen for the base alloy. All samples were ground and polished to obtain smooth bonding surfaces. Afterwards, the smooth surface of the coating alloy was placed on the smooth surface of the base alloy in combination with two crucibles, which were placed on opposite sides. This stack was then placed in a fixture, which can be seen in Fig. 4, such that a sufficient pressure can be applied for an efficient contact. Following this, the fixture was placed in a furnace at low pressure or inert gas atmosphere and annealed for a certain time to investi- gate the interdiffusion effects. Finally, the sample was removed from the fixture, mounted in a resin and metallographically prepared for concentration measurements with EPMA.

Especially Cr diffuses from the base alloy into the coating during interdiffusion. Therefore, the Cr-concentration profiles obtained are presented in Fig. 5. Fig. 5a displays the Cr-concentration profile after a diffusion couple test performed at 1062°C for 150 h with the base alloy BA0 and coating alloy CA0. Fig. 5b shows the concentration profile for Cr under the same test conditions for the base alloy BA0 and coating alloy PA2. By comparing both diagrams qualitatively, the amount of Cr diffused is obviously lower for PA2 compared to CA0. Namely, the diffusion length and the highest Cr concentration in the coating alloy are enormously decreased in the presence of Nb. The major reason for these optimized characteristics of the coating PA2 lies in the re- duced activity gradients of Cr. The consequence is a conservation of the mechanical properties of the PA2-coupled base alloy due to a strongly reduced chromium depletion.

The amount of Cr diffused can also be quantitatively determined, as shown in Fig. 6. For that, the initial contact plane has to be determined. This can be done by Matano-Boltzmann analysis which serves as an approximation of the initial contact plane. After determination of the approx- imated contact plane, the area below the graph is determined by integration. The area corre- sponds to the amount of Cr, which diffused into the coating alloy. The area can be taken as comparative value for chromium diffusion.

Fig. 6 shows the results of diffusion couple tests performed with alloys CAO, PA1 and PA2 cou- pled with the base alloy BAO at two different temperatures i.e. 1012°C and 1062°C for 150 h. At the lower temperature, the amount of diffused Cr is reduced by 10.9% for alloy PA1 and by 14.0% for alloy PA2 compared to CAO. As diffusion is a temperature-activated process, the amount of diffused Cr increases for all coating alloys with increasing temperature. Thereby, the amount of diffused Cr is even more strongly reduced at the higher temperature of 1062 °C: By 19.2% for alloy PA1 and by 68.2% for alloy PA2. The higher difference in diffused Cr for PA1 with increasing temperature compared to CAO can be ascribed to the lower Cr activity gradient. Similar considerations apply for coating alloy PA2, for which the Cr diffusion declines by 14.0% at 1012°C and by 68.2% at 1062°C compared to CAO. Besides the lower activity gradient com- pared to PA1 , the PA2 alloy forms an Nb-rich G-Phase, which shows a beneficial impact, as it exhibits a marginal Cr-solubility. Therefore, the active diffusion cross section is reduced and the precipitations act as a partial diffusion barrier.

Fig. 7 displays the results of diffusion couple tests performed with alloys CAO, PA1 , PA2 and PA3 coupled with pure chromium at two different temperatures i.e. 1012°C and 1062°C for 24 h. The results show a similar behavior as explained in Fig. 6. The alloy PA2 containing Nb exhibits for both temperatures the lowest chromium uptake, which is ascribed to the lowered activity gradient as well as the beneficial Nb-rich G-phase with low Cr-solubility. The alloys PA1 and PA3 containing Mo and Ta, respectively, also exhibit a lower chromium diffusion compared to CAO at 1012°C. At elevated temperature PA3 displays a lower Cr uptake compared to PA1 due to the better adapted Cr activity gradient.

Fig. 8 shows the influence of Fe on the Al-activity in the base alloy BA1. As the amount of this element increases, the Al-activity within the substrate rises without an increase of the actual Al content and thereby aligns to the Al-activity within the coating (1.3^*10-4 at 1100°C). By adding 16 wt.-% Fe to the substrate, the Al-activity raises from 2.1 ^*10⁵ to 7.1 ^*10⁵.

Fig. 9 shows the influence of Al on the Si-activity in the base alloy BA2. As the amount of this element increases, the Si-activity within the substrate rises without an increase of the actual Si content. The Si-activity within the coating is approximately 2.7^*10⁴ at 1000°C. By adding 16 wt.-% Al to the substrate, the Si activity raises from 5.9^*10⁶ to 5.2^*10⁵. Examples

Example 1

Reduction of chromium diffusion from the substrate into the catalytic coating

First, pure chromium is used, which exhibits the highest possible chromium activity, i.e. 1.0. Second, a multi-component base alloy (designation BAO) containing 45% nickel and about 35% of chromium is utilized, which displays a chromium activity of 0.79 at 1012°C. The chromium- rich nickel-base alloy is an austenitic alloy, which is centrifugally cast and especially used for pyrolysis furnace tubes. The high levels of nickel and chromium are used to achieve higher creep strength. Therefore, the interdiffusion between base alloy and diffusion coating needs to be limited to maintain the exceptionally high mechanical properties. In particular, the Cr-diffusion has to be retarded to prevent dissolution of the chromium carbides, which are par- ticularly responsible for the creep strength of the alloy. The composition is listed as BAO in Ta- ble 1.

The coating formulation is tailored to the chromium-rich nickel-base alloy composition of the substrate (BAO). As a starting basis for the new concept exemplarily a highly functional type of coating for steam crackers has been used (coating composition CA0 in table 1), which is de- scribed in WO 2013/181606 A1. The latter discloses a functionally-graded coating that was de- veloped to protect a metallic alloy component from carburization, oxidation and erosion in hy- drocarbon processing at elevated temperatures. The coating forms outer oxide surface layers, among which the outermost oxide layer gasifies carbon deposits catalytically and is inert to fila- mentous-coke formation. The oxide layers support the hydrocarbon manufacturing process in an effective way. The functions of the coating are achieved by a specific phase structure con- sisting of h-carbides and isolated manganese silicates in a ductile y-matrix in conjunction with MnO, Cr₂O₃ and MnCr₂O₄ as oxide layers. The consolidation of CA0 is completed by a Nii6Si7Mn6 melting phase, which dissolves during the further operation, as well as by Cr- diffusion from the base alloy into the coating. The composition of the coating before consolida- tion is also listed in Table 1.

Table 1

Table 1 reveals that the CA0 composition before consolidation contains no chromium. During consolidation of the coating, chromium migrates from the base alloy steel into the coating. Pa- tent WO 2013/181606 A1 discloses typical ranges for the key constituents in the coating after consolidation, given in Table 2. Table 2

‘denotes constituents provided primarily by base alloy

The high Cr-activity gradient between base alloy and CAO before and after consolidation leads to chromium depletion in the sub-surface zone of the base alloy and in the long-term the me- chanical properties of the base alloy can be affected.

The CAO coating was significantly modified in the development work behind the present patent by using the proposed innovative concept introducing a new approach to decelerate chromium (inter-) diffusion. The composition of the original coating CAO was changed based on thermody- namic calculations, and Mo, Nb, and Ta were introduced as new components of the coating which is displayed in Table 3 (coatings PA1 , PA2 and PA3).

Table 3

The different compositions of the three optimized coatings PA1 , PA2, and PA3 correspond to defined increased “virtual” Cr-activities after consolidation (the actual Cr content after consolida- tion was not changed but only that of the other Cr activity influencing elements) as shown in Table 4. The Cr-activities of the coating were adjusted to the high Cr-activity in the base alloy, which results in a suppression/reduction of Cr diffusion into the coating. Table 4

‘concentration in weight percent of the respective Cr active element in the coating to minimize the activity gradient of Cr between coating and substrate

The results of the development work propose as coating alloys PA1 to PA3 that simultaneously provide decelerated interdiffusion due to adjusted activities, stabilization of the substrate sub- surface zone, protection against carburization, catalytic surface activity concerning carbon and resistance against erosion. Preferably, the coating can be produced by a low-pressure consoli- dation process. For this the coating alloy composition can be deposited on a substrate by a slur- ry-based method with additions of aqueous and organic components. An appropriate heat treatment of the coating has to be used in order to achieve good adhesion to the substrate and high consolidation of the coating. The coating manufacturing route used was equivalent to the one used for CAO and is described in detail in WO 2013/181606 A1.

Diffusion couple tests were performed to compare the amount of diffused Cr with coating alloy CAO. The enhanced coatings PA1 to PA3 were found to exhibit significantly lower Cr migration than CAO.

It is important to highlight that the exceptionally low Cr migration is due to the elements Mo, Nb, and Ta, respectively, and their interaction with the other elements in the coating. Therefore, the coatings come with the desirable Cr activity increase leading to decelerated interdiffusion and a stabilized sub-surface zone of the base alloy.

The addition of Nb leads to the formation of an additional phase, the so-called G-Phase, be- sides the y-matrix and the h-carbides. This Nb-rich G-phase has beneficial effects due to its low solubility for Cr and acts as an additional partial diffusion barrier. The addition of Ta leads to an additional phase formation, namely TaC, besides the y-matrix and h-carbides. The TaC phase precipitates preferentially in combination with manganese silicates. The addition of Mo does not induce any changes in the initial microstructure. Example 2

Reduction of the aluminum diffusion from the coating into the substrate

A nickel-based alloy (designation BA1) containing approximately 60% nickel and around 3.6% aluminum is chosen for theoretical demonstration. The alloy shows an aluminum activity of 2.1^*10^-5 at 1100°C. The composition is listed as BA1 in Table 5. The single crystal alloy is ex- tensively used in industrial turbine blades as well as in aircraft engines. The microstructure of this multicomponent alloy comprises homogeneously distributed cuboidal precipitations of y ’-phase which are surrounded by an austenitic y-matrix. These cuboidal precipitates with a mean edge length of 300-500 nm lead to the superior mechanical properties of nickel-based super alloys. Nevertheless, nickel-based super alloys suffer from high temperature corrosion. Therefore, the application of effective protection coatings is inevitable.

Such protective coatings should ideally form an adherent and slow-growing oxide scale, which should additionally exhibit a high thermodynamic stability under the severe conditions inside the turbine. Thereby, two types of protective coatings are utilized in industry that fulfill these afore- mentioned conditions: Diffusion coatings and overlay coatings. Both make use of the b-phase, which acts as an aluminum reservoir for the formation of a protective alumina scale. However, this disclosure focuses particularly on overlay coatings in the following.

Overlay coatings are mainly manufactured by physical vapor deposition or thermal spraying. A prealloyed NiCrAIY composition CA1 , which is listed in Table 5, is applied on the base alloy BA1. The coating alloy CA1 has an aluminum content of 10 wt.-%. This relatively low amount is sufficient to form an external alumina scale. Especially the chromium contained in the coating leads to the high aluminum activity of 1.3^*10 - . Unfortunately, yttrium was not included in the calculation due to its absence in the thermodynamic database of JMatPro, which is also the case for the Thermo-Calc database TCNI5.

Table 5

The high Al-activity gradient between base alloy BA1 and CA1 leads to an aluminum depletion in the coating zone and thus, in long term to an incapability of the coating to form a protective alumina scale. Furthermore, the microstructure of the base alloy deteriorates due to the alumi- num enrichment in the base alloy. Therefore, the aluminum activity has to be raised in the base alloy. The results of the base alloy’s activity adjustment are shown as coating alloys MBA1 to MBA3 in Table 6. Thereby, solely the amount of nickel was decreased proportionally to the amount of the aluminum activity influencing element added. Table 6

By adjusting the aluminum activities in coating and substrate, it is expected to decelerate the interdiffusion and consequently, to maintain the b-phase as aluminum reservoir. Furthermore, the enrichment of the base alloy with aluminum is prevented which is accompanied with a reten- tion of the superior mechanical properties.

Example 3

Reduction of the silicon diffusion from the coating into the substrate

A further example is conceptionally demonstrated in order to reduce silicon diffusion from the coating into the base alloy. Therefore, a heat resistant stainless steel (designation BA2) and a silicon-rich NiCrAIY coating (designation CA2) are chosen. The stainless steel contains approx- imately 54% Fe, 24% Cr and 19.6% Ni and is characterized by outstanding mechanical proper- ties. It has been the standard alloy for reaction tubes in steam-reformer furnaces and in crack- ing furnaces for ethylene production. The composition is listed in Table 7. The NiCrAIY coating is based on patent US 4 034 142 A and is particularly suited to reduce oxidation and hot corro- sion. The composition of the coating is listed in Table 7.

Table 7

The activities of the coating and base alloy composition before consolidation are listed in Ta- ble 8. The coating alloy CA2 with a silicon content of 7 wt.-% shows a silicon activity being two orders of magnitude higher compared to the base alloy BA2. Therefore, the silicon activity in the base alloy is raised by alloying. Thereby, solely the amount of iron was proportionally decreased by the amount of the silicon activity influencing element added. The results are listed in Table 8. Table 8

Claims

Patent claims

1. An interdiffusion barrier layer-free coated substrate having a coating on the substrate, wherein the substrate is formed by a first alloy comprising a first chemical element El₁ in an amount of a first concentration c₁(El₁) and a relative chemical activity a₁(El₁), wherein the coating is formed by a second alloy comprising the first chemical element El₁ in an amount of a second concentration c₂(El₁), which is different from the first concentra- tion, and a relative chemical activity a₂(El₁), wherein | c₁(El₁) - c₂(El₁) | / (ci(El₁) + c₂(El₁)) ³ 0.3, wherein the first alloy further comprises at least a second chemical element El₂ in an amount of a first concentration c₁(El₂), wherein the second alloy comprises the second chemical element in an amount of a sec- ond concentration c₂(El₂), which is different from the first concentration, wherein | c₁(El₂) - c₂(El₂) | > 0.1 wt.-%, wherein the higher concentration of c₁(El₁) and c₂(El₁) and the lower concentration of c₁(El₂) and c₂(El₂) are in the same alloy, and wherein | a₁(El₁) - a₂(El₁) | at a temperature Ց is below 0.3 and Ց is in the range from 500° C to 1500°C.

2. The coated substrate of claim 1 , wherein El₁ is Cr, El₂ is Nb, | c₁(El₁) - c₂(El₁) / (c₁(El₁) + c₂(El₁)) ³ 0.6 and preferably £ 0.7, | c₁(El₂) - c₂(El₂) > 10 wt.-% and preferably < 13 wt.-%, and preferably the higher concentration of c₁(El₁) and c₂(El₁) and the lower concentration of c₁(El₂) and c₂(El₂) are in the first alloy.

3. The coated substrate of claim 1 , wherein El₁ is Cr, El₂ is Mo or Ta, | c₁(El₁) - c₂(El₁) / (c₁(El₁) + c₂(El₁)) ³ 0.6 and preferably £ 0.7, | c₁(El₂) - c₂(El₂) > 8 wt.-% and preferably < 11 wt.-%, and preferably the higher concentration of c₁(El₁) and c₂(El₁) and the lower concentration of cI(EI₂) and c₂(EI₂) are in the first alloy.

4. The coated substrate of claim 1 , wherein El₁ is Al,

El₂ is Fe or Nb, | c₁(El₁) - c₂(El₁) / (c₁(El₁) + c₂(El₁)) ³ 0.4 and preferably £ 0.6, | c₁(El₂) - c₂(El₂) ³ 14 wt.-% and preferably < 18 wt.-%, and preferably the higher concentration of c₁(El₁) and c₂(El₁) and the lower concentration of cI(EI₂) and c₂(EI₂) are in the second alloy.

5. The coated substrate of claim 1 , wherein El₁ is Al,

El₂ is Hf, | c₁(El₁) - c₂(El₁) / (c₁(El₁) + c₂(El₁)) ³ 0.4 and preferably £ 0.6, | c₁(El₂) - c₂(El₂) ³ 17 wt.-% and preferably < 20 wt.-%, and preferably the higher concentration of c₁(El₁) and c₂(El₁) and the lower concentration of c₁(El₂) and c₂(El₂) are in the second alloy.

6. The coated substrate of claim 1 , wherein El₁ is Si,

El₂ is Al, | c₁(El₁) - c₂(El₁) / (c₁(El₁) + c₂(El₁)) ³ 0.5 and preferably £ 0.7, | c₁(El₂) - c₂(El₂) ³ 14 wt.-% and preferably < 18 wt.-%, and preferably the higher concentration of c₁(El₁) and c₂(El₁) and the lower concentration of cI(EI₂) and c₂(EI₂) are in the second alloy.

7. The coated substrate of claim 1 , wherein El₁ is Si,

El₂ is Cu, | c₁(El₁) - c₂(El₁) / (c₁(El₁) + c₂(El₁)) ³ 0.5 and preferably £ 0.7, | c₁(El₂) - c₂(El₂) ³ 15 wt.-% and preferably < 19 wt.-%, and preferably the higher concentration of c₁(El₁) and c₂(El₁) and the lower concentration of c₁(El₂) and c₂(El₂) are in the second alloy.

8. The coated substrate of claim 1 , wherein El₁ is Si,

El₂ is Ru, | c₁(El₁) - c₂(El₁) / (c₁(El₁) + c₂(El₁)) ³ 0.5 and preferably £ 0.7, | c₁(El₂) - c₂(El₂) ³ 13 wt.-% and preferably < 16 wt.-%, and preferably the higher concentration of c₁(El₁) and c₂(El₁) and the lower concentration of c₁(El₂) and c₂(El₂) are in the second alloy.

9. The coated substrate, wherein q is in the range from 750°C to 1250°C.

10. A catalyst comprising a coated substrate of any of claims 1 to 9.

11. The catalyst of claim 10, wherein the catalyst is a steam cracking catalyst.

12. A method for the preparation of an interdiffusion barrier layer-free coated substrate, wherein the method comprises the steps

(c) determining a chemical element El₂, which is influencing the relative activity of El₁;

(d) adjusting the concentrations of El₁ and El₂ in the substrate and/or the coating and determine the relative activity of El₁ in substrate and coating;

(e) repeating step (d) until the difference of the relative activities in substrate and coat- ing is reduced.

(f) preparing the coated substrate having the concentrations of El₁ and El₂ in the sub- strate and/or the coating resulting from steps (d) and (e).

13. The method of claim 12, wherein the coated substrate of step (f), wherein the first alloy comprises a first chemical element El₁ in an amount of a first concentration c₁(El₁) and at least a second chemical element El₂ in an amount of a first concentration c₁(El₂) and wherein the second alloy comprises the first chemical element El₁ in an amount of a sec- ond concentration c₂(El₁), which is different from the first concentration and wherein the coating is formed by a second alloy comprising the first chemical element El₁ in an amount of a second concentration c₂(El₁), which is different from the first concentration, and a relative chemical activity a₂(El₁), and wherein the first alloy further comprises at least a second chemical element El₂ in an amount of a first concentration c₁(El₂) and wherein the second alloy comprises the second chemical element in an amount of a second concen- tration c₂(El₁), which is different from the first concentration and wherein the higher con- centration of c₁(El₁) and c₂(El₁) and the lower concentration of c₁(El₂) and c₂(El₂) are in the same alloy.

14. The method of claim 12 or 13, wherein at least one of the inequations applies: a) | c₁(El₁) - c₂(El₁) / (c₁(El₁) + c₂(El₁)) ³ 0.3; b) | c₁(El₂) - c₂(El₂) ³ 0.1 wt.-%; c) a₁(El₁) - a₂(El₁) at a temperature q is below 0.3 and q is in the range from 500°C to 1500°C.

15. An interdiffusion barrier layer-free coated substrate obtainable by a method according to any of claims 12 to 14.