CA2052050C

CA2052050C - Alumina coating for the protection of stainless steel

Info

Publication number: CA2052050C
Application number: CA 2052050
Authority: CA
Inventors: Guido A. Capuano; Anh N. Dang; Ferdinando Felli; Umberto Bernabai
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-09-23
Filing date: 1991-09-23
Publication date: 1998-08-18
Anticipated expiration: 2011-09-23
Also published as: CA2052050A1

Abstract

An alumina coating, produced by aluminium electroplating of a stainless steel containing from about 0.02 to about 0.2% of misch metal or 0.2 to 0.8% of titanium, improved the oxidation resistance in air of these stainless steels at least up to 1100°C. In thermal cycling tests carried out between 1000°C and room temperature, these steels maintained an excellent adhesion matrix-coating, for at least up to 500 cycles. In fact, the addition of misch metal or titanium in stainless steel provided good bonding between coating and matrix. The porous nature of thealumina coating was found very beneficial because it favors the absorption of the thermal and growth stresses.

Description

ALUMINA COATING FOR THE PROTECTION OF STAINLESS STEEL

BACKGROUND OF THE INV~NTION

Stainless steels are quite resistant to corrosion, and therefore have a wide range of use, such as in airplane technology, nuclear reactors, human prosthesis, auto parts, etc. However, in particular temperatures, environments and applied stresses, they may undergo stress corrosion cracking.
Under normal service conditions, stainless steels are exposed to un~ents in which they become passive while submitted to a normal elastic stress. However, crevices, pits and application of heavy loads, can behave as concentrators of local stress that favor local crack nucleation. Anodic non protected 15 spots are generated where the passive film breaks. These zones are complemented by a large cathodic area. The combination effect of these phenomena contribute to the initiation of localized corrosion.

The use of a protective metallic coating is a well established 20 terhni-lue for the corrosion prevention of metals. It is also well known that this coating must be tightly adhered on the surface in order to efficiently reduce the corrosion (see, for example, US 4,655,852).

An Al203 coating on stainless steels provides a better protection 25 against r,Yid~tir,n than coatings of other metal oxides, especially at high temperature.
Protection by alumina is particularly useful in flux gases with high impurity contents such as sulphur sodium or vanadium, in salt and in metal melts, in corrosive slags etc.

In Fe-AI alloys, the development of an alumina protective coating is possible only when the aluminium content in the alloy is higher than 7 l 1 wt% in the temperature range of 700 - 1000 ~ C, with a tendency to transform mixed oxides to ~-AI203 when the temperature and the ~lumininm content are increased.

Thermal cycling causes spalling of the alumina coating, and no healing takes place if the ~h~minillm content in the Fe-AI alloy is not sufficient]y high. On h ~ ;3 the other hand, steel fragility becomes a problem when the ~ mininm content exceeds S wt%. The situation is somewhat irnproved with Fe-Cr-A] alloys, in which 3-4 wt% aluminium may be sufficient to insure the formation of an external Al2O3protective coa~ing. This occurs apparently by means of a selective oxidation process.
5 At the beginning of the process, chromium behaves as an oxygen getter, thus avoiding that the latter combines with iron. The transient state is so established where a dense and continuous layer of Al2O3 is formed under a lower partial oxygen pressure.

However, in general the alumina coating grown in this manner does not resist to spalling under the effect of thermal cycling, since oxide scales generally have therrnal ryr~n~ion coefficient lower than the metallic substrates. During temperature variations, the dilatation difference induces thermal stress, which added to the growth stresses, causes cracking and eventual spallation of the scale. In15 spalled area, an aluminium depleted metallic surface comes in direct contact with the e.~uunlne~t~ resulting in an accelerated oxidation of the metallic surface.

Ac~oldl.l~, the rne~h~nic~l molding of the stainless steel into the desired shape must be carried out prior to the plating with ~ minium, since such20 molding is carried out at high temperatures. Such rnllltiplic;lti~n of proce-lures raises the costs of finished goods. Attemps have been made to mold Al2O3 coated stainless steels, but were u~lcuccl~ssful because of the presence of cracking and spallingof the alumina coating. (Singh et al., CORROSION, paper No. 276, (1987)).

An increase in the hot-oxidation rÇC;~t~n(e has been obtained with steels containing both Al and Si (Bernabai et al., OXIDATION OF METALS, 33, 309 (1990)), but there has been no similar results reported yet with stainless steels.

SUMMARY OF THE INVENTION
In accordance with the present invention, there is now provided an ~1u...;..;.~ .. el~lroplated stainless steel having improved oxidation rec;ct~nrç More specifically, the present ill~-,.ltion c~-~.p~ises an all~rninillm electroplated stainless steel, wherein said stainless steel contains from about 0.02 to about 0.2 % of misch ~ ~''J; ~3 metal, preferably about 0.15% or from about 0.2 to about 0.8 % of titanium,preferably about 0.53%.

Preferably, the aluminium electroplated stainless steel containing 5 misch metal is a ferritic type stainless steel, and the electroplated stainless steel containing titanium is an austenitic stainless steel.

IN THE DRA~VINGS

10Figure I illustrates the results of oxidation tests carried out at constant temperature for the aluminium electroplated stainless steel AISI-446 and A~SI-446 cont~inine 0.15% of misch metal (AISI-446 MM.15);

Figure 2 illustrates the results of similar oxidation tests for stainless 15steels AISI-304 and AISI-321. As it is the case in Fig. 1, results from uncoated steel are also illu~lateJj Figure 3 illustrates the results obtained from cyclic oxidation of ~Inl~o~ted and Al cle~l- oplated AISI-446 stainless steel with and without misch metal.
20 The "naked" samples show a rapid scale growing because of spinel formation;

Figure 4 illustrates the results obtained from cyclic oyi~ation of Al electroplated AISI-304 and AISI-321. The AISI-304 sample undertakes spalling after onJy 150 cycles, whereas the Al electroplated AlSI-321 sample is still well adherent 25 after 500 cycles; and Figures 5 and 6 present the diffusion profiles of the elements on the lldh~ dl section of the AISI-446 MM. 15 and AISI-321 Al electroplated samples after 48 hours of oxidation, as determined by Energy Dispersion Sp~ll oscc"~ (EDS) 30 analysis.

" ~ - 3 DETAILED DESCRIPIION OF THE IN~rENTION

It has been unexpectedly founcl that the presence of misch metal or titanium in stainless steel greatly improve the adherence of an alumina coating 5 electroplated thereon, thus improving resistance to hot-oxidation, and allowing the molding of Al-electroplated stainless steel without spalling of the Al coating.

Various merh~nicmc have been put forward to explain this effect:
- i) change in the oxide growth process with reduced co.l")rei,~i.e stresses;
- ii) formation of pegs which anchor the scale into the alloy;
- iii) elimination of voids formation by coalescence of var~nr;pc and ~ iv) scale growth of finer grain size, which promotes Cobles creep me~h~nicm and an earlier relieving.
The ~ ~s;on ~misch metal" use throughout the present dcs~ Jtion is met to include primary c~.. lerc;al form of mixed rare earth metals (95%)prepared by the electrolysis of fused rare earth chloride mixture.

Titanium and misch metal behave as reactive elements for a better adhesion between scale and substrate. Titanium, for example, has the property todissolve large quantities of oxygen and hence to form a family of oxides, the composition of which is related to the activity of the constituents and the temperature at which the oxides are formed. TiO is the most stable in terms of free energy of formation. At elevated temperatures, TiO can also be formed from the reaction between alumina and titanium in solution in the matrix subscale zones:
Al2O3 + 3Ti _ 3TiO + 2AI
If located in the interior of an oxide scale and exposed to high temperatures for a long period of time, titanium diffuses faster than ~luminillm For inct~nrP, titanium has been identified as TiO2 in the scale surface of an alloy containing Fe-Cr-AI-Ti, when the latter was ~lllmini7ed by a pack cementation process. ~Illminillm surface enrichment in stainless steels allows a reduction in the aluminium content in the steel. This avoids the brittleness problem and provideswith a large superficial concentration of ~Illminillm for the formation of a pure ,t~ r;

S

~t2O3 scale at the start of the oxidation process. It is a great advantage for hot working, and for reduction of the room temperature brittleness due to thermal aging.

There are several techniques for the superficial enrichment of aluminium on steels: hot dipping, name spraying, pack cementation, electroplating, vapor deposition etc.

Flame Sprayine Al coatings are obtained by melting an aluminium wire with a flame, atomizing the droplets, and propelling them against the surface to be coated, resulting in a porous coating. The adhesion to the substrate is mainly me~h~nic~l;
therefore, diffusion bondings by high-temperature heat treatment is necessary.
Uneven heating of a thin sheet of steel can generate distortion.

Pack C~ ~ ~P~tC ~
This process is performed at temperatures of about 870 - 1200 ~ C in a pack ro~ ne of a mixture of alllminillm powder, alumina powde} to prevent agglomeration, and volatile halides as the chemical-transfer medium. Diffusion during the process causes direct alloying. The process appears cumbersome for large flat co.~.por.G,.t~, and is rather expensive.

Vapor D~
Aluminium vapor is diffused at about 930 o C into the surface of steel components in a sealed retort, generating an alloyed surface. When ~IIlminjllm vapor is allowed to condense at low temperature under vacuum on the surface of the colpone.... ......t to be coated, diffusion bonding is then required. This process is also quite costly.

Hot Dtpplnp Dipping of steel in a liquid pure-~ minil-m bath does not permit accurate control of the thickness of the brittle intermetallic layer. This type of process is usually carried out for coatings on low~arbon steel to be used under isothermal ~,n~ nc at temperatures under 690 ~ C.

Electro~latlny The electroplating process offers several advantages over all the above coating procc~ses. The aluminium deposits are generally more adherent, thecoating does not affect the structural and mP~h~ni~l properties of the substrate5 since it is applied at room temperature. The electroplating process is conventional, more versatile, cheap, and allows a precise thickness control of the alnminillrncoating. The present inventors used aluminium electroplating in a previous work for high temperature protection of manganese steel (OXIDATION OF METALS, supra).
tl Vl~lt;~
The materials tested were ferritic type AlSI-446 stainless steel, with and without misch metal, and austenitic types AISI-304 and AISI-321 stainless steels.
The latter contains about 0.53% titanium. The AISI-304 was tested without misch 15 metal and titanium.

All these stainless steels are commercially available from W~chingtc.n Steel, U.S~.

Aprotic solvents were used for the preparation of suitable electrolytes.
The process for the electrodeposition of aluminium carried out in the course of the present invention is as desc.il~d in J. Ele t.~l,e.... Soc., 1991. ~, 484~90 (Capuano et al.).

Weight gain in general was measured versus time. This weight gain is caused by the increase of oxide concentration in the electroplated sample.
However, in the preoYid~tion test, a dispersion range was indicated, whereas in the high temperature oxidation tests, weight gain average are reported. Preoxidationinvolved heating in air at 600~C usually for 24 hours (Fig. 1 and 2), in order to m~Yimi7e the diffusion of alllminillm into stainless steel.

The hot~,lo~ion resistance after pre~.Yid~tion was improved for all four Al-electroplated samples tested, and the scale formation is highly reduced. The reduction pl oceedi, when, for example, about 0.15% of misch metal are added to the AISI-446 stainless steel.

" ~ 3 l~icrosco~ic clse ~..tlon~ .

In order to evaluate the coating morphology, compartnP-cc~ thickness, adhesion, phase, and elemental diffusion of the Al-coating, the coated surface and S metallographic cross sections were analyzed with a Hitachi S2500 Scanning Electron Microscope (SEM), and a Kevex 8000 Energy Dispersion Spe tto~;upy (EDS) apparatus.

The following examples are provided to illustrate the invention rather 10 than limit its scope.

A 20 x 8 x 2 mm of AISI-446 stainless steel sample is electroplated 15 with ~ rninillrn in a~ldance with the process described in J. Ele~lluehc~,l. Soc., 1991. ~, 484490 (Capuano et al.). The sample was metallographically polished with up to 1 ,~m alumina paste before coating.

Briefly, the el~:tlol~;s bath consists of a mixture of 50 wt% of 20 ~lllrninillrn bromide, and 50 wt% of a mixture of 1:1 volume toluenc-~tllylbcJIzeAe solvent. Gaseous h~J..,bro.llic acid is added to the bath until a conductivity of 24 x 10-3 S2~lcm-1 is reached. Two aluminium electrodes were used as soluble anodes.
The electroplating was carried out at cathodic current density of about 10 to 20mA/cm2. The resultant Al coating is ductile, porous and obtained with cathodic 2S efl ;r ;~ "~ ;~s approacl,ing 100%. The Al coating th irlrnpcc is preferably in the interval of 11-15 ,um, but can be as high as 40 ,um and as low as 5 ~m.

The coated samples, before being submitted to high temperature rY~ tion tests, were isothermally treated in air at a temperature of 600 - C for 24 hours.

Proceeding in the same manner as in Example 1, the AISI446 MM.
15; AISI-321 and AISI-304 are also electroplated with alurninium.

Oxidatlon and cycllne tests:

S High temperature oxidation tests of each sample of Examples 1 and 2 were performed in air, and the weight gain was continuously recorded by a Cahn1000 thermobalance for 48 hours at temperatures between 1000 and 1100-C. The thermal cycling tests were performed after 20 hours of high temperature oxidation test at 1000 ~ C. The thermal cycle performed was: 15 minutes at 1000 ~ C and S min at room temperature. 500 cycles were performed on each sample.

Ferritic type AISI-446 and AIS1-446 MM.I5 stainless steels:

The results obtained from isothermal oxidation tests of stainless steel AISI-446 MM.15, (Fig. 1), show that Al coated samples possess a much higher r~ t~nre to ~-Y;~ ion than nnc~ated samples.

The thermG~ vuncl. ic curve nattening of the coating material can be .oYpl~ined by the EDS profile analysis of Fig. 5, where an inquination of thealumina layer with chromium and iron up to 900 o C drastically dcc. cases at 1000 ~ C
and furthermore at 1100-C, thus generating a pure ~-AI2O3 layer of constant pos;lion.

However, this does not occur as well in the AIS1-446 stainless steel deprived of misch metal. In fact, when heated between 1000 and 1100-C, this stainless steel can develop its own alumina film under the Al coating by a selective oxidation process. This prevents the bonding between the Al coating and substrate, so that at the end of the isothermal oxidation test at 1100-C, the entire coating detaches itself from the steel substrate. Therefore, the temperature of 1100-C
appears critical for the bonding between the Al coating and the AISI-446 stainless steel deprived of misch metal (Fig. 1).

On the other hand, the pr~sence of misch metal in the matrix of AISI-446 creates a strong bonding between the Al coating and the stainless steel.

Here, the misch metal probably acts as nuclei for the alumina superficial f~lm growth in the first stage of the aluminium oxidation at high temperature.

Cycling oxidation tests (Fig. 3) contribute to explain the beneficial S effect of the misch metal in the bonding formation. In fact, the extreme severity of the thermal excursion does not prevent the misch metal to impart their controlling effect on the bonding between the Al coating and the stainless steel.

The aluminium coating of the AISI-446 MM.15 is strongly adherent 10 and without interfacial voids, whereas the coating on the AISI-446 deprived of misch metal presents some interfacial voids, a greater internal porosity and a higher thickness.

Austenitic AIS1-304 and AIS1-321 stainless steels:
Fig. 6 illustates the EDS profile analysis of the Al electroplated AISI-321 stainless steel in the temperature interval of 600-900-C. The aluminillm in the coating has a tendency to migrate toward the matrix, i.e. the stainless steel, to form intermetallic phases with nickel, whereas the chromium and iron in the matrix, tend 20 to migrate toward the coating, i.e., the aluminium.

In samples isothermally oxidized from 1000 to 1100-C, the ~-A1203 is almost free of Fe and Cr, while Ti is present. It has been observed that the porous nature of the Al coating during the preoYi~l?ti~n stage at 600-C allows the 25 formation of scale film of mixed oxides characteristic of the stainless steel matrix, which in the case of AIS1-321 will be mostly chromium and iron oxides. At highertemperatures, the aluminium coating reduces these oxides.

Samples analysed at 600 - C for 24 hours under vacuum of 10~ mBar 30 have shown the absolute absence of ~Inminil~m diffusion in the matrix or any migration of Fe and Cr toward the aluminium coating, i.e. the absence of any bonding between the matrix. and the coating in the absence of oxygen.

As mentioned previously, titanium plays an important role in the 35 adhesion bonding formation between the stainless steel matriY and the Al coating.

,, ~ J, ~? ~ ~

In the AISI-321 stainless steel, titanium and nickel already promote at 600 ~ C the migration of Al from the coating to the matrix to generate the phase y'Ni3 (AlTi).
This phenomenon has been identified by EDS analysis (Fig. 6).

Above 900 ~ C, the y' phase formed during the preoxidation process is dissolved, and Ti can then migrate into the coating, where it produces a concentration peak at the interphase matrix coating, which justifies the improved adhesion of the coating. At 1100 ~ C, a second peak of Ti appears on the coatingarea.
In the present thermal cycling conditions, as identified in Example 3, the oxidized Al coating is submitted to severe stresses, which is even more elevated for the austenitic stainless steel than for the ferritic stainless steel, due to a higher coefficient of expansion of the former.
The AISI-304 possesses a weak coating matrix bonding caused by the absence of misch metal or titanium, which results in an inferior resistance to thermal cycling. During the cycling oxidation tests, fractures of the Al coating expose the matrix to the atmosphere, causing a fast growing formation of spinel rich in manganese and iron, which exerts adrli~i-n~l stress on the coating and causing undesired spalling (Fig. 4). On the other hand, the Al coating of the AISI-321 stainless steel remains well adherent after 500 cycles.

The critical factors for a good resistance to the cycling test are:
i) the adhesion between the coating and the matrix; and ii) the capacity of the coating to absorb deformation stresses.

In more general terms, concerning the adhesion between stainless steel matrix and the ~1nrninium coating, it should be pointed out that in addition to the essential bonding effect of misch metal and titanium, the porosity of the cl~lluplated ~lnn~ininm layer is also critical, because it reduces the stresses caused by the different expansion coefficients existing between the coating and the stainless steel.

. ~

An aluminium coating produced from electroplated aluminium on ferritic stainless steel type AISI-446, especially with .15% of mischmetal in the stainless steel, and on austenitic stainless steel type AISI-321, especially with 0.53%
of titanium, greatly improved the oxidation resistance in air of these steels at least 5 up to 1100 ~ C in isothermal conditions.

In thermal cycling conditions described above, stainless steel types AISI-446 MM.15 and type AlSI-321 demonstrated a high resistance to oxidation, maintaining an excellent adhesion matrix-coating for up to 500 cycles. In fact, the 10 addition of misch metal on the ferritic stainless steel AIS1-446 provides both a better control of the oxidative kinetic and a better bonding coating-matrix. So does titanium in the austenitic stainless steel AISI-321. Furthermore, the porous nature of the Al coating highly favors the absorption of thermal and growth stresses.

The high-temperature oxidation rei,i~lance of Fe-AI alloys depends mainly on alun~inillm content and partly upon temperature. Protective films of alumina develop in the temperature range 700 - 1100 ~ C, when the Al content is 5-8 W/O. In more aggressive ellVil on.nental conditions, the Al content must be between 8 and 25 W/O. However, alloys with Al contents greater than S W/O are brittle.
20 In addition, while increased Al confers more heat rçc;ct~nrç~ it lowers the spalling resistance of the alumina scale.

Claims

1. An aluminium electroplated stainless steel material, wherein said stainless steel contains from about 0.02 to about 0.2% of misch metal or from about 0.2 to about 0.8% of titanium, and wherein said aluminium plating has an adhesivity to said stainless steel and a porosity such that said plating resists spalling under the effect of thermal cycling.

2. An aluminium electroplated stainless steel material as defined in claim 1, wherein said stainless steel is a ferritic-type stainless steel containing from about 0.02 to about 0.2% of misch metal.

3. An aluminium electroplated stainless steel material as defined in claim 1, wherein said stainless steel is an austenitic-type stainless steel containing form about 0.2 to 0.8 % of titanium.

4. An aluminium electroplated stainless steel material as defined in claim 1, wherein the stainless steel is ferritic-type AISI-446 stainless steel.

5. An aluminium electroplated stainless steel material as defined in claim 1, wherein the stainless steel is an austenitic-type AISI-321 stainless steel.

6. An aluminium electroplated stainless steel material as defined in claim 2, wherein the stainless steel contains about 0.15% misch metal.

7. An aluminium electroplated stainless steel material as defined in claim 3, wherein the stainless steel contains about 0.53% of titanium.

8. An aluminium electroplated stainless steel material as defined in claim 1 or 2 which has been isothermally treated in air at about 600°C.

9. An aluminium electroplated stainless steel material as defined in claim 6 or 7, wherein said thermal cycling comprises submitting said material to a temperature up to 1100°C.