DE1621451A1 - Process for the treatment of the surface of objects made of rust-resistant steel - Google Patents

Description

Patent attorney Dip ^ -Phys, Gerhard Uedl θ Wnchen 22 Steinsdorfstr ^ i-22 M298462 ρ i # ai45i.0; a31bi

EUPAG AG CHIIR; Bahnhofplatz 10, house> ^f Winterthür ": ^; GVH IHl ^ Switzerland

Process for the treatment of the surface of counter parts; the end

rustproof steel

The invention relates to a method for treating the surface of items made of stainless steel, especially those that come into contact with water containing chlorine, as well as tubular heating elements, Water containers or devices containing them.

It is known ^ water-receiving containers or any water heating device, e.g. B. electric tubular heating elements, made of stainless steel in order to have a longer service life and greater resistance to the corrosive attack of the water

Today's documents (απ > s Aoa.ü Nr, ι ouua des Amendmentunssoes; v. 4. elissν € 1U9B20 / 1937 ^

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Rust-resistant steels are preferably used here, which usually contain 1% to 40%, preferably 8% to 12% nickel and 8% to 25% chromium. The use of such steels has significantly improved the corrosion resistance of the objects made from them. However, it has been shown time and again that damage due to corrosive influences occurs, in particular on electric tubular heating elements, for which no explanation has been found. In the investigations which are the starting point of the present invention, it has been recognized that what is involved here is the occurrence of stress corrosion cracking. This was surprising in view of the fact that tubular heating elements are exposed to relatively small mechanical stresses.

According to DIN 50, the term stress corrosion cracking is defined as the formation of cracks in metals under simultaneous action Understanding the means of attack and tensile stresses, being a deformationless one Separation with inter- or transcrystalline course without the formation of visible corrosion products occurs and the tensile stresses can sometimes also be present as internal stresses in the workpiece. The decisive factor here is that the means of attack and the tensile stresses have to act on the material at the same time. The term "Tensile stress" is to be understood broadly, since it also includes torsional and combined stresses Torsional and bending stresses can be dangerous. Compressive stresses, however, are harmless. The indication "deformable Separation "is to be understood macroscopically, so to speak does not exclude small local plastic deformations.

The prerequisites for stress corrosion cracking to occur are generally summarized in the literature as follows:

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a) The material itself, or its condition, must have a special tendency to stress corrosion cracking. This requirement is almost always fulfilled, but the degree of susceptibility can sometimes be very different for one and the same material, e.g. as a result of heat treatment.

b) There must be "tensile" stresses of sufficient magnitude. This condition is only fulfilled if the tensile stresses are sufficient Have height, and in some cases a visual threshold value can be specified. Load and residual stresses are given in \ rated in the same way.

c) A specifically attacking reagent must act. The word

"specifically ¹¹ means that an agent which can be critical for one material can behave harmlessly towards another material. Stress corrosion cracking therefore always occurs in certain systems or combinations of materials and agents Concentration and temperature, of, importance.

The problem of stress corrosion cracking is very complex and multifaceted, because there are numerous parameters and influencing factors. Of the page of the material are primarily to be mentioned: structural condition, Heat treatment, trace elements, decarburized edge zones, grain size, Cold deformation, etc. With regard to the aggressive media, among others following quantities of influence · temperature, concentration, inhibitors etc.

In general, there are two types of stress crack corrosion.

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the will. One is essentially a mechanical one Tearing open the material, the corrosion only removing the obstacles that would otherwise inhibit the progress of the crack. Of the others are based on a constant electrochemical attack, which is concentrated on the bottom of the gap due to the mechanical tension and instead of a round hollowed-out hole forms a narrow crack penetrating the metal. The reason for the preferred attack in the crevice base is according to H. W. Logan, J. Res. nat. Bur. Standards 66 C 347 (1962) in that there are continuous oxide layers due to the increased stresses be destroyed. In contrast, the measurements by T. P. Hoar, J. G. Hines, J. Iron St. lhst. 184,166 (1956); J. M. West, Corr. May be. 1,178 (1961); J. Hines, Corr. May be. 1 21 (1961) that the activation energy the formation of cations in the stainless steel / 42% magnesium chloride solution system is greatly reduced when the metal is removed quickly stretches. Furthermore, Hine's experiments give a satisfactory one Correspondence between the values of the corrosion currents and the rate of crack formation. The even and gradual crack propagation was also examined by Fontana and coworkers. (W. M. Pardue, F. H. Beck, M. G. Fontana, Trans. Americ. Soc. Metals 54 (1961).

Based on the very complex and intricate nature of stress corrosion cracking the measures to prevent the same are also very diverse and correspondingly expensive. For example, from H, H. Uhlig (Corrosion and Corrosion Control (New York-London, 1962; Wiley & Sons) The following measures are given in summary to prevent or reduce the risk of cracking in stainless steels:

a) austenitic steels:

Cathodic protection. In boiling MgCl ^ solution (154 ° C) e.g.

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about 0.03 mA / cm necessary. Removal of Cl from acidic media, of O ₆ and other depolisisers or of Cl from neutral or weakly alkaline media,

Avoid high concentrations of OH ~, especially at crevices or in steam zones, as well as addition of buffer ions, e.g.

Use of alloys with> 50% Ni or a reduction in the content of nitrogen and other harmful components the lowest possible value (e.g. <0.04% N in a steel with 20% Cr, 20% Ni, 0.001% C).

Replacement of austenitic by ferritic steels, e.g. X8Crl7 (AISI 430) or XTCrAl 13 (AISI 405). However, this can go through Hydrogen embrittles or bubbles form in certain media.

b) martensitic steels;

Avoidance of overcurrent with cathodic protection. No galvanic Connection with more active metals.

Tempering to the lowest possible hardness. It is reported that martensitic and precipitation hardening stainless steels only fail in aggressive sea air if they are tempered to a hardness of HRc> 40 (in an industrial atmosphere> 45). The steels Xl5Cr 13 or X20 CrL3 (AISI 410, 420) have in the salt spray test or by hydrogen attack the greatest susceptibility to cracking after a two-hour annealing at 425 to 550C on the lowest susceptibility for a two hour start-up to 26O ⁰ C.

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These measures represent a considerable restriction in practice and in many cases cannot be implemented at all. This applies in particular to the above-mentioned cathodic protection of electrical tubular heaters and containers for washing machines that are intended to come into contact with media containing chlorine ions. Methods for preventing stress corrosion cracking have therefore already been proposed which provide for the introduction of chromium diffusion layers into austenitic steels. Various proposals have been made for the implementation of this Diffusionsverfahren.sind, lh a case be exposed to the heated objects on chromizing about 1000 ⁰ C and then for several hours a chromium-containing gas, namely Chromchlorür. This process requires heat treatment as well as complex facilities and times of several hours for each part to be treated, which increases the price of these objects considerably.

Another diffusion method has also become known in which Chromium-nickel alloys in the outer layers to be treated Parts are diffused. In these processes, a carrier material is used, for which calcium, strontium, barium is preferred and magnesium in the liquid phase are possible. As at the temperatures required to maintain the liquid phase these metals react suddenly with oxygen and burn and form nitrides extremely quickly even under nitrogen, this can be done Process only under the protection of argon, or helium perform similar inert gases. Since the expensive facilities required for this a considerable increase in the cost of the protected Objects, this process, which has a largely laboratory character, can be economically

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not economically carry out.

Based on this state of the art and the problem mentioned It is the object of the present invention to propose a simple and economical method whereby the occurrence stress corrosion cracking is either completely prevented or reduced to such an extent that harmful effects are avoided. These The object is achieved in that to prevent the stress crack corrosion a thin protective layer of nickel is applied to the surfaces of the objects. The protective layer can be galvanized or can be applied chemically.

Particularly good results were obtained when subsequent to the coating of the articles with the protective layer a part of the applied Schxitz layer by heating to a temperature of 900 to 1200 ⁰ C in the surfaces of the articles is diffused.

Compared to the previously known diffusion processes, the process according to the invention brings about a considerable reduction in the effort and expense consequently the price of the objects treated in this way. So For example, the objects can be nickel-plated in a galvanic bath, for a layer thickness of 10 μ about 10 minutes required are. In addition, when the above-mentioned diffusion A part of the nickel layer is to be made into the surface of the objects, only needs the galvanic bath To be connected downstream, in which the parts are held at the mentioned temperature of 900 to 1200 ° C for about 5 minutes. During this time, a diffusion of about half the layer thickness, thus about 5 μ, achieved. It is of particular importance for

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the production of tubular heaters, water-absorbing containers or water heating devices, which are caused by the chlorine-ion-containing Water are particularly at risk. It has been shown that tubular heating elements produced in this way do not suffer from stress corrosion cracking are attacked and have excellent resistance to scaling.

The method according to the invention appears absurd at first glance, because, according to the prevailing opinion, a chrome-nickel steel no longer needs to be nickel-plated in order to increase its corrosion resistance raise. On the contrary, the nickel plating would continue after the previous one opinion represented in literature the surface quality of the object reduce and thus increase the risk of stress corrosion cracking. However, as practical tests have shown, that is exactly what it is On the contrary, the nickel plating according to the invention provides complete protection against stress corrosion cracking.

In the event that the nickel layer according to the invention, for example by a Scratches, the water containing chlorine ions acts directly on the base material, e.g. on a chrome-nickel steel. This However, it is only corroded if the interior necessary for the occurrence of stress corrosion cracking is precisely at the point of the scratch There are tensions. A meeting of these two requirements however, it is very unlikely.

A further increase in the service life can be achieved if according to a preferred embodiment of the method of the invention the stainless steel object coated with the nickel layer is heated to a higher temperature. In this case, part of the nickel diffuses

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layers in the chrome-nickel steel surface and increases in the transition zone the nickel content to over 40% “Practical tests have shown However, from 40% nickel content onwards, stress corrosion cracking no longer occurs »The transition layer is now much harder and against certain media more corrosion resistant than the nickel layer, so they Resists injuries much better. The scratches go accordingly - e.g. with normal use of a washing machine neither on the inside wall of the container and on the tubular heating element front surface like this deep that the original chrome-nickel steel alloy is exposed. Even if this is the case, stress corrosion cracking also occurs only open if there is internal tension precisely at this point »That However, the coincidence of all of these circumstances is unlikely.

The diffusion of the protective layer also has advantages with regard to another phenomenon, which can be _{observed (} tubular heaters which are operated in hard water »Here, an increasingly thick limescale deposit quickly forms around the radiator jacket poor thermal conductivity continuously decreases, so that the temperature gradient between the outer skin of the deposited layer and increases the tube heater jacket more and more. In this way, it is not uncommon that the tubular heating up to 800 ⁰ C reached coat temperatures. at this temperature, however, the oxygen contained in the lime deposit is able to oxidize the nickel surface, whereby the resulting nickel oxide bonds with the lime layer at the now again metallically bare places again. In this way a nickel

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layer which does not partially diffuse into the jacket surface is to be completely worn away over time, so that the tubular heater would again be exposed to the effects of corrosion of the type described. The diffusion layer is, however, due to the chromium contained in the stainless steel against the oxygen contained in the calcium deposit resistant, so that the life-increasing effect of the surface treatment according to the invention even under such extreme conditions Operating conditions are maintained. Provided that the method according to the invention is carried out so that part of the applied metal layer diffuses into the surface of the tubular heater jacket, so not only can the stress corrosion cracking suppress, but increase the scale resistance of the rust-resistant steel used considerably.

The nickel plating now brings another significant advantage. If z. B. tubular heating elements with a jacket made of rust-resistant chromium-nickel steel are annealed under an exothermic gas, then the surface turns gray as a result of the formation of chromium oxide. The tubular heating elements then have to be stained again so that the surface is attractive again and optimal corrosion resistance is achieved. However, this incurs costs. Accordingly, one sometimes switches to glowing in a vacuum. The creation of the vacuum, however, also entails a very considerable expense. Another way is to use hydrogen as a protective gas, but this is very expensive. The use of ammonia cracked gas is also known. In the latter process, however, the nitrogen penetrates the surface of the object and binds chromium there, for example up to 12 % of the same in an alloy with 18% chromium. If hydrogen or ammonia cracked gas is used to glow the tubular heating element, there is still a considerable risk of explosion to be accepted.

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It can now be seen that the nickel-plating of the chrome-nickel steel surface the annealing problems can be solved excellently. An exothermic reaction derived from propane gas, for example, can occur without difficulty Gas mixture can be used without the risk of discoloration the surface is due to oxidation.

The results of tests carried out are given below, which compared to the superiority of electrical tubular heating elements made of chrome-nickel steel treated by the method according to the invention show untreated tubular radiators made of chrome-nickel steel.

In order to examine the resistance to stress corrosion cracking, short-term tests are carried out in such a way that the samples to be examined are suspended in a saturated aqueous 47% magnesium chloride solution. In order to increase the aggressiveness of the magnesium chloride solution is maintained at a temperature of 15O ⁰ C. If no cracks appear after a test duration of 500 hours, the material is considered to be resistant to stress corrosion cracking.

Two series of tests were carried out in which tubular heating elements bent in the shape of a hairpin were used, the jacket of which consisted of one Steel with the standard designation XlOCrNiTi 189, material no. 4541, existed. The tubular heater legs had a distance from each other of 25 mm, the tube diameter was 8.5 mm, the leg length 250 mm. In the first series of tests, the tubular heating elements were unloaded investigated, in the second series of tests they were bent together until the tubular heater legs touched each other subjected to bending stress.

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The following table shows the length of time after which The first cracks appeared in both test series.

treatment

Test series

annealed and bright pickled

nickel-plated layer thickness 10 μ

nickel-plated layer thickness 10μ scratched surface

I Π

after 35-50 hours after 25-30 hours

> 500 hours,

> 500 hours

> 500 hours

> 500 hours

It was carried out a further attempt with tubular heating elements, in which due to an annealing treatment at a temperature of 900 ⁰ C to 1200 ⁰ C, a portion of the deposited layer of nickel was diffused into the outer surface of the tubular heater. The tests that were carried out with magnesium chloride solution analogously to those described above showed the same result as the tests that were carried out with tubular heating elements without annealing treatment. In addition, however, these tubular heating elements were subjected to a long-term test in which they were installed in hot water heaters in which water containing chlorine ions with a hardness of about 35 dH was heated to 85 C. Even after repeated flaking of the rapidly developing deposit layer, there was neither stress corrosion cracking, nor scaling, nor any of the above-described corrosion phenomena on the surface of the tubular heating element treated in this way.

It is understandable that within the meaning of the invention, all objects

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should be protected, which consist of a stainless steel and their surface is provided with a protective layer, by diffusion annealing between the protective layer and the stainless steel Transition zone can be created.

In a particularly advantageous embodiment of the present method, the protective layer is applied in a thickness of 10-20μ, the diffusion annealing is carried out at 900 to 120 ° G and then the part of the protective layer that has not diffused in is pickled. The objects obtained in this way then have a more matt and darker surface due to the lack of the mostly shiny metal protective layer. This is particularly advantageous on tubular heating elements, as the darker surface significantly increases the radiation capacity. The same effect can also be achieved if the metal protective layer, for example the nickel layer, is only applied in a die of 2 to 8 μ, preferably 5 μ, and the tubular heating element treated in this way is ^{subjected to diffusion annealing at 900 to 1200 °} C. again. As a result, the entire protective layer diffuses into the surface of the treated tubular heating element.

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Claims (1)

1 BZH Claims 1 "Method for treating the surface of objects stainless steel, especially those that come into contact with water containing chlorine ions, characterized in that for To prevent stress corrosion cracking, a thin protective layer of nickel is applied to the surface of the object. 2. The method according to claim 1, characterized in that the protective layer is applied galvanically or chemically. 3. Process according to Claims 1 and 2, characterized in that the object provided with the protective layer is heated to a temperature of 900 to 1200 C in order to diffuse in a part to effect the applied protective layer in the surfaces of the object. 4. The method according to claim 1 and one of the subsequent claims, characterized in that the steels contain up to 40%, preferably 8% to 12%, nickel. 5. The method according to claim 3, characterized in that the heating takes place under a protective gas containing hydrocarbons. 6. The method according to claim 1 and one of the following, characterized characterized in that the protective layer is applied in a thickness of 1 to 20 μ will. 109820/1937 16.2U51 7. Tubular heating element in which one is embedded in an insulating mass Heating coil is enclosed by a jacket tube made of rust-resistant chromium-nickel steel, characterized in that the jacket tube has a thin protective layer of nickel on its surface. 8 "tubular heater according to claim 8, characterized in that the Alloy of the jacket tube preferably contains 8 to 25% chromium. 9. Container made of rust-resistant chromium-nickel steel for holding water containing chlorine ions, characterized in that its inner surface is lined with a thin protective layer of nickel. 10. Device for heating water by means of a tubular heater, e.g. hot water boiler, pressure boiler, washing machine or dishwasher, characterized in that the inner surfaces of the rust-resistant chromium-nickel steel, for receiving the Water-specific container parts or the tubular heating element with a jacket tube made of rust-resistant chromium-nickel steel or both wear a thin protective layer of nickel, which can be completely or partially diffused into the stainless steel. ³¹⁸¹ 109 820/1937