DE1521004A1 - Process for the production of metal parts resistant to stress corrosion - Google Patents

Description

Process for the production of stress corrosion resistant Metal parts A large number of metallic materials and alloys, e.g. Brass, stainless steel or light metal alloys are susceptible to stress corrosion. In many cases, for technical or economic reasons, it is not possible to use another material that is not sensitive to stress corrosion instead of the susceptible alloy is.

Measures must then often be applied that reduce the risk of To reduce or eliminate stress corrosion. One such measure is e.g. the application of metallic, e.g. galvanic coatings on the endangered Construction parts. To previous proposals are ductile coatings or multiple layers applied. But, as is often the case, name the construction part may still be deformed after metallizing, e.g. when installing a part of the device must, then - especially in the case of severe deformation - the protective effect of such coatings against stress corrosion in many cases greatly reduced or completely destroyed.

It has now been found that a stress corrosion resistant Coating on metal parts that have a coating of at least one metal, is obtained when the coated metal part is subsequently heated. This measure is particularly advantageous if the structural parts are added later be greatly deformed. Such a long service life of the treated parts is achieved, that this equates to an unlimited shelf life in practice. If afterwards is no longer deformed by the heat treatment, the protective effect of metallic Coating in extraordinary; iron increased.

It has also been found that it is particularly advantageous to use multiple layers, mainly nickel-silver layers to apply. This is preferably done by galvanic deposition. Coated with galvanic multiple layers and subsequently heated construction parts made of brass, for example, which are under tensile stress and exposed to the risk of stress corrosion as a result of the effects of ammonia and moisture have a significantly longer service life.

The term "service life" is understood here to mean the period from the beginning of the Check until the first cracks appear. Under the "relative service life" becomes the V £ rhäi, tnia of the life of a protected Sample too understood the service life of an unprotected reference sample.

The protective effect of subsequently deformed galvanic nickel or cadmium coatings as well as nickel-silver double layers are produced by the heat treatment increased in an extraordinary way. Nickel coatings that need to be deformed become only usable through this measure. They make an excellent one Protection. If, for example, heat treatment is carried out at 350o C for one hour, 12 / um is already guaranteed thick nickel coatings provide adequate protection in the event of severe deformation. Still essential The success is better if the heat treatment is carried out at 300o C for 48 hours will. Heat-treated nickel-silver layers also show up in one application 6 / um silver has a very good protective effect even with severe deformation. Will after the heat treatment is not deformed, it will significantly increase the service life achieved, which is possibly due to the formation of a diffusion layer.

The base metal can be made of steel, precious metal alloys, magnesium-aluminum-nickel or titanium alloys. Particularly good results were achieved with structural parts Made of brass.

To protect the base metals, e.g. coatings made of silver, nickel, Cadmium, lead, tin, zinc or multiple layers such as nickel-silver double layers or metal alloys, such as s.B. Tin-nickel alloys, preferably electroplated are applied, suitable. The metallic protective layers can be customary Wise upset. The heat treatment takes place in one suitable Furnace, e.g. a muffle furnace, which ensures that the desired temperature is precisely maintained guaranteed over a longer period of time. for the implementation of the invention In the process, it is often advantageous to heat in the presence of a protective gas. During the heat treatment, it should also be noted that the strength of the Material can decrease as a result of recrystallization. This is caused by a drop in hardness displayed. if this is inadmissible or undesirable, the temperature becomes so low chosen so that a reduction in strength is avoided. The effect of the heat treatment the metallic coating can often be applied by extending the duration of the coating can be increased. Home coating of metal with a metal coating or a Multiple layers and subsequent heating, preferably at a temperature of 300 to 350o C according to the invention becomes an excellent protection against stress corrosion achieved. The heat treatment according to the invention is then particularly great Use when a structural part, for example a contact angle made of brass, is still deformed after the galvanic treatment.

The invention is illustrated below by means of the examples.

In the examples, the service life and the relative service life of loops made of Ms 63 (loop samples according to DIN 50908), which are protected by heat-treated galvanic coatings, are given when testing for stress corrosion in an ammonia atmosphere. In order to clarify the effect of the heat treatment, tables are also included for the service life of non-heat-treated brass loops with the same galvanic coatings with and without deformation. In the following, the terms "moderate" and "severe" deformation will be used. It means: moderate deformation: brass strips are bent into an "open loop" in accordance with DIN 50908 and galvanized in this way. The open loop is then screwed together to form a "closed loop." This results in a deformation of the galvanic coating corresponding to an expansion of 1 to 2 96.

strong deformation: brass strips are electroplated and then bent into a closed loop in accordance with DIN 50908 and screwed together. A deformation corresponding to an elongation of approx. 6% occurs of the galvanic coating.

Example 1 Strips made of brass (Ms 63) were placed in a nickel electrolyte containing 330 g / 1 NiS04. 7 H20 45 g / 1 NiC12 9 6 H20 37.5 g / 1 H3 B03 galvanically nickel-plated at pH 4 to 4.5 and 50 to 60o C and then kept in a muffle furnace at 250o C for 120 hours. The strips were bent into loops and tested for stress corrosion in ammonia vapor over a buffered solution with pH 10 according to AEBI published in Zeitschrift für Metallkunde 49 (1958) pages 63 to 68. The result of this test is shown in Table I. Table I. 'Layer thickness, = duration of the heat deformation service life; rel. treatment (hrs.) (hrs.) Life. ' duration i 12 / around 120 strong 63 '4.5 For comparison, nickel coatings were produced in an analogous manner on strips and exposed loops made of brass (Ms 63). The coated strips and open loops were bent or screwed together to form closed loops and subjected to the stress corrosion test without subsequent heating. The result of this test can be seen from Table II. Table II Layer thickness Deformation Service life (hours) ° Relative service life without 70 i 5 6 / around moderate 14 1 `strong 14 1 to without 126 9 6th 9 @ at ! moderate 14 '1 strong 14 1 v without 168 12 12 / µm; moderate 42 3 strong 14 1 a 'without 532 38 20 / To moderate 14 1 strong 14 1 EXAMPLE 2 Strips and open loops made of brass (Ms 63) were galvanically nickel-plated in a nickel electrolyte of the composition given in Example 1 and then treated in a muffle furnace at 3000 ° C. for 12 to 48 hours. The samples were then bent or screwed together to form finished loops and subjected to the test for stress corrosion in an ammonia atmosphere in the manner indicated above.

The result of this test is shown in Table III. Table III Duration of the layer thickness deformation Lifetime relat. Heat treatment (hours) service life tree moderate 42 3 strong 28 2 12 hrs. 9 / at moderate 1260 90 i strong 28 2 `12 / um moderate 2268 t h '@ strong 56 4 T 3 #, hrs. 12 / um strong 308 121um moderate i 980 strong> 1736 i 1 'a 48 hrs. 20 / um moderate; 1736> i 1 strong > 1736 'r 1 EXAMPLE 3 Strips and open loops made of brass (Ms 63) were galvanically nickel-plated in a nickel electrolyte of the composition given in Example 1 and then treated in a muffle furnace at 300 ° C. for 1 to 15 hours. The samples were then bent into loops and subjected to the test for stress corrosion in an ammonia atmosphere in the manner already indicated.

The result of this test is shown in Table IV. Table IV Duration of the layer thickness' Deformation Service life relat. Heat treatment j food duration 6 to moderate 364 26 strong 28 9 / around 700 ! 50 strong 56 4 1 hour 12 at moderately gc-q # - '@ strong 280 20 @ at moderately 36e0 strong 616 moderate 1 1'l6 @ f34 5th hours) 12 1 at 44 strong 364 @? ri 15 hours * 12 / um strong 616 no r - rr a 44 with this duration of the heat treatment already a strengthening abatement a. EXAMPLE 4 Strips and open loops made of brass (MS 63) were galvanically nickel-plated in a nickel electrolyte of the composition given in Example 1 and then treated in a pouring furnace for 4 to 6 minutes at 400.degree. The samples bent or screwed into loops were subjected to the test for stress corrosion in ansonia vapor, as described above. The result of this test is shown in Table V. Table V Duration of the layer thickness' deformation ' service life' relat. Heat treatment (hours) food duration moderate 462 33 4 minutes 121um strong 14 1 '6 minutes 12 to moderate 5.3: j -42 strong 3f @ `16 Example 5 strips and open loops of brass (Ms 63) were electroplated with nickel in a nickel electrolyte having the composition indicated in Example 1 and then in a Nuffelofen at? 50o C under nitrogen, then were bent, the samples to produce loops or bolted together and the Tested for stress corrosion in ammonia vapor in the manner indicated above.

The result of the test is shown in Table VI. Table VI 'Duration of the deformation of the layer thickness; Service life relat. Heat treatment (: 3td.) Service life 10 seconds 121um moderate 84 6 i 15 seconds 12 / around moderate 180 30 20 seconds 121um moderate 944 60 strong 84 6 25 seconds 121um moderate 1932 13.1 strong 84 d iU seconds * '21um moderately, > 1960 > 141 strong 112 moderate " 1960) 1 = @ 0 40 seconds 12 / um strong 462 50 seconds *) 12 / um strong 658 15 minutes * *) 12 / um strong >3598>, 15.1 with this duration of the heat treatment a certain amount occurs Hardness drop a. -e) During this duration of the Würnebehaadlung there is a sharp drop in hardness Example 6 Strips and open loops made of brass (Na 63) were placed in a nickel electrolyte of the composition given in Example 1 and then in a silver electrolyte containing 40 g / l K Ag (CN) 2. 53 g / 1 KCN galvanically nickel-plated b $ w. silver plated. The samples were then kept in a Nuftel oven at 3500 Ω for 1 hour.

The samples were bent au loops or auca "engesahraubt der'hrüiung and to stress corrosion in Ammonishdupi, as previously described, subject. The result of this test is shown in Table VII. Table VII Duration of the layer thickness I deformation service life reut. Heat treatment; service life 6 / around the Nile moderately 630! 45 6 / around Ag strong 518 37 1 hour 12 / around Nile moderate 868 6 / around Ag strong 868 62 20 / um Ni / moderate 1680 1 r3 () 6 / um Ag strong 952 G.1 I. Example 7 Closed (finished) loops made of brass (Ms 63) were galvanically nickel-plated or silver-plated in a nickel electrolyte of the composition given in Example 1 and then in a silver electrolyte of the composition given in Example 6. The samples were then exposed to a temperature of 350 ° C. for 1 hour in a muffle furnace. The samples were not deformed.

The result of the subsequent test for stress corrosion in ammonia vapor in the manner indicated above can be seen from Table VIII. Table VIII Duration of the layer thickness Lifespan (hours) Relative life - Heat treatment 6 / around NNile 4200 300 6um Ag 12 around the Nile 1 hour 6 / around Aeg 4200 300 20 / around Nile 4200 300 6 to AG For comparison, a nickel-silver coating (double layer) was applied in an analogous manner to strips, open loops and closed loops on brass without subsequent heat treatment. The strips and open loops were bent or screwed together to form closed loops and subjected to the test for stress corrosion in ammonia vapor as described. The result is shown in Table IX. Table IX Layer thickness deformation; Lifespan (hours) relative lifespan without 154 11 6 / um Ni / 6 / around Ag moderate 84 'strong 14 without 378 27 12 / around Ni / moderate 84 6 6 / around Ag strong 14 Example 8 Closed (finished) loops made of brass (read 63) were finished in a commercially available galvanic bright cadmium bath. They were then subjected to the test for stress corrosion in ammonia vapor, as indicated above.

The result of the test is shown in Table X. Table X_ , Duration of the layer thickness service life (hours) relative service life Heat treatment duration 4 hours i 12 / at 462 33 8 hours 12 / at 840 R 6 In comparison to this, closed (finished) loops made of brass were cadmed in a commercially available galvanic gloss admium bath. The samples were then subjected to the test described for stress corrosion in ammonia vapor.

The result of the test is shown in Table XI. Table, XI Layer thickness deformation; Service life (-hours) relative service life 12 / um without 112 (8 m . - mr.r..r.rm i It can be seen from Tables II, IX and XI that simple galvanic coatings without heat treatment and without deformation have a partly good protective effect , but that this protective effect is in most cases even completely undone by deformation . In the case of double layers, for example nickel-silver coatings, the protective effect is also practically destroyed in the event of severe deformation. In contrast, the coatings heat-treated according to the invention provide excellent protection even in the event of severe deformation, in many cases even significantly better than non-heat-treated layers without deformation.

Claims (5)

Claims 1. A method for the production of anti-stress corrosion resistant metal parts that have a coating of at least one metal, characterized in that the coated metal part is subsequently heated. 2. The method according to claim 1, characterized in that one with a nickel coating provided metal part is heated. 3. The method according to claim 1, characterized in that a metal part provided with a nickel-silver layer is heated . 4. The method according to any one of claims i to 3, characterized in that it is heated to 300 to 3500 C. 5. The method according to any one of claims 1 to 4, characterized characterized in that it is heated in the presence of protective gas.