CORROSION RESISTANCE OF VACUUM CHROMIZED IRON PARTS FOR HERMETICAL RELAYS L. Koller, M. Jenko, E. Perman Keywords: electromagnetic relays, hermetical relays, miniature relays, components, vacuum chromizing, corrosion properties, corrosion resistance, professional electronics, testing, experimental results Abstract: Corrosion resistance of vacuum cfiromized layers on pure iron to the defined corroding medium of synthetic sea water (3% NaCI solution) was studied. Two different temperatures (25°C and 93°C) and three different times (48, 100 and 200 hours) of testing were used. An attempt was made to define the coating thickness and the content of chromium which will assure optimal corrosion resistance of surface alloys. Chromizing was performed in medium and high vacuum (3x10'^ mbar and 2x10'® mbar, respectively). Vacuum chromized iron parts (both sintered iron Vacofer S2 and the relay iron ReFeSO) with the layer thickness of 70 |.im (obtained in high vacuum at 2x10'® mbar at the temperature of 1100°C in 12 hours) showed very good corrosion resistance. Korozijska obstojnost vakuumsko kromanega železa za miniaturne hermetične releje Ključne besede: releji elektromagnetni, releji hermetični, releji miniaturni, deli sestavni, kromanje vakumsko, lastnosti korozijske, odpornost korozijska, elektronika profesionalna, preskušanje, rezultati eksperimentalni Povzetelc Študirali smo korozijsko odpornost vakuumsko kromanih plasti na čistem železu proti korozijskem mediju sintetične morske vode (3% raztopina NaCI). Delali smo pri dveh različnih temperaturah (25°C in 93°C) ter pri treh časih (48, 100 in 200 ur). Poskušali smo tudi določiti debelino plasti in vsebnost kroma, ki bi zagotavljala obstojnost površinskih nanosov. Vakuumsko kromanje je potekalo pri srednjem in visokem vakuumu (3x10'^ mbar in 2x10'^ mbar). Zelo dobro korozijsko odpornost so pokazali vakuumsko kromani železni deli, tako tisti iz sintranega železa VACOFER S2 kot tudi tisti iz relejnega železa ReFeSO, pri katerih je bila debelina plasti == 70 |.im (dobljena v visokem vakuumu 2x10'^ mbar pri temperaturi 1100°C in času 12 ur). INTRODUCTION When studying the properties of vacuum chromized layers (2, 5, 7, 13) on pure iron we found out that they cannot be tested by the methods suitable for the galvanic or chemically deposited layers. An attempt was made to define the coating thickness and the content of chromium which will assure optimal corrosion resistance. Besides, there was the open question of the appropriate method for testing the corrosion resistance (1,3,4,6,8-12,14,15)tothe defined corroding medium (16,17). Vacuum chromized layeron very pure sintered iron VACOFER S2 VACUUMSCHMELZE and the relay iron REFeSO USAB-MUNKFORS were studied. EXPERIMENTAL The resistance to attack from salt water is often accepted as a sufficient indication of the corrosion resistance of stainless steel and vacuum chromized iron against the milder and normal atmospheric conditions. This and similar corrosion tests in water and/or different saline solutions are frequently used for stainless and chromi- zed steels. Weight decrease per square cm (mg/sq.cm) and time unit is the measure of the degree of corrosion. Most steels and irons resist corrosion in destiled, river and tap waters while the corrosion in salt water is more severe, since dissolution enhancing the electrolytic effects are predominant. The results give a good indication fairly quickly of the general corrosion resistance of the above mentioned materials. In our investigations corrosion resistance in the defined corroding medium (synthetic sea water (16), often 3% sodium chloride solution) was studied for vacuum chromized test pieces of very pure sintered iron VACOFER S2 (dimensions 50mm x 20mm X 1.2mm) and relay iron REFeSO USAB-N/IUNK-FORS (dimensions 50mm x 20mm x 0.8mm). The test samples were vacuum chromized at the temperature of 1100°C for 3, 6, 9 and 12 hours in a vacuum 3x 10'^ mbar and 2x10"^ mbar (Table 1). The corrosion test times were 48, 100 and 200 hours at the temperature 93°C. For the vacuum chromized sintered pure iron the weight decrease per square cm is listed in Tables 2, 4 and plotted in Figures 1-4. The corresponding data of the vacuum chromized relay iron are also given in Tables 3, 5 and Figures 1-4. tit E d a S 2,8 o ' •o - 2,6 Ä bi C 2,4 S 2,0 1,8 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 O L o 48 100 200 time (h) Fig, 1: Rate of corrosion of vacuum chiromized parts (3x10 mbar, room temperature, 3% NaCI solution) _pure sintered iron VACOFER S2 ----relay iron REFeSO 1 ,2, 3, 4 types of layers (Table 1) M 3 a " 2,8 aj u 2,6 o •c - 2,4 "m S 2,2 S 2,0 1,8 1,6 1,4 1,2 1,0 0,8 1- j 0,6r j 0,4^ i 0,2^ O L o Fig. 2: 48 100 200 time (h ) Rate of corrosion of vacuum chromized parts (3x10^ mbar, 9fC, 3% NaCI solution) ___pure sintered iron VACOFER S2 ----relay iron REFeSO 1, 2,3, 4 types of layers (Table 1) type of Cr layer time of chromi-zing (hours) pressure (mbars) thickness (m) 1 3 3x10'^ 20 2 6 3x10'^ 34 3 9 3x10"^ 39 4 12 3x10"^ 50 r 3 2x10"^ 28 2' 6 2x10^^ 70 3' 9 3x10'^ 71 4' 12 2x10^^ 90 Table 1: Parameters of the vacuum chromized parts and the definition of types type of layer time (hours) (3% NaCI solution) weight decrease (mg/sq.cm) room tempera- 93° C ture 1 48 0.210 0.185 100 0,985 1,360 200 1.935 2.130 2 48 0.205 0,175 100 0.725 1,195 200 0.995 2.105 3 48 0.080 0.020 100 0.690 1.015 200 1.110 1.995 4 48 0.105 0.160 100 0.235 1.045 1 200 0.285 2.100 1 Table 2: Corrosion of vacuum chromized pure iron VACOFER S2 (3x10"2 mbar) Fig. 3: Rate oj corrosion of vacuum chromized parts (2x10' mbar, room temperature, 3% Nad solution) _pure sintered iron VACOFER S2 - — relay iron REFeSO 12', 3', 4 ■ types of layers (Table 1) Fig. 4: Rate of corrosion of vacuum chromized parts (2x1 a^ mbar, 9:fC, 3% NaCI solution) _pure sintered iron VACOFER S2 - - - - relay iron REFeSO 1', 2', 3', 4' types of layers (Table 1) type of layer time (hours) (3% NaCI solution) weight decrease room temperature (mg/sq.cm) 93° C type of layer time (hours) (3% NaCI solution) weight decrease room temperature (mg/sq.cm) 93° C 48 0.405 0.235 r 48 0.200 0.050 100 1.770 1.155 100 0.215 0.125 1 200 2.615 1.900 200 0.420 0.540 2 48 0.205 0.175 2' 48 0.295 0.050 100 0.890 1.065 100 0.295 0.050 200 1.435 1.885 200 0.410 0.460 3 48 0.200 0.200 3' 48 0.050 0.060 100 0.530 0.995 100 0.280 0.025 200 0.795 1.795 200 0.290 0.440 4 48 0.205 0.075 4' 48 0.105 0.085 100 1.040 0.815 100 0.195 0.100 200 1.320 1.440 200 0.245 0.430 Table 3: Corrosion of vacuum chromized relay iron REFeSO (3x10"^ mbars) Table 4: Corrosion of vacuum chromized pure iron VACOFER S2 (2x10^^ mbar) type of layer time (hours) (3% NaCI solution) weight decrease room temperature (mg/sq.cm) 93° C r 48 0.635 0.200 ICQ 0.655 0.180 200 0.945 0,565 2' 48 0.200 0.165 100 0.290 0.150 200 0,580 0.395 3' 48 0.255 0,155 100 0.315 0.135 200 0.545 0.350 4' 48 0.055 0.055 100 0.260 0.070 200 0.430 0,310 1 Table 5: Corrosion of vacuunn chromized relay iron REFeSO (2x10'® mbars) RESULTS AND DISCUSSION Corrosion of metals in solutions can be described with the laws of electrochemistry. The electrode potentials of metals and ions as well as the pH of aqueous solutions must be known to understand the corrosion processes taking place. When the corrosion properties are in question the vacuum chromized soft iron may be compared with a stainless steel containing 13% content of chromium. The standard electrode potential of this steel is -0.32 eV. In the medium of neutral salt water with a slightly negative electrode potential of iron some various parallel anodic reactions can take place. Iron is ionized into ferri and ferro ions according to the following reactions: Fe.....>Fe^^-t-3e" -0.036 V Fe.....>Fe2^-f-2e^ -0.440 V Fe^"--> Fe^"-f e^ -0.771V Ferro and ferri hydroxides react with the oxygen dissolved in water giving Fe'" hydroxide. In the neutral salt solution hydrolysis that gives a dilute solution of HCl is obtained which reacts with the ferro ions giving the chloride of bi-valent iron. Fe^^ + 2 H2O —-> Fe(0H)2 -h2 H" Fe^" + 2 HCl.....> FeCl2 4- 2 H" Our experiments undoubtedly show (see Tables 2-5 and Figures 1-4) that vacuum chromized layers obtained in high vacuum on both substrates have better corrosion resistance than the layers obtained in medium vacuum. 2 CONCLUSIONS - Strong corrosion effects were found when studying the properties of vacuum chromized layers obtained in medium (3x10"^ mbar) and high (2x10"® mbar) vacuum for the layers of type 1 and 1' with the layer thickness of 20 |am and 28 jam, respectively. It is evident that the thickness of these vacuum chromized layers was less than 50 (.im. - Corrosion resistant layers of type 2 and 3 (layer thickness of 34).im and 39 |im, respectively) obtained in medium vacuum are better but the pitting corrosion was detected at points where profile analysis showed that the concentration of Cr under the surface was less than 13%. The layers of type 2', 3', 4 and 4' with a thickness greater than 50 |.im showed excellent corrosion resistance in all mediums both when deposited on very pure sintered iron VACOFER 82 and when deposited on the relay iron REFeSO. - It was shown that for the vacuum chromized corrosion resistant layers obtained in high vacuum 2x10'® mbar (layers of types 1', 2', 3' and 4') the thickness was greater than for the layers obtained during the same deposition time in medium vacuum 3x10 "" mbar (layers of types 1, 2, 3 and 4). The corrosion resistance of the former are therefore better. REFERENCES 1. F.L La Que, H R Capson, Corrosion Resistance of Metals and Alloys, p 86, Chapman and Hale, Ltd, London 1963. 2. R.L.Samuel and N.A Lockington, Iron and Coal Trades Revicte 1954, 169, 803; Industrial Finishing, 1954, 6, 896. 3. M.Milenkovic etal,, Korozija i zaščita, Tehniška knjiga, Beograd 1966. 4. S.Mladenovič et al., Korozija i zaščita materiala, Rad, Beograd 1985. 5. M.Jenko et al.. Študij poboljšanja magnetnih lastnosti železa z difu-zijskim kromiranjem , URP Report 1984, 1985. 6. L. Koller et al.. Študij površinske zaščite mehkega železa z dituzijski-mi postopki, URP Report 1985, 1986. 7. M.Jenko, A.Kveder, R.Tavzes, E.Kansky, Diffusion Chromium Coating of Iron Magnetic Circuits Parts for Relays, Journal of Vacuum Sei, Tech. A3, 6 (1985). 8. 1986 Anual Book of ASTM Standards Section 3, Vol, 03.02 Erosion, and Wear; Metal Corrosion, Philadelphia, 9. G,Herbsleb und P,Schwab, Werkstoffe und Korrosion 37, 24 (1986). 10. F.W.Hirth, H,Speckhardt und K,Stallmann, Galvanotechnik D 7968Saulgan 73, 110(1982), 11. F.W.Hirth, H,Speckhardt und K.Stallmann, Galvanotechnik D 7968 Saulgan 74, 426 (1983). 12. H Schwitter und H Bohni, Werkstoffe und Korrosion 31, 703 (1980), 14. R.Wiederman, Werkstoffe und Korrosion 32, 269 (1981). 15. E.Meckelburg, Galvanotechnik D 7968 Saulgan 72, 953 (1981). 16. R.L.Samuel, N.A.Lockington and H.Dorner, Metal Treatment and Drop Forging, Recent Development in Chromium Diffusion, Part III., p. 336 (1955). 17. A.R.Rudnik and D.L.Ljubinski, Tehnologija Miniatjurnyh Rele, Energoizdat, Leningrad 1982. Lidija Koller Inštitut za elektronil