The studies of corrosion resistance of AISI 316Ti SS in Ringer s solution after electropolishing and passivation in nitric acid

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1 Available online at WSN 98 (2018) EISSN The studies of corrosion resistance of AISI 316Ti SS in Ringer s solution after electropolishing and passivation in nitric acid Krzysztof Rokosz 1,a, Tadeusz Hryniewicz 1,b, Grzegorz Solecki 1,2,c 1 Division of BioEngineering and Surface Electrochemistry, Department of Engineering and Informatics Systems, Faculty of Mechanical Engineering, Koszalin University of Technology, Racławicka 15-17, PL Koszalin, Poland ABSTRACT 2 BerlinerLuft Sp. z o.o., Chocimska 7, PL Białogard, Poland a-c address: rokosz@tu.koszalin.pl, Tadeusz.Hryniewicz@tu.koszalin.pl, grzegorz.solecki.sg@gmail.com In present work, the continuation of general and pitting corrosion analysis of austenitic AISI 316Ti (EN ) stainless steel in Ringer's solution, is presented. The corrosion was studied by using the ATLAS 98 potentiostat with platinum EPT-101 and calomel reference EK-101P electrodes. The three types of specimens, i.e. as received (without any pretreatment), after abrasive mechanical polishing (MP), and after electrochemical polishing (EP), were used. The best pitting corrosion resistance was recorded for electropolished and passivated (in 20% vol. HNO 3 for 30 minutes) surface, i.e. the pitting potential was equal to 761 mv SCE (855.4 ± 58.5 mv SCE ), while the worst one was recorded for mechanically ground samples and the pitting corrosion potential was equal to 270 mv SCE (378.8 ± 60.3 mv SCE ). Keywords: AISI 316Ti/EN , austenitic stainless steel, electropolishing, passivation, pitting corrosion, potentiodynamic corrosion measurements ( Received 02 April 2018; Accepted 21 April 2018; Date of Publication 22 April 2018 )

2 1. INTRODUCTION The pitting corrosion of duplex steels was described in reference [1], however due to the price of this steel in some applications austenitic stainless steel may be used. Specifically this substitutions may be used in the food processing equipment, brewery equipment, chemical and petrochemical equipment, laboratory benches, coastal balustrading, boat fittings, chemical transportation containers, heat exchangers, and mining screens. The most frequently used austenitic steels are AISI 304 or AISI 304L, which contain mainly iron, chromium, and nickel. The more expensive austenitic steels containing molybdenum, i.e. AISI 316 or AISI 316L have better corrosion resistance than AISI 304/304L. The addition of titanium, what is observed in the case of AISI 316Ti stainless steel, results in structure stabilization at temperatures over 800 C, what prevents carbide precipitation at the grain boundaries and protects the metal from corrosion [2]. It should be pointed out that still the steels [3-4] as well as titanium and its alloys [5-10] are applicable in industry. To obtain better surface roughness and corrosion resistance of steels surfaces, the electropolishing (EP) [10-13] has been effectively used. It is worthy noticing that recently some modifications of the process were proposed by magnetoelectropolishing (MEP) [14-28], high-current-density electropolishing (HDEP) [29-31], or high-voltage electropolishing (HVEP) [32]. The aim of this work is a comparative analysis of general and pitting corrosion of austenitic AISI 316Ti (EN ) stainless steel in Ringer's solution. Three states of the steel surface have been taken into account, as-received (AR) after rolling operation, after mechanical/abrasive polishing (MP), and after electropolishing (EP), all of them passivated in 20% vol. HNO METHOD AISI 316Ti (EN ) austenitic stainless steel samples (cuboid with dimensions of mm) were used for the study. The main elements constituting the steel are: chromium (16-18%), molybdenum ( %), nickel ( %), titanium (max 0.7%), silicon (max 0.75%), phosphorus (max 0.05%), sulfur (max 0.08), including carbon (max 0.08%), and iron as the rest of the steel composition. The electrolytic polishing operations were performed at the current density of 50 A/dm 2. The main elements of the Electropolishing (EP) setup were a processing cell, a DC power supply RNG-3010, the electrodes and connecting wiring. The studies were carried out in the electrolyte of initial temperature of 40 C, with the temperature control of ±5 C. Generally, the final electrolyte temperature was increased up to 55 C. For the studies, the electrolyte being a mixture of two acids, i.e. H 2 SO 4 :H 3 PO 4 equal to 2:3, was used. The passivation was performed in the solution of 20% by volume nitric acid (HNO 3 ) for 15 and/or 30 minutes in temperature of 25 ºC. The corrosion potentiodynamic polarization tests were carried out in Ringer s solution (8.6 g/dm 3 NaCl, 0.3 g/dm 3 KCl, 0.48 g/dm 3 CaCl 2 ) on the ATLAS 98 testing device using the POL 98 software. The tests were carried out with a potential of 400 mv relative to the saturated calomel electrode (SCE) in the anodic side with a potential step of 5 mv (potential change rate of 0.5 mv/s) up to current density of 1000 μa/cm 2. The scan in cathodic direction was performed with the potential change rate of 1 mv/s. As counter, reference and working electrodes a platinum plate with a surface area of 25 mm 2 (EPT-101), calomel reference (EK- -47-

3 101P), and AISI 316Ti stainless steel (examined sample), were used, respectively. For each run, the electrolytic cell made of glass was used, containing up to 500 ml of the electrolyte. 3. RESULTS AND DISCUSSION In Figure 1, there are presented potentiodynamic corrosion results of AISI 316Ti stainless steel, as received (AR). The corrosion potential of general corrosion was changing in the range from 108 mv SCE up to 57 mv SCE (range: 165 mv SCE ), while pitting corrosion potential was in the range of from 587 mv SCE up to 728 mv SCE (range: 141 mv SCE ). In Figure 2, there are presented potentiodynamic corrosion results of AISI 316Ti stainless steel only after passivation for 15 minutes in 20% (vol.) HNO 3. The corrosion potential of general corrosion was changing in the range from 246 mv SCE up to 76 mv SCE (range: 322 mv SCE ), while pitting corrosion potential was in the range of from 511 mv SCE up to 659 mv SCE (range: 148 mv SCE ). In Figure 3, there are presented potentiodynamic corrosion results of AISI 316Ti stainless steel only after passivation for 30 minutes in 20% (vol.) HNO 3. The corrosion potential of general corrosion was changing in the range from 249 mv SCE up to 134 mv SCE (range: 115 mv SCE ), while pitting corrosion potential was in the range of from 575 mv SCE up to 925 mv SCE (range: 350 mv SCE ). In Table 1, there are corrosion results related to AISI 316Ti stainless steel without any treatment (as received) after passivation for 15 and 30 minutes in 20% (vol.) HNO 3. Corrosion potential for non-passivated sample was equal to 33.6 ±59.3 mv SCE (median: 40.5 mv SCE ), while the pitting corrosion potential ±40.2 mv SCE (median: mv SCE ). In case of passivated sample in 20% (vol.) HNO 3 for 15 minutes the corrosion potential was equal to 68.7 ±142.8 mv SCE (median: 57.5 mv SCE ), while pitting corrosion potential was ±41.6 mv SCE (median: 573 mv SCE ). The corrosion potential of sample passivated in 20% (vol.) HNO 3 for 30 minutes was equal to ±34.8 mv SCE (median: mv) SCE, while pitting corrosion potential was ±102.9 mv SCE (median: 635 mv SCE ). In Figure 4, there are presented potentiodynamic corrosion results of AISI 316Ti stainless steel after abrasive polishing (MP) using abrasive paper No The corrosion potential of general corrosion was changing in the range from 214 mv SCE up to 102 mv SCE (range: 112 mv SCE ), while pitting corrosion potential was in the range from 270 mv SCE up to 462 mv SCE (range: 192 mv SCE ). In Figure 5, there are presented potentiodynamic corrosion results of AISI 316Ti stainless steel after abrasive polishing and passivation for 15 minutes in 20% (vol.) HNO 3. The corrosion potential of general corrosion was changing in the range from 265 mv SCE up to 87 mv SCE (range: 178 mv SCE ), while pitting corrosion potential was in the range of from 365 mv SCE up to 495 mv SCE (range: 130 mv SCE ). In Figure 6, there are presented potentiodynamic corrosion results of AISI 316Ti stainless steel after abrasive polishing and passivation for 30 minutes in 20% (vol.) HNO 3. The corrosion potential of general corrosion was changing in the range from 293 mv SCE up to 156 mv SCE (range: 137 mv SCE ), while pitting corrosion potential was in the range of from 452 mv SCE up to 576 mv SCE (range:124 mv SCE ). -48-

4 Fig. 1. Potentiodynamic curves of AISI 316Ti (AR as received) in Ringer s solution Fig. 2. Potentiodynamic curves of AISI 316Ti stainless steel (as receved) after passivation for 15 min. in 20% (vol.) HNO 3 obtained in Ringer s solution -49-

5 Fig. 3. Potentiodynamic curves of AISI 316Ti after passivation for 30 min. in 20% (vol.) HNO 3 obtained in Ringer s solution Fig. 4. Potentiodynamic curves of AISI 316Ti stainless steel after abrasive polishing (MP) obtained in Ringer s solution -50-

6 Table 1. Results of potentiodynamic measurements of AISI 316Ti stainless steel (AR as received) and after passivation in 20% (vol.) HNO 3 for 15 and 30 minutes Sample 316Ti-AR 316Ti-AR-20% HNO 3-15min 316Ti-AR-20% HNO 3-30min E pit E cor E pit E cor E pit E cor Average Stand. dev Median Max Min Range In Table 2, there are corrosion results related to AISI 316Ti stainless steel after abrasive polishing and after passivation for 15 and 30 minutes in 20% (vol.) HNO 3. Corrosion potential for non-passivated sample was equal to 160 ±30.8 mv SCE (median: mv SCE ), while the pitting corrosion potential was ±60.3 mv SCE (median: mv SCE ). In case of passivated sample in 20% (vol.) HNO 3 for 15 minutes, the corrosion potential was equal to 172 ±58 mv SCE (median: 168 mv SCE ), while pitting corrosion potential was ±38.1 mv SCE (median: mv SCE ). The corrosion potential of sample passivated in 20% (vol.) HNO 3 for 30 minutes was equal to ±36.3 mv SCE (median: 198 mv SCE ), while pitting corrosion potential was ±37 mv SCE (median: 547 mv SCE ). -51-

7 Fig. 5. Potentiodynamic curves of AISI 316Ti stainless steel after abrasive polishing and passivation for 15 min. in 20% (vol.) HNO 3 obtained in Ringer s solution Fig. 6. Potentiodynamic curves of AISI 316Ti stainless steel after abrasive polishing and passivation for 30 min. in 20% (vol.) HNO 3 obtained in Ringer s solution -52-

8 Table 2. Results of potentiodynamic measurements of AISI 316Ti stainless steel after abrasive polishing (MP) and passivation in 20% (vol.) HNO 3 for 15 and 30 minutes Sample 316Ti-MP 316Ti-MP-20% HNO 3-15min 316Ti-MP-20% HNO 3-30min E pit E cor E pit E cor E pit E cor Average Stand. dev Median Max Min Range In Table 3, there are corrosion results related to AISI 316Ti stainless steel after electrochemical polishing and after passivation for 15 and 30 minutes in 20% (vol.) HNO 3. Corrosion potential for non-passivated sample was equal to ±31.4 mv SCE (median: mv SCE ), while the pitting corrosion potential was ±128.7 mv SCE (median: 900 mv SCE ). In the case of passivated sample in 20% (vol.) HNO 3 for 15 minutes the corrosion potential was equal to ± 89.1 mv SCE (median: 175 mv SCE ), while pitting corrosion potential was ±142.1 mv SCE (median: mv SCE ). The corrosion potential of sample passivated in 20% (vol.) HNO 3 for 30 minutes was equal to ±25 mv SCE (median: 193 mv SCE ), while pitting corrosion potential was ±58.5 mv SCE (median: 854 mv SCE ). -53-

9 Table 3. Results of potentiodynamic measurements of AISI 316Ti stainless steel after electrochemical polishing (EP) and passivation in 20% (vol.) HNO 3 for 15 and 30 minutes Sample 316Ti-EP 316Ti-EP-20% HNO 3-15min 316Ti-EP-20% HNO 3-30min E pcit E cor E pit E cor E pit E cor Average Stand. dev Median Max Min Range In Figure 7, there are presented potentiodynamic corrosion results of AISI 316Ti stainless steel after electrochemical polishing. The corrosion potential of general corrosion was in the range from 286 mv SCE up to 174 mv SCE (range: 112 mv SCE ), while pitting corrosion potential was changing from 650 mv SCE up to 1072 mv SCE (range: 422 mv SCE ). In Figure 8, there are presented potentiodynamic corrosion results of AISI 316Ti stainless steel after abrasive polishing and passivation for 15 minutes in 20% (vol.) HNO 3. The corrosion potential of general corrosion was equal in the range from 237 mv SCE up to 47 mv SCE (range: 284 mv SCE ), while pitting corrosion potential was in the range from 485 mv SCE up to 915 mv SCE (range: 430 mv SCE ). -54-

10 Fig. 7. Potentiodynamic curves of AISI 316Ti 2205 stainless steel after electrochemical polishing (EP) obtained in Ringer s solution Fig. 8. Potentiodynamic curves of AISI 316Ti stainless steel after electrochemical polishing and passivation for 15 min. in 20% (vol.) HNO 3 obtained in Ringer s solution -55-

11 In Figure 9, there are presented potentiodynamic corrosion results of AISI 316Ti stainless steel after abrasive polishing and passivation for 30 minutes in 20% (vol.) HNO 3. The corrosion potential of general corrosion was changing in range from 220 mv SCE up to 141 mv SCE (range: 79 mv SCE ), while pitting corrosion potential was in the range of from 761 mv SCE up to 979 mv SCE (range: 218 mv SCE ). Fig. 9. Potentiodynamic curves of AISI 316Ti stainless steel after electrochemical polishing and passivation for 30 min. in 20% (vol.) HNO 3 obtained in Ringer s solution 4. CONCLUSION The passivation of AISI 316Ti stainless steel in 20% by volume nitric acid (HNO 3 ) during 15 and 30 minutes without any pre-treatment (as received) as well as after abrasive and electrochemical polishing may change the pitting corrosion resistance of the formed passive layers, what was shown in present paper. The best pitting corrosion protection may be understood as a minimal potential of pitting corrosion, which was found based on ten measurements. This value of the best pitting corrosion protection in Ringer s solution was found for the steel surface after electropolishing and passivation in 20% vol. HNO 3 for 30 minutes, i.e. the pitting potential was equal to 761 mv SCE (855.4 ± 58.5 mv SCE ). The worst pitting corrosion resistance was recorded for mechanically ground samples and the pitting corrosion potential was equal to 270 mv SCE (378.8 ± 60.3 mv SCE ). Summing up, one should notice that the electrochemical treatments, such as electropolishing and passivation, may change the composition of passive layers on AISI 316Ti, what is bound with pitting corrosion resistance. In industry the AISI 316Ti stainless steel should be treated only by passivation or, in case of a scratched surface, it should be electropolished and passivated. -56-

12 Fig. 10. Comparison of corrosion behavior of AISI 316Ti stainless steel after different treatments: AR as received, MP after abrasive polishing, EP after electropolishing -57-

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