CHAPTER 7 STUDIES ON CORROSION RESISTANCE OF STAINLESS STEEL CLADDINGS

Transcription

1 134 CHAPTER 7 STUDIES ON CORROSION RESISTANCE OF STAINLESS STEEL CLADDINGS 7.1 INTRODUCTION Corrosion is the destructive result of chemical or electrochemical reactions between a metal or metal alloy and its surroundings. The nature of this reaction depends not only on the chemistry of the system but also on the structure of the metal. The grain boundaries, which are imperfect and high energy regions, generally weaken the corrosion resistance of materials due to the depletion of corrosion resistance alloying elements on the grain boundaries. There are several test methods for determining the Pitting and Inter Granular Corrosion (IGC) of stainless steel claddings. The weight loss acid test in which the Pitting and IGC rates are determined by measuring the weight loss of the sample as per ASTM G-48-practice-A and ASTM A-262- practice-c respectively for the stainless steel cladding. Another test method of measuring the degree of sensitization to intergranular corrosion involves electrochemical reactivation of the samples as defined in ASTM G-108. This reactivation process is called Electrochemical Potentiokinetic Reactivation (EPR) and has been developed in to two types: Single loop (SLEPR) and Double loop (DLEPR). The SLEPR test is usually accounted to detect the susceptibility of the cladding towards pitting corrosion resistance and the

2 135 DLEPR test is usually accounted to detect the susceptibility of the cladding towards IGC resistance. 7.2 PLAN OF INVESTIGATION The investigations are carried out in the following sequence: 1. Conducting weight loss tests namely Total immersion ferric chloride test and the Boiling nitric acid or Huey s test for detecting the susceptibility of Pitting and Intergranular corrosion attack in stainless steel claddings as per ASTM G-48 / Practice-A and ASTM A-262 / Practice-C respectively, for the as cladded and liquid nitrided specimens. 2. Conducting the Single Loop (SLEPR) and Double Loop (DLEPR) tests to detect the susceptibility of Pitting and IGC attack in stainless steel claddings as per ASTM G-5 and ASTM G-108 respectively, for the as cladded and nitrided specimens. 3. Results and discussions. 7.3 WEIGHT LOSS TESTS TOTAL IMMERSION FERRIC CHLORIDE TEST AS PER ASTM G Preparation of the test specimen and test solution Four test specimens were prepared from overlay plates cladded at low (4.10 KJ/mm), high (6.81 KJ/mm), optimum (4.61 KJ/mm) heat input condition as well as at optimum dilution (4.61 KJ/mm) and liquidnitrided condition for conducting the test. The top surface of the specimens were ground flat to facilitate maximum surface exposure to the corrosive test solution. The test solution was prepared by adding 100 gm of Ferric Chloride

3 136 (Fecl 3 ) to 900 ml of distilled water (6% Fecl 3 by wt) as per ASTM standards. The solution was considered appropriate because, the effect of Fecl 3 was more pronounced and also aggressive in the environments that could formulate pitting corrosion Experimental procedure The total immersion ferric chloride test was conducted as per ASTM standards to detect the susceptibility of pitting corrosion attack in austenitic stainless steel. Samples of size 3.7 x 1.9 x 0.5 cm 3 with a surface area of cm 2 were cut from the specimens weld cladded with different heat inputs and at optimum condition. They were polished with 120 grit abrasive paper, washed and dried by dipping in acetone. After taking the initial weighed of the sample in a calibrated digital balance it was placed in a glass cradle having holes and kept inside an Erlenmeyer flask fitted with a condenser which dissipated the heat developed during the test period. The flask was filled with 100 ml (5ml/cm 2 ) of the test solution to cover the entire specimen surface. Cooling water was passed through the condenser for dissipating the heat generated and the flask is electrically heated and maintained at 40 C thereby keeping the test solution boiling throughout the test period. The test period was of 72 hours duration and after the end of the test period the specimen was rinsed with water and scrubbed with a nylon brush under running water to remove any adhering corrosion products. Then the specimen was dried by dipping in acetone and weighed in a calibrated digital balance. The difference in weight is recorded for estimating the corrosion rate.

4 BOILING NITRIC ACID TEST (HUEY S TEST) AS PER ASTM A-262-C The boiling nitric acid or Huey s test is used for detecting the susceptibility of stainless steel cladding towards intergranular corrosion attack and conducted as per ASTM A-262-Practice-C. It is conducted for detecting the susceptibility of intergranular corrosion attack in stainless steel claddings. This procedure can also be used to check the effectiveness of the stabilising elements and the effect of carbon content in reducing the susceptibility to intergranular corrosion attack in Cr-Ni stainless steel Preparation of the test specimen and test solution The entire lateral surfaces of the four prepared test specimens were finely grinded to facilitate better surface exposure to the corrosive test solution. A 65 % by weight nitric acid solution was prepared by adding distilled water to concentrated nitric acid (HNO 3) of reagent grade with specific gravity 1.42 at the rate of 108 ml of distilled water per litre of concentrated nitric acid as per ASTM standards. The solution was considered appropriate because of the effect of HNO 3 being more pronounced and aggressive in the environments that could formulate intergranular corrosion Experimental procedure Intergranular attack in nitric acid is associated with the intergranular precipitation of chromium carbides. The specimen was polished with 120 grit abrasive paper and weighed initially. It was placed in a glass cradle is presented in Figure 7.1 (a) and kept inside the Erlenmeyer flask fitted with an Allihn condenser with four bulbs as presented in Figure 7.1 (b) to dissipate the heat developed during boiling of the acid.

5 138 (a) (b) Figure 7.1 Huey s test setup showing: (a) the glass cradle and (b) Erlenmeyer flask fitted with an Allihn condenser The flask was filled with sufficient quantity of the test solution to cover the specimen and to provide a volume of 20 ml/cm 2 of the specimen surface. Cooling water was passed through the condenser for dissipating the heat generated and the flask is electrically heated and maintained at 60 C thereby keeping the test solution boiling throughout the test period. The test period was of 48 hours duration and after the end of each test period the specimen was rinsed with water and scrubbed with a nylon brush under running water to remove any adhering corrosion products. Then the specimen was dried by dipping in acetone and weighed in an analytical balance. The difference in weight is recorded for estimating the corrosion rate. This test procedure was repeated for five consecutive boiling periods with duration of 48 hours for each period for every specimen. Fresh test solution was used every time during the entire testing period.

6 SINGLE LOOP EPR TEST AS PER ASTM G Preparation of the test specimen and test solution Four already prepared cladded test specimens were used for conducting EPR tests. Figure 7.2 shows the surface of these specimens (I) before and (II) after the EPR test. (I) (II) Figure 7.2 Specimen with: (a) low heat input, 4.10 KJ/mm (b) high heat input, 6.81 KJ/mm (c) optimum heat input, 4.61 KJ/mm and (d) optimum (4.61 KJ/mm) and liquid nitrided condition, (I) before and (II) after Single loop EPR test The top surface of the specimen was ground flat to facilitate 1 cm 2 of the surface was exposed to the corrosive test solution. It was first polished by a 600 grit SiC paper and further wet polished with 1 µm alumina slurry on a micro cloth mounted polishing wheel to a surface roughness of 6 m as per ASTM E13 standards. Then they were washed with distilled water and dried in a stream of cool air before immersing them in to the corrosive test solution. The corrosion medium for the present investigation has been selected based on the basis of corrosion environments as cited in the available literatures.

7 140 A 3.56 % sodium chloride (NaCl) solution was prepared by dissolving 34 g of sodium chloride in 920 ml of deionised water Experimental procedure The potentiodynamic polarisation test was conducted to predict the pitting corrosion resistance of the specimens cladded at various heat input conditions as per the ASTM G-5 standard. The schematic and experimental set up of ACM Gill 5500 potentiostat instrument with a flat cell in three electrode configuration is shown Figure 7.3 (a) and (b). (a) (b) Figure 7.3 EPR test set up showing: (a) Schematic diagram and (b) Experimental set up

8 141 The cell consists of a glass cylinder clamped horizontally between two end plates housing the working electrode, WE (AISI 316 L stainless steel cladded surface) and the auxiliary electrode, AE, (platinum gauze). A saturated calomel electrode (SCE) using 0.1 M KCl was used as the reference electrode (RE). The cell was filled with 250 ml of test solution and all the tests were conducted at room temperature, 30 ± 2 C. All the three electrodes are connected to corrosion measuring instrument through the leads provided in the flat cell. Polarisation test was commenced by measuring the rest potential after the samples were immersed for 50 minutes in non deaerated chloride solution to allow for rest potential to settle. The potential was anodically scanned at a rate of 60 mvmin -1 from mv to mv. The current density was measured continuously using the data acquisition software provided with the instrument. 7.5 DOUBLE LOOP EPR TEST AS PER ASTM G-108 Four already prepared test specimens cladded were used to conduct the test. The double loop EPR test was done according to ASTM G-108 standard and the recommendations made by Majidi and Streicher (1984). The standard solution was modified to suit the austenitic stainless steel and consisted of 2M H 2 SO MNaCl MKSCN at 30 ± 1 C and a scan rate of 15 V/h. The test was performed by running the sample from a potential lower than Ecorr in the cathodic region. The potential is scanned in the anodic direction from Ecorr to a point of V in the middle of the passive region. The scanning direction is then reversed and the potential is reduced back to the cathodic region. Two loops are generated, an anodic loop and a reactivation loop. The peak activation current (Ia) and the peak reactivation current (Ir) were measured during the forward and backward scans, respectively. The degree of sensitization was measured as the ratio of peak activation current to the maximum current densities generated in the double loop test (Majidi and Streicher, 1984).

9 RESULTS AND DISCUSSION Weight loss tests Total immersion ferric acid test The corrosion rate was measured by determining the weight loss of the specimen after the test period and the weight loss was calculated for each specimen. The corrosion rate was calculated by using the relation, Corrosion rate = 7290 x W / A t, mm/month, where W= the total weight loss of the specimen in grams, A= the area of the specimen exposed in cm 2, = the density of the overlay material in grams / cm 3 and t= the time of exposure in hours. The corrosion rate was calculated for each test period and the average corrosion rate was referred against the ASTM acceptance limits for all the four specimens cladded at different heat input conditions. The results of the Total immersion ferric chloride test are presented in Table 7.1.

10 143 Table 7.1 Results of total immersion ferric chloride test No Thermal history of the specimen 1 Cladded at low heat input (4.10 KJ / mm) 2 Cladded at high heat input (6.81 KJ / mm) 3 Cladded at optimum heat input (4.61 KJ / mm) 4 Cladded at optimum heat input (4.61 KJ / mm) and liquid nitrided condition Initial weight of the specimen, gm Final weight of the specimen, gm Difference in weight, gm Corrosion rate, mm/month ASTM acceptance limit, mm / month From Table 7.1 it is found that cladding produced at low heat input and optimum conditions are having lower corrosion rates than that of other claddings in ferric chloride solution Boiling nitric acid test (Huey s test) The results of the Huey s test are presented in Table 7.2.

11 144 Table 7.2 Results of Huey s test No Description of the specimen Cladded at low heat input (4.10 KJ / mm) Cladded at high heat input (6.81 KJ / mm) Cladded at optimum heat input (4.61 KJ / mm) Cladded at optimum heat input (4.61 KJ / mm) and liquid nitrided condition Total weight loss, gms Total corrosion rate, mm / month Average corrosion rate, mm / month ASTM acceptance limit, mm / month It is observed that the corrosion rate in boiling nitric acid of nitrided claddings produced at optimum dilution condition is lower compared with all other claddings. Also it is evident from tables that corrosion rate increases with the increase in heat input which may be attributed to increased dilution. The scanning electron micrograph (SEM) of the nitrided cladding produced at optimum heat input condition (4.61 KJ/mm) and at high heat input (6.81 KJ/mm) condition after Huey s test are shown in Figure 7.4. A stepped type microstructure is noticed for the nitrided specimen cladded at optimum heat input condition. This is because of the reason that the lower heat input promoted faster cooling rates thereby forming finer grains with stepped structures. Finer grains with stepped structures possess excellent corrosion resistance and tensile properties combined with good bonding strength between adjacent grains (Aydogdu and Aydinol 2006, Mirko Gojic et al 2008). This in turn promotes excellent ductility and toughness of the cladding which will widen their potential applications.

12 145 Figure 7.4 SEM photomicrograph of nitrided claddings produced at optimum heat input (4.61 KJ/mm) condition after Huey s test showing a stepped structure, X500 The stepped type microstructure is presented at a higher magnification for clearly visualising the corrosion debris after the Huey s test, in Figure 7.5. Figure 7.5 SEM photomicrograph of nitrided cladding produced at optimum heat input (4.61 KJ/mm) condition after Huey s test showing a stepped structure, X2000

13 146 A ditched type microstructure noticed in the specimen cladded at high heat input (6.81 KJ/mm) condition after Huey s test is shown in Figure 7.6. Figure 7.6 SEM photomicrograph of high heat input specimen (6.81 KJ/mm) after Huey s test showing a ditched structure, X500 The slower cooling rates of the cladding due to higher heat input promoted a coarser grain structure which does not have the normal mechanical and metallurgical properties. Also, the slower cooling rates promoted the formation of coarser grains with ditched structure. Their bonding strength may not be evenly distributed due to the formation of coarser grains with a lathy morphology (Arikan and Doruk 2008). The SEM images reveal that the ditched type microstructure are highly prone to the intergranular corrosion attack or sensitization than the stepped type structure. The ditched type microstructure is presented at a higher magnification for clearly visualising the corrosion debris after the Huey s teat, in Figure 7.7.

14 147 Figure 7.7 SEM photomicrograph of high heat input specimen (6.81 KJ/mm) after Huey s test showing a ditched structure, X Single Loop EPR test as per ASTM G-5 Graphs were plotted keeping current density in logarithmic scale along X-axis and potential along Y-axis using the analysis software. Typical potentiodynamic anodic polarization curves of the specimens cladded at corresponding conditions are shown in Figure For each specimen, the test was repeated twice in different areas and the average value was recorded for the analysis. The current density was measured continuously using commercial data acquisition software provided with the instrument. Corrosion behaviour was investigated using potentiodynamic polarisation measurements in 3.5 wt. % NaCl.

15 148 Figure 7.8 Single Loop EPR curve for a specimen cladded at low heat input (4.10 KJ/mm) condition Figure 7.9 Single Loop EPR curve for a specimen cladded at high heat input (6.81 KJ/mm) condition

16 149 Figure 7.10 Single Loop EPR curve for a specimen cladded at optimum heat input (4.61 KJ/mm) condition Figure 7.11 Single Loop EPR curve for a specimen nitrided and cladded at optimum heat input (4.61 KJ/mm) condition

17 150 The rest potential (the potential at which the current becomes zero) and the pitting potential are considered as a measure of the material dissolution from the surface being tested. It has been referred that the passive film on the surface was destroyed progressively with time and as a result more and more of the metal (which is active) is exposed in the electrolyte. The results are presented in Table 7.3. All potentials are vs. saturated calomel electrode. Table 7.3 Results of Single Loop EPR test Sample description Low heat input of 4.10 KJ/mm High heat input of 6.81 KJ/mm Optimum heat input 4.61 KJ/mm Optimum heat input 4.61 KJ/mm and nitrided Rest Potential, mv Pitting Potential, mv Corrosion current density, (Icorr) A cm -2 Corrosion rate, mm/year Corrosion rate, mils/yr X X X X Prasad Rao et al (1986-a) investigated the pitting potential for AISI 316L stainless steel claddings in 3.5% NaCl aqueous solution at a controlled temperature of 30 ± 2 C were between -120 to +430 mv. Pulino-Sagradi et al (1997) observed the same between -130 to +420 mv.

18 151 Kamachimudali et al (2000) investigated the pitting potential between140 to +450 mv. The results show that the pitting potential of the AISI 316L cladded specimens in 3.5% NaCl aqueous solution at a controlled temperature of 30 ± 2 C lie between -120 to +410 mv for various heat inputs. The above observations indicate that the values of pitting potentials obtained in the present study are in agreement with the literature. The optical and SEM micrographs of the nitrided cladding produced at optimum heat input condition showing stepped structure with pits are presented in Figure 7.12 and 7.13 respectively. Figure 7.12 Optical micrograph of the nitrided cladding produced at optimum heat input condition after Single Loop EPR test

19 152 Figure 7.13 SEM micrograph of the nitrided cladding produced at optimum heat input condition after Single Loop EPR test The optical and SEM micrographs of the nitrided cladding produced at high heat input condition showing ditched structure are presented in Figure 7.14 and 7.15 respectively. Figure 7.14 Optical micrograph of the cladding produced at high heat input condition after Single Loop EPR test

20 153 Figure 7.15 SEM micrograph of the cladding produced at high heat input condition after Single Loop EPR test A step type microstructure with pits was noticed for the specimen cladded with optimum heat input (4.61 KJ/mm) and nitrided condition and a ditched type microstructure with pits was noticed for the specimen cladded with high heat input (6.81 KJ/mm). Stepped type microstructure might have formed due to the faster cooling of the cladding at low heat input conditions. The ditched type microstructure might have formed due to the slow cooling of the cladding produced during higher heat input condition. It is found that the stepped type microstructure possesses comparatively better resistance to pitting corrosion than the ditched type microstructure Double Loop EPR test as per ASTMG -108 Polarisation graphs were plotted keeping current density in logarithmic scale along X-axis and potential along Y-axis using the analysis software. Typical potentiodynamic anodic polarization curves of the claddings produced at the specified heat input conditions are presented in Figure

21 154 Figure 7.16 Double Loop EPR curve for a specimen cladded at low heat input (4.10 KJ/mm) condition Figure 7.17 Double Loop EPR curve for a specimen cladded at high heat input (6.81 KJ/mm) condition Figure 7.18 Double Loop EPR curve for a specimen cladded at optimum heat input (4.61 KJ/mm) condition

22 155 Figure 7.19 Double Loop EPR curve for a specimen nitrided and cladded at optimum heat input (4.61 KJ/mm) condition For each specimen, the test was repeated twice in different areas and the average value was recorded for the analysis. The current density was measured continuously using commercial data acquisition software provided with the instrument. All potentials are vs. saturated calomel electrode. The degree of sensitization was measured from the ratio of maximum current densities generated in the double loop test (Majidi and Streicher 1984). The results of the double loop EPR test is presented in Table 7.4.

23 156 Table 7.4 Results of Double Loop EPR test No Description Low heat input of 4.10 KJ/mm High heat input of 6.81 KJ/mm Optimum heat input 4.61 KJ/mm Optimum heat input 4.61 KJ/mm and nitrided Activation peak potential, E a,(mv) Activation peak current density, I a, (ma/cm 2 ) Reactivation peak potential, E r,(mv) Reactivation peak current density, I r,(ma/cm 2 ) Passivation current density, I pass,(ma/cm 2 ) Degree of sensitization ( Ir /Ia x100) % SUMMARY From the weight loss test with ferric chloride it can be concluded that the nitrided cladding deposited at optimum heat input condition possessed better pitting corrosion resistance. In the Huey s test, the claddings deposited at low and optimum heat input conditions possessed better resistance to IGC. Also from the single loop EPR test an increase in pitting potential is noticed in the cladding deposited with optimum heat input condition. The positive value of pitting potential indicates that a stable film is formed over the surface of the cladding which confirms that the material is more nobler with increased pitting corrosion resistance. In the double loop EPR test the ratio of the degree of sensitisation (Ir/Ia) was found to be very lower in the cladding deposited at

24 157 optimum heat input condition, which reveal that the cladding possesses better resistance to IGC.