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Corrosion Compatibility Studies on Carbon Steel and Type 304 Stainless Steel in Acid Permanganate and Organic Acid Mixtures

August 2, 2007

By Rajesh, Puspalata Kumar, Padma S; Suresh, Sumathi; Chandran, Sinu; Et al

Type 304 stainless steel and carbon steel are extensively used as structural materials in the reactor coolant systems of nuclear power plants. The corrosion of these structural materials during normal operation and the subsequent neutron activation of their corrosion products leads to the build-up of radiation on the out-of-core surfaces of the coolant systems. The Dilute Chemical Decontamination (DCD) process Is one of the methods used to remove the activity that becomes embedded in the oxide layers formed over these structural materials. Permanganate based oxidising reagents are generally used to dissolve chromium from oxide films containing iron, chromium and nickel on the surfaces of Type 304 stainless steel and other chromium-containing alloys. This study has investigated the behaviour of carbon steel In oxidising permanganate based acidic media. The corrosion/decomposition rate varied with temperature. With permanganate concentrations above 1.5 mM for permanganic acid and greater than 6 mM for nitric acid and potassium permanganate (NP), passivation of carbon steel surfaces occurred with negligible loss of weight of the carbon steel and insignificant decomposition of the permanganate. The effects of additions of Chromate to permanganate on the passivation of carbon steel have also been evaluated. In the case of permanganic acid, the presence of even 50 ppm of Chromate was sufficient to protect the carbon steel surface whereas a minimum of 150 ppm of Chromate was required for passivation In the NP medium. Electrochemical polarisation experiments were also carried out to support these observations. When the concentration of permanganic acid was above 200 ppm, the open circuit potential of carbon steel in permanganic acid was + 1.0 V (SCE) and it was observed to be passive whereas at lower concentrations the carbon steel was actively corroding at a potential of -0.695 V (SCE). Keywords: pre-oxidation, carbon steel, corrosion, passivation, sensitisation, intergranular attack

Introduction

Stainless steel and carbon steel are important structural materials in the coolant systems of nuclear reactors. Stainless steel is extensively used in Pressurised Water Reactors (PWRs), Boiling Water Reactors (BWRs) and in some components of Pressurised Heavy Water Reactors (PHWRs). In the latter type of reactor, carbon steel is the main structural material and a few minor components made of carbon steel and other low alloy steels are present in BWRs and PWRs. The corrosion film formed on carbon steel internal surfaces is mainly iron oxide whereas on stainless steel it is a mixed oxide of iron, chromium and nickel. The composition of the oxide film is mainly determined by the prevailing water chemistry in the system. Oxidising chemistry conditions favour the release of chromium from the oxide film on stainless steel, and hence, it contains less chromium. Such conditions are also favourable for the formation of ferric oxides. Thus, in BWRs operating under a normal water chemistry regime and exhibiting an oxidising chemistry condition, approximately 200 ppb of dissolved oxygen is present due to the radiolysis of water, and the oxides grown on stainless steel contain less chromium while the deposited oxides are mainly alpha- Fe^sub 2^O^sub 3^. In contrast, in PWRs, which operate under reducing chemistry conditions, hydrogen dosing reduces the dissolved oxygen content to

The main risk associated with decontamination is corrosion of the base metal by the decontaminating chemical reagents. General corrosion and localised corrosion such as pitting, Intergranular corrosion, (IGC) and intergranular stress corrosion cracking (IGSCC), are various forms of corrosion that can arise out of the interaction of the chemical decontaminant with the structural materials of the reactor. Thus, it is essential that the chemical reagents to be used as decontaminants are tested for compatibility with the structural materials of the reactor coolant system. The compatibility of stainless steel with acid permanganates (NP and HMnO^sub 4^) has been investigated6-8 and it has been shown that the acidic permanganate reagents are compatible with stainless steel surfaces. The decomposition of both permanganic acid and NP in the presence of stainless steel under boiling conditions have already been studied and shown to follow the same slope. The dissolution of the oxide film present on the carbon steel surfaces does not require an oxidising treatment. However, in a system containing both stainless steel and carbon steel, exposure of the carbon steel to NP or HMnO^sub 4^ cannot be avoided. Hence, a detailed study has been undertaken on the corrosion compatibility of carbon steel with permanganic acid and NP.

Carbon steel is prone to corrosion under acidic conditions. The presence of H+ in acid permanganate tends to corrode the steel while the permanganate ion, which is a powerful oxidising agent, tends to passivate it. Thus, the two mutually opposing processes compete with each other, making the corrosion behaviour of carbon steel heavily dependent on the concentrations of both the H+ and permanganate ions. The dominant reaction on the surface (whether corrosion or passivation) is reflected in the electrochemical potential of the carbon steel. Thus, measurement of the open circuit potential (OCP) is helpful in establishing the conditions required for maintaining passivity.

Chromate ion (CrO^sub 4^^sup 2-^) is another well known passivator for carbon steel. The effect of the combined presence of Chromate and permanganate has been the subject of detailed investigation in the present study with a view to exploiting it for the protection of carbon steel. As Chromate is one of the products generated in situ by the reaction between the oxide on stainless steel and permanganate, it is necessary to establish the minimum concentration of oxidant required to protect carbon steel before applying it to systems containing both stainless steel and carbon steel. The passivation and protection of carbon steel by permanganate, and mixtures of permanganate and Chromate, is dictated by the film formed by these reagents on the carbon steel surface. Use has been made of impedance spectroscopy to understand the nature of the film formed over the carbon steel surface and its properties.

Table 1 Heat Treatments given to the Carbon Steel and Stainless Steel before Testing

During decontamination, subsequent to treatment with acid permanganate, the steel surfaces are treated with a mixture of organic acids to remove the chromium-depleted iron and nickel oxide layer. Since the presence of a reducing agent aids the dissolution of iron (III) oxides, an organic reducing agent such as oxalic acid or ascorbic acid is added as a constituent of the organic acids mixture. Other constituents include complexing agents such as ethylene diamine tetra-acetic acid (EDTA), nitrilo triacetic acid (NTA), citric acid (CA), etc. In this study, an organic acid mixture containing NTA-ascorbic acid (AA) has been evaluated for compatibility with stainless steel and carbon steel. The corrosion rate of stainless steel is very low in these mild acid conditions. However, the possibility of any localised corrosion must be ruled out before such mixtures are used for decontamination. Carbon steel undergoes general corrosion in such media. However, corrosion of carbon steel can be controlled by the use of corrosion inhibitors.

Experimental

A stainless steel coupon was characterised by energy dispersive X- ray fluorescence (ED-XRF, using an X-ray energy of 30 keV, a current of 4 mA, an aluminium filter and a silicon pin diode detector). The carbon content of the steel was determined using direct reading optical emission spectrometry so that the temperature region for sensitisation could be decided. The analyses showed the sample to have the composition (wt-%): 74.3 Fe-0.04 Co-17.0 Cr-8.0 Ni.

Specimen Preparation

The heat treatments used to normalise the carbon steel coupons, which was to ASTM 106 Gr. B (CS), and to sensitise the AISI Type 304 stainless steel coupons, are given in Table 1 .

The coupons were polished using successively finer grades of waterproof abrasive paper to an 800 grit finish, followed by final polishing with 1.2 [mu]m diamond paste. They were then cleaned and degreased with analytical reagent (A. R.) grade acetone. The carbon steel coupons were weighed to an accuracy of 0.01 mg, both before and after exposure. Preparation of permanganic acid and the permanganate-nitric acid mixture

Permanganic acid was prepared by passing a solution of potassium permanganate through a strong acid cation exchange resin (Tulsion T- 33). The flow rate was optimised to avoid decomposition of the resin and to achieve complete conversion to permanganic acid. The nitric acid-permanganate mixture was prepared by dissolving potassium permanganate in nitric acid solution of appropriate concentration.

Corrosion experiments

Corrosion experiments on the carbon steel coupons were carried out by immersion in a three-necked flat bottomed flask of 1 litre capacity with provision for attaching a water-cooled condenser to minimise the loss of water vapour and an aperture through which the test coupons could be inserted. This contained 500 mL of permanganic acid at concentrations in the range 50-1000 ppm. The temperature was maintained at 90[degrees]C unless otherwise specified. Samples of the corrodent were removed at pre-determined time intervals and, after filtering through 0.22 [mu]m membrane filter paper, were analysed using a spectrophotometer to determine the decomposition rate of the permanganic acid. The extent of decomposition of the permanganic acid corresponded to the amount of iron released from the metal. Thus, the corrosion rate of the mild steel could be determined by monitoring the permanganic acid concentration as a function of time, and using a standard curve fitting procedure. At the end of each experiment, the weight loss of the carbon steel was recorded following removal of the manganese dioxide film deposited over it by cleaning with citric acid.

A similar procedure was followed for tests in the nitric acid- permanganate (NP) solutions. The concentrations of nitric acid and potassium permanganate were varied in the range of 1 mM – 15mM HNO^sub 3^ and 1 mM – 12mM KMnO^sub 4^, respectively. At the end of all of the corrosion experiments, the change in weight of the carbon steel was recorded.

Polarisation and electrochemical impedance spectroscopy studies

Polarisation studies and electrochemical impedance spectra (at the open-circuit potential) were measured using an Autolab instrument. A platinum foil served as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode.

Inhibitor studies

Rodine-92 B, a commercial corrosion inhibitor, was evaluated for its efficiency in inhibiting the corrosion of carbon steel. Experiments were carried out in a mixture containing 5 mM NTA and 2.5 mM Ascorbic acid together with the Rodine-92 B at concentrations of 0-200 ppm. The change in weight of the carbon steel following exposure to the mixture was measured to calculate the efficiency of the inhibitor.

Characterisation of sensitised stainless steel

Sensitisation of the Type 304 stainless steel coupons was confirmed following standard ASTM procedures which included the optical examination of coupons that had been electrolytically etched in 10 % oxalic acid, and subjected to Strauss test and electrochemical potential reactivation (EPR) tests. The area beneath the polarisation curve in the EPR test indicates the amount of charge associated with corrosion of the chromium-depleted regions surrounding the chromium carbide decorated grain boundaries i.e. the degree of sensitisation.

1 Corrosion rate of carbon steel with different concentrations of permanganic acid, with and without Chromate

Test for intergranular attack (IGA)

After confirming the presence of sensitisation, the sensitised coupons were exposed to I mol dm-3 oxalic acid (OA), permanganic acid (at concentrations of 100 ppm and 300 ppm), nitric acid- permanganate (NP), and NAC (a mixture of nitrilo triacetic acid (6.85 mM), ascorbic acid (8.5 mM) and citric acid (7 mM)) at 104[degrees]C separately to see whether any intergranular attack occurred. Some of the specimens were examined by scanning electron microscope after exposure to these chemical reagents to aid the detection of any grain boundary attack.

Results and discussion

Corrosion of carbon steel in permanganic acid

Carbon steel corrodes under mildly acidic conditions and in permanganic acid it was found to undergo corrosion that led to general wall thinning. Weight loss measurements in permanganic acid at 90[degrees]C indicated that the rate of corrosion depended on the concentration of the acid. The average corrosion rate of the carbon steel increased with increasing initial concentration of permanganic acid until the concentration reached approximately 150 ppm. However, at concentrations above 150 ppm the permanganic acid caused passivation of the carbon steel by forming a protective passive film and the corrosion rate dropped to below the detection limit of the weight loss technique (see Figure 1). This is a commonly observed characteristic of passivating solutions.

The reaction taking place on the carbon steel was different from the reactions that usually take place during corrosion in acid media. In permanganic acid media the corrosion was also associated with the simultaneous decomposition of permanganic acid via the following reaction:

HMnO^sub 4^ + Fe + H^sub 2^O[arrow right]Fe(OH)^sub 3^ + MnO2 (1)

Thus, one mole of iron was released from the carbon steel for every mole of decomposed permanganate. In this process, the permanganate is reduced to manganese dioxide. The iron released to the solution would be converted to its hydrous oxide. Hence, by measuring the concentration of permanganic acid as a function of time, the rate of decomposition of permanganic acid and hence the corrosion rate of carbon steel could be monitored. It was assumed that the contribution of iron release from normal acid corrosion by H+ is insignificant.

2 Concentration of permanganic acid versus time in the presence of carbon steel coupons at 90 [degrees]C

4 Potentiodynamic Anodic Polarisation curves for carbon steel in (a) 100 ppm permanganic acid, (b) 100 ppm permanganic acid with 50 ppm Chromate, (c) 1000 ppm permanganic acid and (d) Pt electrode in 1000 ppm permanganic acid at 90 [degrees]C

At permanganic acid concentrations below 200 ppm, the decrease in permanganic acid concentration with time (and hence the extent of corrosion), was directly dependent on the permanganic acid concentration. Figure 2 shows the decrease in the concentration of permanganic acid over times of up to 4 h at 90[degrees]C during the exposure of carbon steel. The variations in permanganic acid concentration with time shown in Figure 2 were converted into iron equivalents and differentiation of these data with respect to time yielded the corresponding corrosion rates.

The corrosion rate of carbon steel is shown as a function of permanganic acid concentration for initial concentrations of 50, 100 and 150 ppm in Figure 3. The linear variation in the corrosion rate with the permanganic acid concentration and the similarity of the slopes for data points collected at three different initial concentrations confirms the direct dependence of the corrosion reaction on permanganic acid.

It is known that permanganate undergoes self-decomposition, the rate of which is strongly dependent on temperature:

3 Variation in the corrosion rate of carbon steel as a function of concentration of permanganic acid (for three different initial concentrations) at 90 [degrees]C

However, even at 90[degrees]C, the rate of this reaction is very slow (100 ppm/24 h) as compared to the rate of the corrosion reaction and hence the error introduced in the corrosion rate calculation due to this reaction can be ignored.

Electrochemical studies

Electrochemical polarisation studies confirmed the observations made using the conventional methods, as discussed above. With higher concentrations of permanganic acid (above 300 ppm) the open circuit potential (OCP) of carbon steel was in the positive region viz. + 1.11 V versus SCE, whereas at lower concentrations the potential was -0-695 V versus SCE (see Figure 4). In the latter Figure, the potentiodynamic anodic polarisation curve for the Pt electrode immersed in the 1000 ppm solution of permanganic acid is shown for reference purposes. The open circuit potential of platinum, being an inert material, reflects the redox conditions of the solution. At concentrations less than 200 ppm, the permanganic acid concentration is insufficient to passivate the carbon steel and the potential of – 0-695 V corresponds to a mixed potential due to several possible redox reactions in the medium (e.g. the reduction of permanganate and hydrogen evolution) and is closer to the redox potential for the simple corrosion reaction of carbon steel in acid:

2 HMnO^sub 4^ + Fe[arrow right]2Mn07 + Fe^sup 2^+ + H^sub 2^ (3)

With increasing concentration of permanganic acid (an oxidiser), permanganate reduction becomes the major cathodic reaction. The actual reversible potential for MnO^sub 4^^sup -^ / MnO^sub 2^ reduction is raised in accordance with the Nernst equation. This would have been expected to result in an increased corrosion rate. However, the observation of negligible corrosion of carbon steel at higher concentration of permanganic acid can be explained by the fact that permanganic acid is an anodic polariser which polarises the anodic reaction of metal corrosion by modifying the electrode surface. This results in an increase in the anodic Tafel slope such that the intersection with the cathodic curve occurs in the passive range, resulting in a higher mixed corrosion potential, and self- passivation. At such a noble potential, oxygen evolution is the common anodic reaction and hence no weight loss was observed.

The rate of reaction is affected to some extent by the hydrodynamic conditions, especially when the solution condition approaches those required for passivation of the metal. Less corrosion was observed for carbon steel under stirred conditions (in 150 ppm permanganic acid at 90[degrees]C, the corrosion rate of carbon steel was 0-011 pm h^sup -1^ under stirred conditions whereas it was 0-234 pm h^sup -1^ when there was no stirring). This is usually observed in metals corroding under a diffusioncontrolled cathodic reaction and in passivating media.14 The limiting diffusion current density is increased by agitation of the solution and it exceeds the critical current density where the metal becomes passive and corrodes at very slow rate corresponding to the passivation current. Thus, it is very clear that permanganic acid will start corroding carbon steel once its concentration falls below 200 ppm at 90[degrees]C. Normally, for the chemical decontamination of stainless steel surfaces, permanganic acid is used at concentrations of 300-400 ppm.13 As permanganic acid is used up in the dissolution of chromium oxides, its concentration can soon reduce to less than 200 ppm, thereby causing corrosion of any adjacent carbon steel parts of the system. Also, the permanganic acid would be consumed in the corrosion reaction instead of its intended purpose of oxidising the chromium oxide layer. Hence, it is essential to modify the condition of the permanganic acid solution in such a way as to prevent the corrosion of carbon steel. This could be achieved by adding an inhibitor to permanganic acid. Oxidising anions including Chromate, tungstate, molybdate and nitrite14-19 are considered to be good inhibitors for iron alloys. They shift the electrochemical potential (ECP) by several tenths of a volt in the noble direction and are readily reduced giving a shallow cathodic polarisation curve. Chromates belong to the most efficient oxidising type of corrosion inhibitor, being effective in both aerated and deaerated neutral, alkaline and acid solutions. Hence, corrosion experiments on carbon steel were carried out with permanganic acid containing various concentrations of Chromate. This inhibits the anodic reaction as follows:

2Fe + 2Na,2CrO^sub 4^ + 2H^sub 2^O [arrow right] y-Fe^sub 2^O^sub 3^ +Cr^sub 2^O^sub 3^ +4NaOH (4)

In acid solutions, the reaction can be written as:

Fe^sub surface^ + CrO^sup 2-^^sub 4^ +8H^sup +^[arrow right] 2Cr^sup 3+^ +4H^sub 2^O + O^sub 2^ .O^sub adsorbed^ ^sub on^ ^sub Fe^ (5)

Thus an adsorbed layer of oxygen is formed which is chemisorbed, rendering the entire active dissolution site inoperative. The experiments confirmed that Chromate is effective in preventing the corrosion of carbon steel by permanganic acid. Also, it was found that even 50 ppm of Chromate is sufficient to protect a carbon steel surface from corrosion by permanganic acid. Figure 1 clearly reveals that by adding 50 ppm Chromate, the corrosion of carbon steel in permanganic acid solution can be totally prevented.

This was also confirmed by the polarisation studies. Figure 4 shows that the open circuit potential of carbon steel was shifted from -0-695 V (SCE) to +0-891 V (SCE) by the addition of 50 ppm Chromate to 100 ppm permanganic acid at 90[degrees]C. Thus it can be concluded that the addition of Chromate to low concentrations of permanganic acid modifies the surface film by repairing the gaps that are not covered by permanganate, thereby giving carbon steel complete protection from corrosion.

5 Impedance spectra of carbon steel in (a) 100 ppm per-manganic acid, (b) 100 ppm permanganic acid with 50 ppm Chromate and (c) 1000 ppm permanganic acid at 90[degrees]C

Similar behaviour was also observed in the electrochemical impedance studies. Two semi-circles were obtained in the Nyquist plot (Figure 5), both for 1000 ppm permanganic acid and for 100 ppm permanganic acid containing 50 ppm Chromate (lines c and b, respectively). Due to corrosion of the carbon steel in 100 ppm permanganic acid, together with heavy precipitation of MnO^sub 2^, the system reflected a dynamic condition with constant perturbation (line a). Hence the impedance spectrum in the lower frequency region was noisy and only one semicircle was obtained in the higher frequency region. However, with 1000 ppm permanganic acid, the impedance spectrum in the lower frequency region was smooth, as there was not much change in the solution. The film formed can be attributed to a double-layer model.20 The curves were fitted to equivalent circuits and values of the fitted parameters are given in Table 2. The film consists of a comparatively thin outer layer, L^sub 1^, the capacitance of which is measured in nanofarads, and a thicker inner layer L^sub 2^, the capacitance of which is measured in microfarads, as shown in Figure 6. With increasing permanganic acid concentration a decrease in the resistance values (which can be attributed to the pore resistance in the L^sub 1^ layer i.e. R^sub L1^) was observed. This is because the solution becomes more oxidising at higher concentrations. The outer film L^sub 1^ is thicker in 100 ppm permanganic acid compared to that in the chromate- containing solution for the same system. Due to the less oxidising condition prevailing in the former solution, electrons are more readily available for permanganate reduction, leading to the formation of a thicker manganese dioxide film. This is reflected by the lower capacitance and higher pore resistance for the 100 ppm permanganic acid solution. The resistance R^sub L2^ for the inner layer obtained in the lower frequency region in the case of the 100 ppm permanganic acid plus Chromate and 1000 ppm permanganic acid solutions can be attributed to the charge transfer reaction between the oxide film and the solution interface. R^sub L2^ is higher in the case of the chromate-containing solution than with the higher concentration of permanganic acid, which implies that the film formed on the carbon steel surface in a system containing 100 ppm of permanganic acid and 50 ppm of Chromate is more adherent, uniform and protective than the film formed with higher concentrations of permanganic acid.

6 Schematic diagram of the double layer model of the film formed on carbon steel on exposure to permanganic acid

7 Variation in the permanganate concentration versus time in NP with increasing HNO^sub 3^ concentration

8 Plot of initial corrosion rates of carbon steel in NP with increasing HNO^sub 3^ concentration

Corrosion of carbon steel in NP

The corrosion of carbon steel in nitric acid and permanganate (NP), with varying concentrations of both HNO^sub 3^ and potassium permanganate (ImM – 15mM HNO^sub 3^ and ImM – 12mM KMnO^sub 4^) in 1:1 and 1 :3 ratios was investigated by weight loss measurements. The decrease in permanganate concentration as a function of time was also measured. The weight losses obtained at the end of the experiments were not in stoichiometric equivalence with the extent of permanganate decomposition. This is attributed to the simultaneous corrosion of carbon steel by nitric acid and potassium permanganate. The decomposition rates of potassium permanganate with increasing concentrations of HNO^sub 3^ can be implied from Figure 7. The decomposition rate increased with increasing nitric acid concentration. No passivation of carbon steel coupons was observed even at 15 mM HNO^sub 3^. A plot of initial corrosion rate versus nitric acid concentration is given in Figure 8.

The decomposition rates of potassium permanganate with increasing concentrations of potassium permanganate in NP are given in Figure 9. The decomposition rate remained quite low as the potassium permanganate concentration was increased. With 9 mM or more potassium permanganate, passivation of the carbon steel coupons was observed. A plot of initial corrosion rate versus potassium permanganate concentration is given in Figure 10.

Table 2 Equivalent circuit parameters and sum of least squares Values (chi2), obtained by non-linear fitting of the Impedance spectra shown in Fig. 5

To investigate the inhibition of carbon steel in 1:1 NP (1 mM KMnO^sub 4^ + 1 mM HNO^sub 3^) solution, experiments were carried out with different concentrations of Chromate. About 1 50 ppm of Chromate was required for complete inhibition (Figure 1 1).

Though activities of the H^sup +^ ion in NP (ImM KMnO^sub 4^ + ImM HNO^sub 3^) and permanganic acid (1 mM HMnO^sub 4^) are the same, a higher concentration of Chromate is required for passivation of carbon steel in NP. This is due to the presence of the more reducible, aggressive nitrate ion which depolarises the cathodic tafel slope by providing an alternative additional reduction reaction. This results in a higher corrosion rate and hence a larger amount of Chromate is required to cover more of the active sites. Therefore, during the oxidising pretreatment step of decontamination with NP, a higher concentration of potassium permanganate is required to supply an adequate concentration of Chromate (by the dissolution of chromium-containing oxides) to protect the carbon steel portions of the reactor system.

9 Variation in the concentration versus time of permanganate in NP with increasing potassium permanganate concentration

11 Plot of corrosion rates of carbon steel in NP (1 mM KMnO^sub 4^ – 1 mM HNO^sub 3^) with varying concentrations of Chromate

Open circuit potentials (OCPs) of other structural materials used in nuclear reactors, such as Incoloy 800, Zircaloy 2 and Monel exposed to same concentration of permanganic acid at 90[degrees]C were also found to be in the noble region (Figure 12). Large fluctuations in the current density observed for Zircaloy in the passive region can be attributed to the dissolution and reformation of the surface film.

Corrosion of sensitised Type 304 stainless steel in organic acids

Sensitised stainless steel is prone to intergranular attack (IGA). It is essential that the chemical decontaminant should not cause IGA of sensitised stainless steel. Hence, sensitised stainless steel specimens were prepared and standard procedures (ASTM A262, practices A and E) were used to test for sensitisation in the coupons.22 Scanning electron micrographs of the grain boundaries exposed by the oxalic acid etching test (practice A) in the case of both sensitised and solution-annealed coupons are shown in Figure 13 (a) and (b), respectively. After electrolytic etching in 10 % oxalic acid, the sensitised coupons showed a ditch structure whereas annealed coupons revealed a step structure. Another standard method, ASTM G 108 (the electrochemical potentiokinetic reactivation test), was used to detect quantitatively the degree of sensitisation.23 As shown in Figure 13 (c), for sensitised coupons the area beneath the curve was found to be one hundred times greater than that for solution annealed coupons. 10 Plot of initial corrosion rates of carbon steel in NP with increasing potassium permanganate concentration

12 Polarisation curves for (a) Monel (b) Incoloy 800 and (c) Zircaloy in 1000 ppm of permanganic acid at 90 C

Sensitised Type 304 stainless steel specimens exposed to NAC at 9O[degrees]C showed no general corrosion or intergranular corrosion whereas coupons exposed to oxalic acid (1 mol dm^sup -3^ at 104[degrees]C) had extensive attack on the surface. Scanning electron micrographs of the exposed coupons are given in Figure 14. Coupons exposed to oxalic acid showed uniform surface attack.

Similarly, sensitised coupons of Type 304 stainless steel were exposed to permanganic acid at concentrations of 100 ppm and 300 ppm, and to equivalent concentrations of nitric acid permanganate (NP). Microscopic examination of the exposed specimens indicated that there was no attack. The surfaces of the specimens were very clean and had retained the original lustre from diamond polishing. Thus, pretreatment with either permanganic acid or nitric acid permanganate does not aggravate the corrosion behaviour of sensitised stainless steel. Also, it was clearly established that the sensitised Type 304 stainless steel does not undergo intergranular attack in NAC solution.

13 Demonstrations of sensltlsatlon of Type 304 stainless steel: (a) and (b) scanning electron micrographs of sensitised and annealed coupons, respectively, after the etch test; (c) EPR curves for (1) solution annealed material and (2) and (3) materials sensitised at 621 [degrees]C for 24 h and 650 C for 10h, respectively

Corrosion of carbon steel in NTA-ascorbic acid

Carbon steel undergoes corrosion in organic acid solutions such as a mixture of nitrilo triacetic acid and ascorbic acid. However, the corrosion loss to carbon steel can be minimised by adding a corrosion inhibitor. Rodine-92 B can be used as a corrosion inhibitor for carbon steel. Experiments were carried out to determine the minimum concentration of the inhibitor required to reduce the extent of corrosion to low value. For a mixture containing 5 mM NTA and 2.5 mM AA it was observed that 50 ppm of Rodine-92 B is sufficient to protect the carbon steel from corrosion (Figure 15).

14 Scanning electron micrographs of sensitised coupons of Type 304 stainless steel exposed to (a) NAC solution and (b) oxalic acid solution

Conclusions

1. Under certain conditions, carbon steel corrodes in permanganic acid. However, at concentrations above 200 ppm, permanganic acid passivates the carbon steel.

15 Plot of initial corrosion rates of carbon steel in solutions containing NTA (5mM) and AA (2.5mM) with varying concentrations of Rodine-92 B

2. In nitric acid-permanganate (NP), the corrosion of carbon steel is dependent on the concentration of both nitric acid and potassium permanganate. Nitric acid hinders the passivation of carbon steel by permanganate. Hence, a much higher concentration of permanganate is required to passivate carbon steel than is required for permanganic acid.

3. The presence of Chromate helps to protect carbon steel from the corrosion by both permanganic acid and nitric acid – permanganate.

4. The characteristics of the passive film formed on carbon steel in permanganic acid in the presence of Chromate is superior to the passive film formed at high concentrations of permanganic acid.

5. The corrosion of carbon steel in a nitrilo triacetic acid – ascorbic acid mixture can be mitigated by the addition of a corrosion inhibitor such as Rodine-92 B.

(c) 2007 Institute of Materials, Minerals and Mining

Published by Manay on behalf of the Institute

Received 8 October 2006; accepted 27 March 2007

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22. Practice for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels ASTM A 262-91.

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Puspalata Rajesh, Padma S. Kumar, Sumathi Suresh, Sinu Chandran, H. Subramanian, S. Velmurugan and S. V. Narasimhan*

Water and Steam Chemistry Laboratory, BHABA Atomic Research Centre, Kalpakkam – 603 102, Tamilnadu, India

* Corresponding Author. Tel: 91 44 27480203. Fax: 91 44 27480097. E-mail: SVN@igcar.gov.in

Copyright Institute of Materials Jun 2007

(c) 2007 British Corrosion Journal. Provided by ProQuest Information and Learning. All rights Reserved.




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