Go To Review

Characterization of High and Low Mo Stainless Steels in a Simulated Biological Environment

Dr. Jamal Alhajji & Dr. Manikam Valliappan
Corrosion and Materials Laboratory
Mechanical Engineering Department
Kuwait University
P. O. Box: 5969 - Safat - 13060 - Kuwait
Tel : 965-481-1188 Ext. 5981
Fax : (965)-484-7131

e-mail alhajji@kuc01.kuniv.edu.kw

Abstract:

Susceptibility to MIC (Microbially Induced Corrosion) decreases as the pitting resistance of the alloy increases. However, some MIC studies have indicated an increase in corrosion rate of stainless steels with high Mo content than the lower one. Apart from the common method, ferric chloride used for the determination of pitting resistance of stainless steels, careful reproduction of real world situations in the laboratory should be made before the selection of suitable stainless steels for seawater systems. This paper is focused on the characterization of high and low Mo containing SS in a simulated environment of biological consequences that result in low pH generation under biofilm or tubercles. Simulation studies at the pH value of 5 done with acetic acid, on biological approach, and HCl, on non-biological approach indicated the influence of age of the surface film and molybdenum content. These two parameters have allowed the high Mo SS (PRE=45) to show more noble pitting potential values in the acidified systems. DC polarization and AC impedance data for lower PRE stainless steel i. e., 23 selected for this investigation showed the decrease in pitting resistance in the acidified systems.

Stainless steels have been considered as promising and economic materials for aggressive corrosion environments because of its spontaneous passivity. However, Stainless Steels are sensitive to pitting1 and other types of localized corrosion in chloride containing media. One common way of selecting materials for severe seawater applications with high pitting tendency is choosing the material with high pitting resistance equivalence number (PRE). This can be attained by improving the passivity of stainless steels through alloying of iron with other elements mainly chromium, nickel, molybdenum and nitrogen by properly adjusting the composition of each alloying element according to the severity of the application.

The main parameters which play the lead role in deciding the passivity of stainless steels were documented2 as (i) the passivation potential or oxidizing power of the solution (ii) the pH of the test electrolyte (iii) local pH within highly hydrated passive films (iv) the age of the passive film (v) the location of the ion relative to the external surface. Among other variables, biofilm3 and under tubercles or discrete biological deposit4,5 play the lead role in producing conditions favorable to localized corrosion.

Microbially induced corrosion failures of SS have been reported in the literature and sulfate reducing bacteria have been implicated as either the cause or a contributing factor6-8. Recent advancements in analyzing the corrosion phenomena through the measurement of metal/solution interfacial pH variations have directed several MIC researchers to consider the pH variations that could take place through biofilms formed at sea on the different materials9,10. However, no general agreement has yet been reached on the effects of the biofilm on pH variations. Excluding this type of biological analysis, the common method used for the evaluation of stainless steels in aggressive environments is Ferric Chloride test11.

The selection of stainless steels for seawater service system based on the accelerated test i.e., ferric chloride test some times may not be worthwhile because of the profound effects of MIC on the unexpected failures of stainless steel components. This calls for the simulation of biogenic attack on materials before the selection of materials based on the severity of the application. Such simulation studies were applied to acid production under a simulated tubercle12. It has been correctly pointed out that the reason for low corrosion rates obtained in the laboratory studies is the problem in accurately reproducing the real world situation. It appears therefore necessary to simulate the conditions favorable to pitting before the selection of materials for particular service. This investigation is focused on the effects of acidification on stainless steels with low and high Mo using acetic acid, on biological approach, and hydrochloric acid, on non-biological approach, to the deaerated system of synthetic seawater after the initial and one day period of immersion of stainless steels in seawater solution.

The composition of stainless steels selected for this paper are given in Table I with PRE number based on the formula

The test medium used was synthetic seawater and kept as such or deaerated with nitrogen or adjusted to the pH value of 5 using acetic acid or HCl according to the following simulation steps.

Simulation I : Stabilization period of one hour in synthetic seawater.

Simulation II : Incubation time of one hour in deaerated synthetic seawater.

Simulation III : Immersion period of one hour in deaerated synthetic seawater, pH adjusted to 5 using acetic acid for the biological approach and HCl for the non-biological approach.

One Day Exposure Studies :

Simulation I : Incubation time of one day in synthetic seawater (pH = 9.2)

Simulation II : Stabilization period of one day in synthetic seawater and then deaerated with nitrogen to simulate the biogenic condition of initial exposure to biological free system and onset of deaerated environment as the result of biological sequences.

Simulation III : Stabilization period of one day in synthetic seawater (pH = 9.2) and then the medium was deaerated with nitrogen and acidified with acetic acid or hydrochloric acid to the pH value of 5.

DC polarization studies at a scan rate of 0.5 mV/sec and AC impedance measurements for the sinusoidal voltage of 5 mV over the frequency range of 100 KHz to 10 mHz were performed at the room temperature (19± 1o C) using the computer controlled potentiostat (EG&G model 273 A) and high frequency response analyzer (Schlumberger SI 1255).

Polarization resistance, Rp from linear polarization method (Figures 1 and 2) indicated the influence of chromium and molybdenum on the pitting resistance of stainless steels.

The initial exposure studies revealed the effect of high chromium content with low Mo content (compared with high Mo content on the pitting susceptibility of stainless steel in blank and deaerated synthetic seawater solutions. However, increase in Rp values was noted for high Mo SS under the acidified conditions (pH = 5). The stabilization period of one day and the simulation of biogenic attack, has allowed the high Mo Stainless steel to passivate in the selected medium. This results in high Rp values as to that compared with other SS, showed the beneficial effect of Mo except in the acidified system of HCl. The pitting potential value from the anodic polarization curves (Figure 3 and 4) for the A grade SS (Table 1) is high in the blank seawater solution and deaeration with nitrogen resulted in the decrease of pitting potential for the initial exposure period. Previous results13 indicated most noble potentials for the high concentration of oxygen and the removal of oxygen resulted in the decrease of pitting potential.

The biogenic simulation studies done with acetic acid and hydrochloric acid further decreased the pitting potential of grade A stainless steel. This situation in quite clear for one day exposure studies (Figure 4). The results from this investigation are some what different from the general approach, because of the change in the experimental approach, in very dilute hydrochloric acid and organic acid solutions14.

It has been stated that passivity is possible in very dilute hydrochloric acid, and organic acids can be corrosive to stainless steels when in the form of aqueous solutions, among which acetic acid is less corrosive. AC impedance spectra obtained in the form of Bode plot (Figures 5 and 6) have also shown the influence of acidification on the pitting mode of stainless steel (Grade A) in the biologically simulated systems where decrease in impedance values ½z½ was noted.

The above situation was not reflected for the grade B and C stainless steels (Table 1) where more positive pitting potential values were obtained for the acidified system for the initial exposure period (Figures 7 and 11).

However, one day stabilization period Figure 8 changed the pitting mode of stainless steel B in hydrochloric acid added system. The acidification using acetic acid resulted in the noble pitting potential as compared to that in other systems of deaeration and blank seawater solutions.

The AC impedance measurements are not in agreement with DC polarization studies for the initial exposure studies where decrease in impedance values was observed for the acidified system (Figure 9).

Figure 10 yields the lower impedance values for the blank system in the low frequency range. The initial exposure studies for grade C SS (Figure 11) indicated the influence of HCl and deaeration on the pitting susceptibility of high Mo stainless steel. But aging of the surface film has allowed the acidified system to show more noble pitting potential values (Figure 12).

In contrast to the potentiodynamic observations, acidification led to the decrease of impedance values for high Mo SS, notably HCl system (Figure 13 and 14). Because of the initial dissolution of Mo in acidified system of HCl the better pitting resistance was observed for high Cr steel (Grade B). It is well know that increase in Cr and Mo content increases the pitting resistance. However beneficial effect of Mo is questionable in some studies15 and attributed to the complexing of compounds that Mo can form depending on the experimental conditions.

Another study16 indicated that the addition of Mo to 18% Cr steel upto the solubility limit of 3.5% increase pitting resistance in 1 molar NaCl in the temperature range 1 to 70o C and the pitting resistance of a 4.7% Mo alloy is lower than that of a 3.5 % Mo alloy. Another important point to be noted for high Mo stainless steel is that copper has no effect in the absence of Mo, but a slightly detrimental effect in the presence of molybdenum17 (Table 1-Grade C). Earlier studies18 related to the pH influence on pitting potential, revealed relatively little effect of pH in the range 1.6 to approximately 10. In contrast to this effect, the critical pH range of 4.5 to 5 has the certain influence on the pitting resistance19 and related to the nature of sulfides present in the stainless steel that led to the pH assisted MnS dissolution. The results from this paper indicated that the increase in PRE number is not always beneficial on the biological simulation point of view. It calls for the careful reproduction of real world conditions such as biological consequences before the selection of suitable materials for the aggressive environments.

1. Z. Szklarska-Smialowska, Pitting Corrosion of Metals, NACE, Houston, TX., 1986.
2. Clive R. Clayton, and Ingemar Olefjord, Passivity of Austenitic Stainless Steels in Corrosion Mechanisms in Theory and Practice, eds., P. Marcus and J. Oudar, Marcel Dekker NewYork, 1995, p 176.
3. H. A. Videla, M. F. L. de Mele and G. J. Brankevich, Corrosion, 44, 1988, pp 423-426.
4. D. J. Duquette, Electrochemical aspects of Microbiologically Induced Corrosion in Biologically Induced Corrosion-NACE 8, ED., S. C. Dexter, Houston, TX: NACE, 1986.
5. R. E. Tatnall, Corrosion /84, paper no.95, Houston, TX: NACE, 1984.
6. A. K. Tiller, Is Stainless Steel Susceptible to Microbial Corrosion?" Microbial Corrosion, The Metals Society, 1983, p 104.
7. R. E. Tatnall, Materials Performance, 22, 8, 1983, P41.
8. J. F. D. Stott, Metals and Materials, 4, 1988, p 224.
9. S. C. Dexter and S. H. Lin, Mechanism of Corrosion Potential Enoblements by Marine Film in Proc., 7th International Congress on Marine Corrosion and Fouling, universidad politecmia, Espana, 1988.
10. B. Little et. al., Corrosion/90, paper no. 150, Houston, TX: NACE, 1990.
11. ASTM G 48-76, Annual Book of ASTM standards,Vol. 03.02, ASTM, USA, 1990.
12. R. E. Tatnall, R. A. Corbett, B. D. Krantz and S. C. Dexter, Corrosion/95, paper no. 191, Houston, TX: NACE, 1995.
13. B. E. Wilde and E. Williams, J. Electro Chem.Soc., 116, 1969, p 1539.
14. A. Sedriks, ed., Stainless Steels , John Wiley and Sons, USA, 1979, p 221 and 227.
15. T. Horvath and H. H. Uhlig, J. Electro Chem.Soc., 115, 1968, p 791.
16. E. A. Lizlorsand and A. P. Bond, J. Electro Chem.Soc., 116, 1969, p 574.
17. A. Moskowitz et al., ASTM STP -418, ASTM, USA, 1967, p3.
18. S. Szklarska-Smialowska, Localized Corrosion, NACE, Houston, TX., 1986, p312.
19. Bernard Baroux, Further insights on the Pitting Corrosion of Stainless Steels, in Corrosion Mechanisms in Theory and Practice, eds., P. Marcus and J. Oudar, Marcel Dekker, New York, 1995, p 282.

(0)