stainless steels and alloys: why they resist corrosion an how they fail

Stainless Steels: Why They Resist Corrosion and How They Fail Dr. Jianhai Qiu

School of Materials Engineering Nanyang Technological University

Nanyang Avenue, Singapore 639798

ABSTRACT

This paper discusses the corrosion resistance properties of stainless steels with a focus on why stainless steels resist corrosion and how they fail under service conditions. It is hoped that the discussions would help designers, specifiers, engineers and maintenance personnels understand the corrosion resistance characteristics of stainless steels and hence aid their specification, design, selection, evaluation and efficient applications.

Introduction

Stainless steels are a family of special grade of iron-based alloys that contain at least 11% of chromium in their composition. As their names suggest, stainless steels can retain the "stainless" appearance as opposed to the rusty look of common carbon or mild steels. Car manufacturers often coat steel car body with 6 or more layers of metallic and non-metallic (paint) coatings to protect the substrate steel. Many of us have the experience that deep scratched car body, if not touched up promptly, would become rusty at scratched areas. If the car body is made of stainless steel, protective coatings become unnecessary and scratches on the stainless steel car body would not lead to rust stain formation. This is because stainless steels have remarkable resistance to atmospheric corrosion. The color insert in reference 1 showed a 1936 Deluxe Ford Sedan with a bare (not coated) stainless steel body[1]. Other non-stainless steel components such as brake, clutch, gears and the engines had to be replaced three or more times, the surface condition of stainless steel body (after about 60 year's atmospheric exposure) is essentially the same as when the car left the assembly line.

Stainless steels have found applications in almost every industrial sector such as petrochemical, oil and gas, food processing, pharmaceutical, power generation and marine. In recent years, stainless steels are increasingly specified and used in the construction industry for their excellent corrosion resistance in the concrete environment. For instance, to achieve a designed life of 750

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years for the historic building of Guildhall in London, 140 tons of 304 stainless steel reinforcement are used instead of the conventional steel reinforcement [2].

General Classification

The properties of stainless steels, like any other materials, are primarily determined by their compositions which further determine the structure [3]. Stainless steels are generally classified according to their microstructure as ferritic, austenitic, duplex, martensitic and precipitation-hardening. The relationship between composition, structure and properties of different grades of stainless steels is summarized in Fig.2.

Fig. 2 Composition, Structure and Property Relationship of Stainless Steels

Effect of Alloying Elements

Each grade of stainless steel has its own unique property due to modifications to its composition or structure. The common requirement for all grades of stainless steels is that the chromium content must be greater than 11% (wt) in the composition. This is the minimum amount of chromium that can maintain the "stainless" appearance of a steel by forming a compact chromium-rich ultra thin surface oxide, know as "passive film".

Another major alloying element commonly found in austenitic and duplex stainless steels is nickel. As a more noble element than iron, nickel in stainless steels help improve the corrosion resistance. Fig.3 shows the polarization behavior of the nickel-containing austenitic 304 stainless steel and two ferritic grades in sulphuric acid [4,8,9].


Fig.3 Polarization Behavior of Austenitic and Ferritic Stainless Steels

It is clearly seen from the above figure that the 9% nickel in 304 stainless steel has a corrosion potential that is over 400 mV more positive or noble than the ferritic Fe17Cr stainless steel. This shift of corrosion potential in the noble direction indicates an increased thermodynamic stability of the metal/solution system. Another marked feature observed from this polarization diagram is that the peak passivation current density for the nickel-containing 304 steel is reduced by over 2 orders of magnitude when compared with the ferritic Fe17Cr. Nickel in the alloy is also able to reduce the passive current density within the passive potential range.

Molybdenum addition in stainless steels increases the resistance to localized corrosion such pitting and crevice. The polarization curve in Fig.3 also showed that 4% molybdenum can reduce the peak passivation current density by an order of magnitude.

If we compare the corrosion behavior of stainless steels and carbon steel in natural sea water


(Fig.4), the effects of alloy composition are even more pronounced. Both 304 and 316 stainless steels showed natural passivation behavior in sea water with similar passive current density of ~ 1 uA/cm2. The passive potential range for 316 (Mo-alloyed) is significantly broader than that of 304 (Mo-free) while the conventional carbon steel showed no passivation at all. The substantial difference in the corrosion potentials among the three steels is entirely due to the different compositions! The corrosion rates determined from Fig.4 are roughly 0.010 mm/y for 316/304 stainless steels and 0.25 mm/y for carbon steel in natural sea water (taken from Tuas, Jurong).

Fig.4 Polarization Behavior of Stainless Steels and Carbon Steel in Natural Sea Water [5]

Composition of Passive Film

It has been well established that passivity is caused by the formation of a compact ultra-thin surface oxide film - the passive film. The corrosion resistance of stainless steels under service conditions is essentially determined by the stability of such a passive film, which is further determined by the composition, thickness and structure. The relationship between the corrosion resistance, passive film property and alloy composition is illustrated in the following Figure.


Fig.5 The relationship between the corrosion resistance of stainless steels, passive film property and alloy composition.

The passive film on stainless steels has its own composition which is strongly enriched in chromium. For an alloy containing 15% Cr in its composition, the chromium content in the top layer of passive film can be as high as 60% (Fig.6)! In Fig.6, the chromium enrichment factor (defined as the ratio of chromium content in the passive film to that of the alloy) is plotted against the film thickness for two alloys passivated in 0.1M sulphuric acid at 920 mV (SHE) for 1 hour. The chromium content was measured using X-ray Photoelectron Spectroscopy (XPS). It is clearly seen that the 4% molybdenum in alloy Fe15Cr4Mo markedly increased the chromium content in the passive film! It is also seen from the figure that the chromium enrichment factor approach unity at a thickness of about 3 nm, which is considered to be the passive film thickness.


Fig. 6 Chromium enrichment in the passive film as a function of thickness

In addition to the dependence of alloy composition, the chromium content in the passive film is also dependent on the passivating potential. Fig. 7 shows that the passive film formed at a higher passivating potential (+920 mV) has a higher chromium content than that formed at a lower passivating potential (+750 mV).


Fig. 7 The potential dependence of chromium content in passive films

An interesting observation for the nickel-containing stainless steel is that nickel was actually depleted in the passive film and in some cases was found to be enriched in the metallic form immediately beneath the passive film. Surface analysis results in Fig.8 showed that chromium was enriched in the passive films while nickel was depleted in the nickel-containing steel.


Fig. 8 Enrichment of chromium and depletion of nickel in the passive films

Localized Corrosion

Breakdown of passive film often leads to localized corrosion such as pitting and crevice corrosion. Figure 9 shows pitting corrosion in a Duplex stainless steel after exposure to NaCl solution [7].


Figure 9 Pitting corrosion of Duplex stainless steel (SAF2304) in NaCl solution.

The pitting events can sometimes be indicated by an electrochemical potential noise as illustrated in Figure 10 where the potentials for 3 different grades of duplex stainless steels are being monitored. SAF2304 showed clear pitting signals compared with SAF2507 and SAF 2205.

Figure 10

The susceptibility to pitting corrosion can be quantified with the electrochemical technique - the cyclic pitting scan. Figure 11 showed the comparison between three duplex steels in NaCl


solution.

Figure 11

The trend observed in figure 11 is similar with Figure 10. Both figures revealed that SAF2304 is less resistant to pitting than the other two. Simple and rapid measurements of this kind can be used to evaluate and select the right grade of stainless steel for a specified application. Critical temperature, critical pitting potential and the protection potential can be determined in a given process condition such that pitting corrosion of stainless steels and alloys can be minimized or avoided.

It was reported that the pitting potential for type 304 stainless steel is directly proportional to the concentration of chloride ion on the semi-log scale [12-14]:

Epit= A + B log[Cl-]

At the design stage, knowledge of the nature of environment and particularly the concentration level of chloride ions would be useful in assessing the suitability of a specific grade of stainless steel for the intended application.

In addition to the environmental considerations, the pitting and crevice corrosion resistance of stainless steels is also determined by alloying elements such as chomium, molybdenum, nitrogen and tungsten. The synergistic effects of these alloying elements help to stabilise the passive film, and in case of breakdown, rapid repassivation can take place to heal the damaged area. This


compositional effect is commonly represented by the "pitting resistance equivalent number, PREN":

PREN = Cr% + 3.3x(Mo%) + 16 x (N%) + 1.65 (W%)

The numerical value derived from the chemical compositions of a specific grade of stainless steel is sometimes used as an indication to the pitting and crevice corrosion resistance. For example, a PREN value of 40 is required for a stainless steel to be resistant to localised corrosion in deoxygenated sea water. Super duplex and super austenitic stainless steels (6Mo) meet this compositional requirement and in fact are resistant to pitting and crevice corrosion in deoxygenated sea water.

Stress Corrosion Cracking

Stainless steels are susceptible to stress corrosion cracking in environments containing certain chemical species such as chloride. Welding of unstabilised stainless steel may cause sensitization and result in weld decay - a form of intergranular stress corrosion cracking. Figures 12 and 13 showed two structures with and without sensitization.

Figure 12 Unsensitized structure for 304 stainless steel


Figure 13 Sensitized structure of 304 stainless steel

The degree of sensitization can be rapidly quantified with the double-loop electrochemical potentiokinetic reaction method (DL-EPR). Figure 14 showed the DL-EPR for a 304 stainless steel sensitized at 700oC with and without protection by ceramic insulating paste [6].


Figure 14 DL-EPR measurements on 304 stainless steels

The section protected by ceramic heat insulating paste (green curve) showed similar current density peaks as the control sample (blue curve) indicating the effectiveness of the paste in reducing the degree of sensitization.

Other Forms of Corrosion Other forms of corrosion observed for stainless steels under service conditions include hydrogen-induced cracking, microbiologically influenced corrosion, galvanic corrosion and knifeline attack. The nature of environment including the operating conditions and the specific grade of alloy will ultimately influence the form and degree of attack.

Concluding Remarks The corrosion resistance of stainless steels depends on the stability of an ultra thin passive film on their surfaces, which is in turned determined by the alloy composition and the nature of the environment. A good design should give due consideration to the nature of the environment under which a stainless steel component operates. Breakdown of passivity often leads to the localized corrosion such as pitting or crevice corrosion. Welding or hot-working of stainless steel may introduce sensitized structure and subsequently lead to weld decay or intergranular stress corrosion cracking. Chloride, hydrogen and hydrogen sulfide are common species that induce cracking of stainless steels. Proper tests should be performed to evaluate the suitability of a


specific grade of stainless steel for a specific application. A number ASTM standards and NACE recommended practices exist for this purposes [10].

References: William D. Callister, Materials Science and Engineering, 4th edition, John Wiley & Sons, Inc., p428 1996. C. J. Abbott, Concrete, May 1997, p28 A. J. Sedriks, Corrosion of Stainless Steels, John Wiley & Sons, Inc., 2nd Edition, 1996, p17 J. E. Castle and J. H. Qiu, Corrosion Science, Vol.30, No.4, p429-438, 1990. J. H. Qiu, to be published. J. H. Qiu, British Corrosion Journal, Vol. 33, No.4, pp 318-320, 1998. K. S. Seow, T. Y. Song and J. H. Qiu, Anti-Corrosion Methods and Materials, Vol.48, No.1, pp31-36, 2001 M. P. Seah, J. H. Qiu, P. J. Cumpson and J. E. Castle, NPL Report, DMM(A)95, 1993 M. P. Seah, J. H. Qiu and J. E. Castle, Metrologia, U.S.A., Vol.31 pp93-108, 1994 Annual Book of ASTM Standards, Vol.03.02, ASTM 100 Barr Harbour Drive, PA 19428 NACE International Standards, NACE International, Houston, TX 77216-1009 H. P. Leckie and H. H. Uhlig, J. Electrochem. Soc., Vol.113, p1262, 1966 J-H. Wang, C. C. Su and Z. Szklarska-Smialowska, Corrosion, Vol.44, p732, 1988 H. H. Uhlig and J. Gilman, Zeitschrift fuer Physikalische Chemie, Vol.226, p127, 1964

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stainless steels and alloys: why they resist corrosion an how they fail

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