stainless steels and alloys - why they resist corrosion and how they fail.docx

7/28/2019 Stainless Steels and Alloys - Why They Resist Corrosion and How They Fail.docx

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Stainless Steels and Alloys: Why They Resist Corrosion

and How They Fail*Dr. Jianhai Qiu

School of Materials Engineering

Nanyang Technological University

Nanyang Avenue, Singapore 639797ABSTRACT

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 [1], 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 photo showed a 1936 Deluxe Ford Sedan with a bare (not coated)stainless steel body. 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.


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General ClassificationThe 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 asferritic, 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 ElementsEach 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


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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.


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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 steelsand 0.25 mm/y for carbon steel in natural sea water (taken from Tuas,

Jurong, Singapore).


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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


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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 factorapproach 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 showsthat the passive film formed at a higher passivating potential (+920 mV) has a


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higher chromium content than that formed at a lower passivating potential

(+750 mV).

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


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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].


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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.


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Figure 10The 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

-]


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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 crevicecorrosion 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 CrackingStainless 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.


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Figure 12 Unsensitized structure for 304 stainless steel

Figure 13 Sensitized structure of 304 stainless steel


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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 steelsThe 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 CorrosionOther 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.


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Concluding RemarksThe 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 shouldgive 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 thispurposes [10].If you want to know more about this topic presented here, there is a short course

entitled"Stainless Steels and Alloys: Why They Resist Corrosion and How They Fail". It

can be taken as in-house training course, online course or distance learning course.References:

1. William D. Callister, Materials Science and Engineering, 4th edition,John Wiley & Sons, Inc., p428 1996.

2. C. J. Abbott, Concrete, May 1997, p283. A. J. Sedriks, Corrosion of Stainless Steels, John Wiley & Sons, Inc., 2nd

Edition, 1996, p17

4. J. E. Castle and J. H. Qiu, Corrosion Science, Vol.30, No.4, p429-438,1990.

5. J. H. Qiu, to be published.6. J. H. Qiu, British Corrosion Journal, Vol. 33, No.4, pp 318-320, 1998.7. K. S. Seow, T. Y. Song and J. H. Qiu, Anti-Corrosion Methods and

Materials, Vol.48, No.1, pp31-36, 20018. M. P. Seah, J. H. Qiu, P. J. Cumpson and J. E. Castle, NPL Report,

DMM(A)95, 1993

9. M. P. Seah, J. H. Qiu and J. E. Castle, Metrologia, U.S.A., Vol.31 pp93-108, 1994

10.Annual Book of ASTM Standards, Vol.03.02, ASTM 100 Barr HarbourDrive, PA 19428

11.NACE International Standards, NACE International, Houston, TX 77216-1009

12.H. P. Leckie and H. H. Uhlig, J. Electrochem. Soc., Vol.113, p1262, 1966
http://www.corrosionclinic.com/corrosion_courses/stainless_steels_why_resist_corrosion.htmhttp://www.corrosionclinic.com/corrosion_courses/stainless_steels_why_resist_corrosion.htmhttp://www.corrosionclinic.com/corrosion_courses/stainless_steels_why_resist_corrosion.htmhttp://www.corrosionclinic.com/corrosion_courses/stainless_steels_why_resist_corrosion.htm


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13.J-H. Wang, C. C. Su and Z. Szklarska-Smialowska, Corrosion, Vol.44,p732, 1988

14.H. H. Uhlig and J. Gilman, Zeitschrift fuer Physikalische Chemie,Vol.226, p127, 1964

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