Stainless Steel Corrosion

2Chemical resistance of NIROSTA steels

General corrosion

Stainless steels are defined asbeing characterized by particularlyhigh resistance to chemical attack byaqueous media. In general, they con-tain at least12% by weight of chromeand a maximum of 1.2% carbon.

The reason for their high resist-ance to corrosion is a passive layerthat forms on the surface. This consistsof a metal oxide or hydroxide layer,rich in chrome, only a few ngstromunits thick, separating the actual metalfrom the attacking medium.

After sufficient time has passed,the passive layer of a stainless steelexhibits a constant composition andremains in a state of equilibrium withthe surrounding medium. Once form-ed, such a layer cannot therefore betransferred to another medium. Fol-lowing any mechanical damage ofthe surface, a new layer can generallybe expected to form spontaneously atthat point.

If in some medium a satisfactorypassive layer cannot form, or if anexisting layer is locally damaged orcompletely destroyed, corrosion canoccur.

The decisive element responsiblefor the formation of a passive layer ischrome. A chrome content above thequoted value of some12% inhibits rust-ing under normal atmospheric condi-tions. Further increases in the chromecontent and, according to the appli-cation, the addition of molybdenumand other alloys permit corrosionresistance to be extended to muchmore agressive media.

Only those alloy contents dissolv-ed in the metal are effective in achiev-ing passivation. The highest resistanceto corrosion is thus given with a seg-regation-free matrix whose chromeor molybdenum contents are not re-duced by precipitations of the forma-tion of non-metallic phases. The rightheat treatment for achieving an idealstructure is described in the particularmaterial sheets.

that even slight additions, e. g. ofoxidising or reducing materials, canweaken or intensify corrosive attack.Deposits, such as those occasionallyfound on the walls above the bathsurface or at other points, and con-densation in the steam chamber ofan enclosed apparatus can undercertain circumstances greatly modifythe conditions for corrosive attack.

Exact knowledge of corrosiveconditions is thus vital in selecting theright grade of steel. The best (andsometimes only) way of gaining infor-mation on the resistance of a materialin the corrosive medium in question isto carry out tests on a specimenunder actual service conditions,taking into consideration not only thecomposition and concentration of thecorrosive medium but also the tem-perature, the pH value and othervariables.

We would be pleased to pro-vide specimens of the relevant mate-rials for test purposes.

Classification of NIROSTA-/ASTM grades by group

Gr. 1

4000/410 S4002/4054003/A240/A240M4006/4104021/(420)4024/(410)4028/(420)4031(420)4034/(420)

4313

4512/(409)

4589

Gr. 4

4301/3044303/(305)

4306/304 L

4307/304 L4310/(301)4311/304 LN4315/304 N4318/301 LN4541/321

4544

4546

4550/347

Gr. 3

4113/4344521/4444568631

4465

4539

4565 S

Gr. 2

4016/4304057/431

4120

4305/3034427/316 F4509/4414510/439

4511

4520

Gr. 5

4401/316

4404/316 L

4429/316 LN4435/316 L4436/3164438/317 L

4439

4462

4501

4561/316 Ti4571/316 Ti

0 = resistant to general corrosion(mass loss rate 11.0 mmthickness reduction/year)

The following warning is providedfor the major forms of localisedcorrosionL = risk of pitting, crevice corro-

sion or stress-corrosioncracking, even in resistanceclass 0

Stainless steels may suffer gen-eral corrosion or various types oflocalised corrosion. Resistance togeneral corrosion is usually classifiedas follows:

General corrosion is to be ex-pected primarily in acids and strongalkaline solutions. Pitting, crevicecorrosion or stress-corrosion crackingare most frequently caused by chlo-ride ions but they may also be in-duced by the rarer halides bromideand iodide, while stress-corrosioncracking can also occur in the pres-ence of other species.

Pitting and crevice corrosion

Pitting corrosion is initiated byinteraction between halide ions andthe passive film, the latter being locallypunctured. Hollows the size of pinpricks are formed and grow into pitsites which can vary greatly in se-verity. The risk of pitting increaseswith increasing concentrations of

halide ions increasing temperature an increase in the electro-

chemical potential of the steelin the relevant electrolyte,caused for example by theeffects of an oxidising agent.

Crevice corrosion arises in crev-ices where fluid exchange with thesurrounding environment is limited.Crevices of this nature are design orproduction related and occur e.g.on flanges, tube sheets, beneathseals or even under scale/deposits.

The corrosion mechanism is largelythe same as for pitting, althoughcrevice geometry and the type ofmaterials forming the crevice exert anadditional influence. Since creviceattack occurs under less seriouscorrosion conditions than pitting,attempts should be made to designout crevices in components to beused in chloride-bearing media.

Assuming a homogeneous distri-bution of alloying elements, a roughguide to the pitting and crevice corro-sion resistance of stainless steel is theefficiency sum (W) of % Cr + 3.3 x% Mo + 30 x % N (see Fig.). Theinfluence of nitrogen as an alloyingelement is, however, more complexthan expressed by this equation. Thehigh efficiency expressed in thefactor of 30 will only apply in fullin the case of high-alloy steels withincreased molybdenum contents.

A materials inherent resistanceto pitting and crevice corrosion canonly be fully achieved if the surfacequality of the material is pristine,i. e. bright metallic. It is thereforeimportant to remove any heat tintingor scale left after welding, iron par-ticles or rust from other sources,grinding residue etc.

Stress-corrosion cracking

Stainless steels in media con-taining specific components in par-ticular chloride ions and subjectedat the same time to tensile stressesmay suffer corrosion attack andcracking, even if the steel displaysadequate resistance to the mediumwhen not under mechanical load.This phenomenon is known as stress-corrosion cracking and is not causedexclusively by service stresses; theblame frequently lies with internalstresses applied during processing,e.g. welding, grinding or cold forming.

As with pitting and crevice cor-rosion, the risk of chloride-inducedstress-corrosion cracking becomesgreater as the temperature and chlo -

20

60

50

40

30

20

10

025 30 35 40 45 50 55

Critical crevice corrosion temperatureas a factor of the efficiency sum

1.4565 S

1.4462

1.4539

1.44391.4429

1.44381.4435

10% FeCl3 6 H2O

Crit

ical

cre

vice

cor

rosi

on te

mpe

ratu

re

C

Efficiency sum (% Cr + 3.3 x % Mo + 30 x % N)

ride concentration increase. Thereare, however, other material-relatedvariables. For example, austeniticsteel grades 18/10 -CrNi and17/12/2-CrNiMo are at particularrisk of chloride-induced stress-corro-sion cracking when temperaturesexceed 50 C. Resistance can bedistinctly enhanced by increasing themolybdenum and in particular thenickel content of the material. Incomparison, ferritic and austenitic-ferritic stainless steels are relativelyinsensitive to corrosion of this type.

How to use the table

Even though the figures pro-vided in the following have beencalculated in laboratory tests usingpickled specimens with the bestpossible microstructure annealed,tempered or quenched they providea basic guide to applicability.

It must however be emphasisedthat under practical conditions agents rarely occur in such pure form, and

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S.01-37 Chem.Best.engl. 23.09.1999 10:55 Uhr Seite 2