Stainless Steel and Corrosion
What is corrosion?
Metals, with the exception of the precious metals such
as gold and platinum, that are found in their natural state
are always extracted from ores; metals have therefore a
tendency to revert to their stable state, which corresponds to
their original state, that is to say their oxide form.
Metal corrosion is essentially an electro-chemical reaction at
the interface between metal and surrounding environment .
Stainless Steel and the passive layer
Steel is an alloy of iron and carbon. Contrary to carbon steel,
the presence of a minimum of 10.5 % chromium in the
stainless steel gives it the property of corrosion resistance.
What are the major factors of corrosion?
Medium
Design
Chemical nature
Surface state
Concentration
Shape
Oxidising power
Assembly (welds, rivets)
pH (acidity)
Mechanical stresses
Temperature
Pressure Proximity to other metals Contact
with a medium
Viscosity
(partial or total immersion)
Solid deposits
Methods of protection
Agitation
Composition
Manufacturing
Metallurgical state
(thermal or mechanical
treatment)
Additives
Impurities
Material
Chromium
>10,5%
factors
Time
What are the 5 principal types of corrosion linked with
the surrounding environment ?
Carbon
<1,2%
Iron
Corrosion
Ageing of the structure
Evolution of stresses
Temperature variability
Modification of coatings
Maintenance frequency
Fe+C=steel
Fe+C+Cr=stainless steel
generalised
localised
Pitting
Indeed, on contact with oxygen, a chromium oxide layer is
formed on the surface of the material. This passive layer
protects it and has the particular ability to self repair.
neutral
chloride
environment
Reaction of steel and stainless steel in contact
with moisture in the air or water.
Steel
Stainless Steel
Formation of iron oxide
(rust)
Formation of chromium
oxide
Fe + C
O2
Crevice
Under
stress
Fe + C + Cr > 10,5 %
Rust
Passive layer
However if this protective layer is damaged, the start of
corrosion can appear
Intergranular
Generalised
acid medium
Generalised corrosion is noticed when stainless steel is
in contact with an acid medium and localised corrosion
is seen in the majority of cases when stainless steel is
placed in a neutral chloride environment
La corrosion par piqûre
In the document which follows we describe the 5 principal
types of corrosion and we rank the majority of Stainless
Europe grades from standard laboratory tests
However, the phenomena of corrosion in real life are always
specific, the data described does not exclude extra trials to
choose the optimal material.
Pitting Corrosion
To understand the phenomenon
Pitting corrosion is a local break in the passive layer of the
stainless steel provoked by an electrolyte rich in chloride
and or sulphides. At the site of the pitting, where the metal
is unprotected, corrosion will develop if the pit does not
re-passivate, in other words if the speed of metal dissolution
enables to maintain a sufficiently aggressive environment to
prevent its re-passivation.
Passive layer
Metal
METAL ATTACK
Chlorides
> If the potential of the stainless steel in the given medium is
superior to the pitting potential =>the stainless steel corrodes
Note: the higher the pitting potential, better shall be the
corrosion resistance of the grade. Outside the pits, the
passive layer is always present to protect the stainless steel.
Figure 1 shows pitting potentials obtained for different
stainless steels in water containing 0.02M NaCl (710mg/l Cl-)
at 23°C.
It shows the influence on the resistance to pitting corrosion
with the content of chromium and molybdenum for
the ferritics, and chromium, molybdenum and nitrogen content
for the austenitics.
mV/SCE
Passive layer
Figure 1
800
K44
Corrosion of the base metal
The metal re-passivates
Passive
Layer
Metal
This dissolution gives rise to metal ions and electrons and
thus the passage of current (of dissolution) which gives rise
to an electrical potential difference between the anodic zone
(pitting) and the cathodic zone (the rest of the metal).
To simulate this type of corrosion under laboratory
circumstances, a sample is immersed in a corrosive electrolyte
to which an increasing potential is applied until the passive
layer is broken.
During this dynamic potential (intensity/potential) scan
the sudden increase in intensity corresponds to the pitting
potential Epitting
IntensitY
I =50µA/cm2
Epitting
POTENTIAL
Passive Field
The pitting potential corresponds to the potential necessary
to initiate stable pits.
> If the potential of the stainless steel in the given medium is
inferior to the pitting potential => pitting does not start
700
17-11MT
18-11ML
600
18-9E/L
18-10T
17-7A/C/E
K41
500
K45
K36
18-7L
17-4Mn
16-4Mn
16-5MnL
2 cases
Pitting potential in water containing 0.02MNaCI pH=6,6 at 23°C (mV/SCE )
Metal
K39
400
K30-K31
K03
K09
K10
300
Martensitic
200
10
12
14
16
18
20
22
24
PREN (%Cr+3,3%Mo+16%N)
Commercial
Designations
Martensitic
K09+
K03
K10
K30-K31
K39
K41
K36
K45
K44
16-4Mn
16-5Mn
17-4Mn
17-7A/C/E
18-7L
18-9L
18-11 ML
17-11MT
Standards
ASTM
Designation: Type
420
409
410S
430
439
441
436
445
444
201.2
201LN
201.1
301
301LN
304L
316 - 316 L
316Ti
EN
1.4021
1.4512
1.4003
1.4000
1.4016 - 1.4017
1.4510
1.4509
1.4526
1.4621
1.4521
1.4372
1.4371
1.4618
1.4310
1.4318
1.4307
1.4401 - 1.4404
1.4571
26
As figures 2 and 3 show, this pitting potential can only be used to rank the grades in a given medium.
It diminishes notedly when the temperature (figure 2) or the concentration of chlorides in the medium increases. (figure 3)
500
K44
600
18-11ML
500
18-9E/L
400
K36
K41
K39
300
K30
200
K03-K09-K10
100
Pitting potential in water containing 0.5M NaCI pH=6,6 at 50°C (mV/SCE)
Pitting potential in water containing 0.02M NaCI pH=6,6 at 23°C (mV/SCE)
700
450
400
350
300
18-9E/L
K36
K41
250
K39
200
150
K30
100
50
0
K03-K09-K10
0
10
12
14
16
18
20
22
24
K44
18-11ML
26
10
12
14
PREN (%Cr+3,3%Mo+16%N)
16
18
20
22
24
26
PREN (%Cr+3,3%Mo+16%N)
Figure 2
Figure 3
In order to map the duplex range, tests in the most severe medium, 0.5M NaCl (17.75g/l Cl-) at 50°C, were carried out.
The results obtained are shown below.
23° 50°
23°
700
23°
Standards
No pits
500
No pits
600
No pits
Pitting potential in water containing 0.5M NaCl, at 23°C and 50°C (mV/SCE)
Figure 4
1200
50°
Commercial
Grades
23°
23°
ASTM
Designation
Type
400
23°
300
50°
50°
200
50°
EN
UNS
DX2202
2202
UNS 32202
1.4062
DX2304
2304
UNS 32304
1.4362
DX2205
2205
UNS 32205
1.4462
50°
100
0
DX2202
DX2205
DX2304
304
316
K44
>
Usually we use the PREN (Pitting Resistance Equivalent Number) of the grades to rank their general piting behaviour. The PREN,
%Cr+3.3%Mo+16%N , demonstrates the major influence of these alloy elements.
Our recommandation
To avoid pitting corrosion:
> We would look to see if it is possible to lower the corrosiveness by lowering the temperature of the medium, limiting
contact time, avoiding stagnant areas and reducing the concentration of halogens and the presence of oxidants.
> We would choose a grade high in chromium or containing molybdenum.
Crevice Corrosion
To understand the phenomenon
A/Initiation of corrosion
In an electrolyte high in chloride, a confined (occluded) zone linked for example to bad design, favours the accumulation of chloride
ions. The progressive acidification of the medium in this zone facilitates the de-stabilisation of the passive layer. When the pH in this
zone reaches a critical value called « depassivation pH »,corrosion starts. The depassivation pH or pHd is used to characterize the
resistance to crevice corrosion initiation.
WHY?
Confined zone
(acidification)
HOW?
Break in the
passive layer
and metal attack
Some pHd values for our stainless steels are given in figure 5.
The lower the value pHd the better the resistance to crevice
corrosion.
Figure 5: Depassivation pH of various stainless steels in NaCl
2M (71g/l Cl-) de-aerated and acidified with HCl at 23°C
This value is sensitive to the alloy elements which improve the
passivity and limit active dissolution, principally molybdenum,
nickel and chromium (see figure 6 ).
The speed of propagation is also a function of local
aggressiveness and temperature of the medium.
4
3,5
3
Figure 6: Critical current «icrit» at the peak of activity for
various stainless steels in NaCl 2M (71g/l de Cl-) deaerated and
acidified with HCl at 23°C.
2,5
2
1,5
DX2205
1
0,5
10
15
20
25
30
35
PREN (%Cr+3,3%Mo+16%N)
B/ Propagation of corrosion
Once corrosion is initiated, its propagation occurs by active
dissolution of the material in the crevice.
In the laboratory, we simulate this type of corrosion by
recording the potentiodynamic scans in chloride mediums of
increasing acidity.
Critical current «icrit» at the peak of activity for various stainless steels in NaCl 2M
+(71g/l de Cl-)
Depassivation pH of various stainless steels in NaCl 2M ( 71g/l Cl-)
de-aerated and acidified with HCl at 23°C
If on a recording we detect a current peak (activity), crevice
corrosion is starting , in the opposite case repassivation takes
place.
Activity peak measurement for a pH lower to the depassivation
pH can then be considered to quantitatively compare the
speed of crevice corrosion propagation for different grades.
2500
K03/K09/K10
2000
Martensitic
K30/K31
K41
K39
1500
K45
1000
164Mn
165MnL
500
174Mn
K44
DX2202
187L DX2304
177A
0
1
3
5
189E
1810T
7
1711MT
1810L 1811ML 1812MS 1813MS
9
DX2205
11
0,2%Cr+%Mo+0,4%Ni
>
Our recommandation
Our first recommendation to avoid crevice corrosion is to optimise the design of the piece to avoid all artificial
crevices. An artificial crevice can be created by a badly made joint, a rough or bad weld, deposits, gaps between
two plates etc.,
If the confined zone is unavoidable, it is preferable to enlarge this zone and not to make it smaller.
If the design of the pièce is not modifiable or if the fabrication process makes difficult to avoid confined zones,
the risk of crevice corrosion is very high . We recommend, in this case, choosing an appropriate grade, in particular
a stainless steel austenitic or duplex when the product will be in contact with corrosive media or part of the
process equipment.
If ferritics with 20% chromium limit the risk of crevice corrosion initiation, they do not, with the exception of
K44 containing 2% molybdenum, curb its propagation unlike austenitic or duplex more highly nickel and / or
molybdenum alloyed.
Intergranular Corrosion
To understand the phenomenom
At temperatures greater than 1035°C , the carbon is in solid
solution in the matrix of the austenitic stainless steels.
However, when these materials are cooled slowly from these
temperatures or even heated between 425 and 815 °C,
chromium carbides precipitate at the grain boundaries. These
carbides have a higher chromium content in comparison to
the matrix.
Consequently, the zone directly adjacent to the grain
boundaries is greatly impoverished.
The sensitisation state takes place in a lot of environments
by privileged initiation and the rapid propagation of corrosion
on the de-chromed sites.
For unstabilized ferritic stainless steels, the sensitisation
temperature is greater than 900°C.
13
>
Our recommandation
In practice , this case of corrosion can be encountered in the welded zones . The solution for the austenitic
consists of using a low carbon grade called « L » (Low C%<0.03%) or a stabilized grade, and the titanium or
niobium stabilized ferritic grades.
The volume of the piece permiting a thermal treatment of the quenching type (rapid cooling) at 1050/1100°C or
a tempering of the welded piece can be done.
Stress Corrosion
To understand the phenomenon
We mean by « stress corrosion » the formation of cracks
which start after a period of long incubation and which
afterwards can propagate very rapidly and provoke downtime
of the equipment by cracking .
This particularly dangerous phenomenon is the result
of the combined effects of 3 parameters:
- temperature, since stress corrosion rarely develops
under 50°C
- the applied or residual stresses
- the corrosiveness of the medium : presence of Cl-, H2S or caustic media NaOH
Aggressive medium
Chlorides
Temperature effects
> Although stress corrosion of ferritics can be provoked by
particularly aggressive tests in the laboratory ,
their body cubic centred structure rarely renders them subject
to this type of phenomena in practice .
> The face cubic centred structure of austenitic stainless
steels can present a risk.
In effect , it favours a mode of planar deformation which can
generate very strong stress concentrations locally. As shows
the graph below, this is particularly true for classic austenitic
stainless steels with 8% nickel; an increase in nickel above
10% is beneficial .
> In austenitic stainless steels, the austenitic stainless steels
with manganese perform worse.
> The austeno-ferritic structure of the duplex gives them
an intermediate behaviour, very close to the ferritics in the
chloride medium and even better in the H2S medium .
1000
cracking
Fissuration
PASSIVE LAYER
No cracking
STAINLESS
contraintes
PASSIVE LAYER
100
Time to crack (hours)
contraintes
10
STAINLESS
CRACKs
The metallurgical structure of stainless steels influences their behaviour in this type
of configuration.
1
0
>
20
40
60
80
Nickel content, wt.%
Effect of nickel content on the resistance to stress corrosion of
Stainless steel containing from 18-20% chromium in magnesium chloride at 154°C
[ From a study by Copson [ref] . Physical Metallurgy of Stress Corrosion Cracking,
Interscience, New York, 247 (1959).]
Our recommandation
To avoid this type of corrosion:
> Suppress the stresses or have a better redistribution, by optimising the design or by
, of the pieces concerned.
a stress relieving treatment after forming and welding
,
> lower the temperature if possible
> If not practicable, choose the grade most adapted, favouring as a solution a
ferritic or duplex but bearing in mind the other corrosion problems encountered .
Uniform Corrosion
To understand the phenomenon
The maximum current reading of the activity peak allows us
to classify the resistance of different grades to this type of
corrosion (see figure 7).
Generally, the higher the current, the faster and greater the
dissolution, thus the less the grade will be resistant.
We see this corrosion in acid mediums. Indeed, below a critical
pH value, the passive layer protecting the stainless steel
is no longer stable and the material suffers a generalised
active dissolution. The more acid the medium, the faster the
corrosion and the loss of thickness of the stainless steel.
In the laboratory, we measure this speed of corrosion in an
acid medium by graphing the polarisation curve (see below).
An increasing potential scan is imposed on the metal and the
corresponding intensity is recorded.
100
10
Polarization curve in an acidic medium
Potential U
1
Transpassivity
0.1
Passivity
0.01
K09
Pre-passivity
Up
B
K03
K30
K41
K39
K45
K44
16-4 18-9E 17-4 18-11
Mn
Mn
ML
Figure 7: Critical current «icrit» at the peak maximum in H2SO4
2M de-aerated at 23°C
Activity
Immunity
M
UP =
Current
density (I)
Cathodic Curve
Anodic Curve
Pitting Potential
In a low oxidising medium, the cathodic curve (M) cuts the anodic curve
below the pitting potential: metal remains intact.
In a strong oxidising medium, the cathodic curve (B) cuts the anodic curve
above the pitting potential: pits appear on the surface of metal.
Aperam Stainless Europe
Le Cézanne - 30, Avenue des Fruitiers
Aperam Stainless Europe
FR-93212 La Plaine Saint Denis Cedex
Le Cézanne - 30, Avenue des Fruitiers
FR-93212 La Plaine Saint Denis Cedex
>
Our recommandation
To avoid this type of corrosion, choose
the appropriate grade in regard to the
acid medium used.
We note the favourable impact of
chromium and molybdenum which
reinforce the existing passive film but also the combined
effect of noble alloys (nickel, molybdenum and copper) which
slow down the dissolution of the material when the stability
of its passive layer is broken.
Information
Information
Tel. : +33 1 71 92Information
06 66
www.aperam.com/stainlesseurope
Fax : +33 1 71 92
07: 98
Tel.
+33 1 71 92 06 66
stainless.europe@aperam.com
www.aperam.comFax : +33 1 71 92 07 98
stainless.europe@aperam.com
www.aperam.com
stainless.europe@aperam.com
September 2013, Aperam Stainless Europe. FT-Corrosion.en. While everycare has been taken to ensure that the information contained in this publication is as accurate as possible, Aperam Stainless Europe cannot guarantee that it is complete nor that it is free from error.
KARA® is a brand of Aperam Stainless Europe, registered in numerous countries. © Aperam.
This is the dissolution of all the affected points on the
surface of the material which are attacked by the corrosive
medium. On the micrographic scale,this corresponds to a
regular uniform loss of thickness or loss of weight (uniform or
generalised corrosion as opposed to localised corrosion).