What is corrosion?
Metals, with the exception of the precious metals such What are the major factors of corrosion? as gold and platinum, that are found in their natural state are always extracted from ores; metals have therefore a Medium Design tendency to revert to their stable state, which corresponds to their original state, that is to say their oxide form. Chemical nature Concentration Surface state Metal corrosion is essentially an electro-chemical reaction at Oxidising power Shape the interface between metal and surrounding environment . pH (acidity) Assembly (welds, rivets) Temperature Mechanical stresses Pressure Proximity to other metals Contact Viscosity with a medium Stainless Steel and the passive layer Solid deposits (partial or total immersion) Agitation Methods of protection Steel is an alloy of iron and carbon. Contrary to carbon steel, Corrosion factors the presence of a minimum of 10.5 % chromium in the Composition Ageing of the structure stainless steel gives it the property of corrosion resistance. Manufacturing Evolution of stresses Metallurgical state Temperature variability (thermal or mechanical Modification of coatings treatment) Maintenance frequency Additives Impurities
Material Time
Chromium What are the 5 principal types of corrosion linked with >10,5% the surrounding environment ?
Iron Carbon Fe+C=steel <1,2% 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 Crevice Reaction of steel and stainless steel in contact environment with moisture in the air or water.
Steel Stainless Steel
Formation of iron oxide Formation of chromium Under (rust) oxide stress
O2
Fe + C Fe + C + Cr > 10,5 %
Generalised Inter- Rust granular Passive layer acid medium However if this protective layer is damaged, the start of corrosion can appear Generalised corrosion is noticed when stainless steel is In the document which follows we describe the 5 principal in contact with an acid medium and localised corrosion types of corrosion and we rank the majority of Stainless is seen in the majority of cases when stainless steel is Europe grades from standard laboratory tests placed in a neutral chloride environment However, the phenomena of corrosion in real life are always specific, the data described does not exclude extra trials to La corrosion par piqûre choose the optimal material. Pitting Corrosion
To understand the phenomenon > If the potential of the stainless steel in the given medium is Pitting corrosion is a local break in the passive layer of the superior to the pitting potential =>the stainless steel corrodes stainless steel provoked by an electrolyte rich in chloride Note: the higher the pitting potential, better shall be the and or sulphides. At the site of the pitting, where the metal corrosion resistance of the grade. Outside the pits, the is unprotected, corrosion will develop if the pit does not passive layer is always present to protect the stainless steel. re-passivate, in other words if the speed of metal dissolution enables to maintain a sufficiently aggressive environment to Figure 1 shows pitting potentials obtained for different prevent its re-passivation. stainless steels in water containing 0.02M NaCl (710mg/l Cl-) at 23°C. Passive layer It shows the influence on the resistance to pitting corrosion Metal with the content of chromium and molybdenum for the ferritics, and chromium, molybdenum and nitrogen content
METAL ATTACK for the austenitics. Chlorides mV/SCE Figure 1 Passive layer 800 K44 Metal
700 2 cases 17-11MT 18-11ML The metal re-passivates Corrosion of the base metal K45 600 18-9E/L Passive Layer 18-10T K36 17-7A/C/E 18-7L Metal K41 500 K39
This dissolution gives rise to metal ions and electrons and 17-4Mn 16-4Mn 16-5MnL thus the passage of current (of dissolution) which gives rise 400 to an electrical potential difference between the anodic zone K30-K31 K03 (pitting) and the cathodic zone (the rest of the metal). K09 300 K10 Martensitic Pitting potential in water containing 0.02MNaCI pH=6,6 at 23°C (mV/SCE ) (mV/SCE 23°C at pH=6,6 0.02MNaCI containing water in potential Pitting
To simulate this type of corrosion under laboratory circumstances, a sample is immersed in a corrosive electrolyte 200 to which an increasing potential is applied until the passive 10 12 14 16 18 20 22 24 26 layer is broken. PREN (%Cr+3,3%Mo+16%N) During this dynamic potential (intensity/potential) scan Standards the sudden increase in intensity corresponds to the pitting Commercial Designations ASTM pitting EN potential E Designation: Type Martensitic 420 1.4021 IntensitY K09+ 409 1.4512 I =50µA/cm2 K03 1.4003 K10 410S 1.4000 K30-K31 430 1.4016 - 1.4017 K39 439 1.4510 K41 441 1.4509 K36 436 1.4526 Epitting POTENTIAL K45 445 1.4621 Passive Field K44 444 1.4521 16-4Mn 201.2 1.4372 16-5Mn 201LN 1.4371 17-4Mn 201.1 1.4618 17-7A/C/E 301 1.4310 The pitting potential corresponds to the potential necessary 18-7L 301LN 1.4318 to initiate stable pits. 18-9L 304L 1.4307 > If the potential of the stainless steel in the given medium is 18-11 ML 316 - 316 L 1.4401 - 1.4404 inferior to the pitting potential => pitting does not start 17-11MT 316Ti 1.4571 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)
700 500
450 K44 K44 600 18-11ML 400 18-11ML 500 350
18-9E/L 18-9E/L 300 400 K36 K41 K36 K41 K39 250 K39 300 200 K30
200 K03-K09-K10 150
100 K30 100 Pitting potential in water containing 0.02M NaCI pH=6,6 at 23°C (mV/SCE)23°C pH=6,6 at NaCI 0.02M containing water in potential Pitting 50 Pitting potential in water containing 0.5M NaCI pH=6,6 at 50°C (mV/SCE) K03-K09-K10 0 0 10 12 14 16 18 20 22 24 26 10 12 14 16 18 20 22 24 26 PREN (%Cr+3,3%Mo+16%N) 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.
1200 23° 50° 23°
700
23° 600 Standards No pits No pits No pits ASTM 500 Commercial
50° Grades Designation EN 23° 23° 400 Type UNS
Figure 4 Figure 23° DX2202 2202 UNS 32202 1.4062 300
50° DX2304 2304 UNS 32304 1.4362 50° DX2205 2205 UNS 32205 1.4462 200 50° 50° 100
Pitting potential in water containing 0.5M NaCl, at 23°C and 50°C (mV/SCE)50°C and 23°C at NaCl, 0.5M containing water in potential Pitting 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? HOW? Break in the passive layer Confined zone and metal attack (acidification) Some pHd values for our stainless steels are given in figure 5. If on a recording we detect a current peak (activity), crevice The lower the value pHd the better the resistance to crevice corrosion is starting , in the opposite case repassivation takes corrosion. 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. 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 ).
4 The speed of propagation is also a function of local
) aggressiveness and temperature of the medium. - 3,5
3
Figure 6: Critical current «icrit» at the peak of activity for - 2,5 various stainless steels in NaCl 2M (71g/l de Cl ) deaerated and acidified with HCl at 23°C.
2
2500 Martensitic 1,5 K03/K09/K10 de-aerated and acidified with HCl at 23°C at HCl with acidified and de-aerated
DX2205 1 2000 K30/K31 K41 Depassivation pH of various stainless steels in NaCl 2M ( 71g/l Cl 71g/l ( 2M NaCl in steels stainless various of pH Depassivation
0,5 K39 1500 ) 10 15 20 25 30 35 - PREN (%Cr+3,3%Mo+16%N)
K45 +(71g/l de Cl de +(71g/l 1000 B/ Propagation of corrosion
Once corrosion is initiated, its propagation occurs by active 2MNaCl in steels stainless various for activity of peak the at » 164Mn crit 500 165MnL DX2202 dissolution of the material in the crevice. 174Mn In the laboratory, we simulate this type of corrosion by K44 187L DX2304 177A 1810T 1711MT recording the potentiodynamic scans in chloride mediums of 189E 1810L 1811ML 1812MS 1813MS
Critical current «i current Critical 0 DX2205 increasing acidity. 1 3 5 7 9 11 13 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 Consequently, the zone directly adjacent to the grain At temperatures greater than 1035°C , the carbon is in solid boundaries is greatly impoverished. solution in the matrix of the austenitic stainless steels. The sensitisation state takes place in a lot of environments However, when these materials are cooled slowly from these by privileged initiation and the rapid propagation of corrosion temperatures or even heated between 425 and 815 °C, on the de-chromed sites. chromium carbides precipitate at the grain boundaries. These For unstabilized ferritic stainless steels, the sensitisation carbides have a higher chromium content in comparison to temperature is greater than 900°C. the matrix. 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 > Although stress corrosion of ferritics can be provoked by To understand the phenomenon particularly aggressive tests in the laboratory , their body cubic centred structure rarely renders them subject We mean by « stress corrosion » the formation of cracks to this type of phenomena in practice . which start after a period of long incubation and which afterwards can propagate very rapidly and provoke downtime > The face cubic centred structure of austenitic stainless of the equipment by cracking . steels can present a risk.
This particularly dangerous phenomenon is the result In effect , it favours a mode of planar deformation which can of the combined effects of 3 parameters: generate very strong stress concentrations locally. As shows the graph below, this is particularly true for classic austenitic - temperature, since stress corrosion rarely develops stainless steels with 8% nickel; an increase in nickel above under 50°C 10% is beneficial .
- the applied or residual stresses > In austenitic stainless steels, the austenitic stainless steels with manganese perform worse. - - the corrosiveness of the medium : presence of Cl , H2S or caustic media NaOH > 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 .
Chlorides 1000 Temperature effects Aggressive medium
PASSIVE LAYER Fissuration cracking
No cracking contraintes STAINLESS contraintes 100
PASSIVE LAYER
Time to crack (hours)crack to Time 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 a stress relieving treatment after forming and welding, of the pieces concerned. > 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
This is the dissolution of all the affected points on the The maximum current reading of the activity peak allows us surface of the material which are attacked by the corrosive to classify the resistance of different grades to this type of medium. On the micrographic scale,this corresponds to a corrosion (see figure 7). regular uniform loss of thickness or loss of weight (uniform or generalised corrosion as opposed to localised corrosion). 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 100 acid medium by graphing the polarisation curve (see below). An increasing potential scan is imposed on the metal and the corresponding intensity is recorded.
10
Polarization curve in an acidic medium
Potential U 1
Transpassivity
0.1
Passivity
0.01 K09 K03 K30 K41 K39 K45 K44 16-4 18-9E 17-4 18-11 Mn Mn ML
Pre-passivity Up B Figure 7: Critical current «icrit» at the peak maximum in H2SO4 2M de-aerated at 23°C Activity
Immunity M Current density (I) 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. error. from nor that it is free that it is complete guarantee cannot Europe Stainless Aperam as possible, is as accurate in this publication contained that the information ensure to has been taken While everycare
Cathodic Curve Anodic Curve Our recommandation UP = Pitting Potential 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 In a low oxidising medium, the cathodic curve (M) cuts the anodic curve reinforce the existing passive film but also the combined below the pitting potential: metal remains intact. effect> of noble alloys (nickel, molybdenum and copper) which In a strong oxidising medium, the cathodic curve (B) cuts the anodic curve slow down the dissolution of the material when the stability is a brand of Aperam Stainless Europe, registered in numerous countries. © Aperam. countries. in numerous registered Europe, Stainless of Aperam is a brand above the pitting potential: pits appear on the surface of metal. of its passive layer is broken. ® September 2013, Aperam Stainless Europe. FT-Corrosion.en. Aperam Stainless Europe. FT-Corrosion.en. September 2013, KARA
Information Aperam Stainless Europe Information Le Cézanne - 30, Avenue des Fruitiers Tel. : +33 1 71 92Information 06 66 Aperam Stainless Europe www.aperam.com/stainlesseurope FR-93212 La Plaine Saint Denis Cedex Fax : +33 1 71 92Tel. 07 : 98+33 1 71 92 06 66 Le Cézanne - 30, Avenue des Fruitiers [email protected] www.aperam.comFax : +33 1 71 92 07 98 FR-93212 La Plaine Saint Denis Cedex [email protected] [email protected]