**4.1 General**

It is well known that nitrogen in high alloyed steels improves the corrosion resistance; this is especially true for pitting and crevice corrosion. Additionally, nitrogen helps to prevent the alloy from stress corrosion cracking, though in an oblique way. Generally, the beneficial effect of nitrogen can be led back to an enrichment of nitrogen at the oxide/metal-interface and its influence on passivation. It has been reported that the enrichment increases with increasing potentials. However, these mechanisms have been discussed controversially [Mudali & Rai, 2004]:


Other intermetallic phases, such as Laves-phase, Z-phase and -phase have been investigated with regards to nitrogen alloying within literature. Generally, nitrogen seems to delay the formation of intermetallic phases. The underlying mechanism has been discussed controversially; the enhanced solubility of Chromium and Molybdenum due to nitrogen or its influence on Gibbs free energy for phase formation [Mudali & Rai, 2004]. The following table will provide an overview about the influence of nitrogen on some

M6C appears instead of

increasing nitrogen contents

intermetallic phases [Mudali & Rai, 2004], [Heino et al., 1998].

precipitation

**Laves** Precipitation is shifted to higher

**4. Corrosion resistance of HNS** 

not be covered.

[Mudali & Rai, 2004]:

reported.

**4.1 General** 

intermetallic phases. [Mudali & Raj, 2004], [Heino et al., 1998]

a formation of nitrides or mixed nitride layer, e.g. Ni2Mo3N

gradient of the passivation film and reject Cl--ions

 Suppresses formation of Shifted to longer times

**Description Influence of nitrogen… Remark** 

Narrows temperature range for

temperature but accelerated **R** See Laves

**M23C6** Suppresses formation of M23C6 Can be replaced by M6C with

Table 3. Overview about the role of nitrogen alloying on the precipitation behavior of some

The role of nitrogen in stainless steel with regards to the corrosion resistance has been previously reported within literature [Pleva, 1991], [Truman, 1988], [Pedrazzoli &Speidel, 1991], [Dong et al., 2003], [Mudali & Rai, 2004]. This chapter will review today´s knowledge and present own data; corrosion fatigue and high temperature corrosion will

It is well known that nitrogen in high alloyed steels improves the corrosion resistance; this is especially true for pitting and crevice corrosion. Additionally, nitrogen helps to prevent the alloy from stress corrosion cracking, though in an oblique way. Generally, the beneficial effect of nitrogen can be led back to an enrichment of nitrogen at the oxide/metal-interface and its influence on passivation. It has been reported that the enrichment increases with increasing potentials. However, these mechanisms have been discussed controversially

enrichment of negatively charged N-ions, i.e. N-. These ions will lower the potential

 formation of Cr2N. A high local Cr-concentration should improve the corrosion resistance. However, this approach is unlikely as nitrogen does not change the matrix composition underneath the passivation film. No Cr depletion whatsoever has been Nitrogen does not show any influence on the thickness of the oxide layer, investigations of various alloys with different nitrogen contents have confirmed an average thickness of 12-22 Å [Mudali & Rai, 2004] within all varieties.

A synergism of nitrogen and molybdenum is suggested by many authors [Mudali & Rai, 2004], [Pickering, 1988], [Pedrazzoli & Speidel, 1991]. Molybdenum shifts the metal dissolution to higher potentials which will consequently lead to an increased enrichment of nitrogen at the metal/oxide-interface. In this case, nitrogen can lower the current density below the critical value for pitting corrosion [Mudali & Rai, 2004].

It has been suggested that Molybdenum and nitrogen support the formation of highly mobile ions that interact with the passivation film. Addtionally, nitrogen seems to have a buffer effect by reacting as follows in oxidizing corrosive media:

$$\text{[N]} + 4H^{+} + 3e^{-} \rightarrow \text{NH}\_{4}^{+} \tag{4}$$

The formation of NH4+ - ions helps to increase the pH value which results in an improved repassivation and reconditioning of the base material [Mudali & Rai, 2004]. It also has impact on the depronotation, which might explain the good performance in acids and halide containing liquids such as Cl-, Br- and I- [Truman, 1988].

Corrosion Resistance of High Nitrogen Steels 69

Fig. 15. Comparison of nitrogen and carbon on Current-density depending of a given alloy.

**electrode- potential, [mV]** 

Fig. 16. Critical Pitting Temperature (CPT) as a function of PRE. [Pedrazzoli & Speidel, 1991]

[Pedrazzoli & Speidel, 1991]

**current density, [ A/cm2]**

#### **4.2 The role of nitrogen on pitting and crevice corrosion**

Pitting corrosion is a very serious and harmful type of corrosion and is classified as local corrosion, characterized by small holes or pits. Usually, a repassivation cannot be achieved so that these pits can initiate cracks. This is the main reason why pitting often comes along with stress corrosion cracking. Pitting can be determined either by current-density-curves or a critical pitting temperature. It has been reported that nitrogen lowers the passivity in the current-density diagram. In austenitic steels, 1 % nitrogen improves the pitting potential by 600 mV [Pedrazzoli & Speidel, 1991]. Crevice corrosion follows generally the same principles; however the conditions are significantly tougher due to the geometric impact (electrolyte concentration in crevice). This will be covered at a later stage.

Fig. 14. Influence of nitrogen on Current-density depending of a given alloy. [Pedrazzoli & Speidel, 1991]

Pitting corrosion is a well-known corrosion problem for stainless steels. It can come along with sensitivation, i.e. a local Cr depletion can support pitting corrosion. Therefore, any segregation, welding joint, heat treatment etc. can have an impact on pitting corrosion.

The critical pitting temperature (CPT) is defined at what temperature pitting occurs. A common range for stainless steels is 10-100 °C and obviously depends on the alloy composition. [Pedrazzoli & Speidel, 1991] has reported that the critical temperature for crevice corrosion (CCT) is approx. 20 ° lower compared to pitting, see fig.16 & 17 for details.

Pitting corrosion is a very serious and harmful type of corrosion and is classified as local corrosion, characterized by small holes or pits. Usually, a repassivation cannot be achieved so that these pits can initiate cracks. This is the main reason why pitting often comes along with stress corrosion cracking. Pitting can be determined either by current-density-curves or a critical pitting temperature. It has been reported that nitrogen lowers the passivity in the current-density diagram. In austenitic steels, 1 % nitrogen improves the pitting potential by 600 mV [Pedrazzoli & Speidel, 1991]. Crevice corrosion follows generally the same principles; however the conditions are significantly tougher due to the geometric impact

> **solution annealed austenitic steel, seawater, room temperature**

Fig. 14. Influence of nitrogen on Current-density depending of a given alloy. [Pedrazzoli &

**electrode- potential, [mV] SHE** 

**pitting corrosion** 

Pitting corrosion is a well-known corrosion problem for stainless steels. It can come along with sensitivation, i.e. a local Cr depletion can support pitting corrosion. Therefore, any segregation, welding joint, heat treatment etc. can have an impact on pitting corrosion.

The critical pitting temperature (CPT) is defined at what temperature pitting occurs. A common range for stainless steels is 10-100 °C and obviously depends on the alloy composition. [Pedrazzoli & Speidel, 1991] has reported that the critical temperature for crevice corrosion (CCT) is approx. 20 ° lower compared to pitting, see fig.16 & 17 for details.

Speidel, 1991]

**current density, [ A/cm2**]

**4.2 The role of nitrogen on pitting and crevice corrosion** 

(electrolyte concentration in crevice). This will be covered at a later stage.

Fig. 15. Comparison of nitrogen and carbon on Current-density depending of a given alloy. [Pedrazzoli & Speidel, 1991]

Fig. 16. Critical Pitting Temperature (CPT) as a function of PRE. [Pedrazzoli & Speidel, 1991]

Corrosion Resistance of High Nitrogen Steels 71

A commonly accepted ranking of alloys in terms of their pitting addiction is the Pitting

It has been suggested by [Pleva, 1991] to use x = 16 for steels containing Mo < 4,5 % and

The PRE does not take any other elements but Cr, Mo and N into account. [Speidel & Theng-Cui, 2003] has suggested a new figure to include also C, Mn and Ni into the equation and

 MARC = Cr (%) + 3,3 Mo (%) + 20 C (%) + 20 N (%) - 0,5 Mn (%) – 0,25 Ni (%) (6) The MARC-equation is the first formula that considers carbon to be beneficial against pitting. [Bernauer & Speidel, 2003] has suggested a high carbon + nitrogen alloyed steel with improved pitting resistance. This is due to the higher thermodynamic stability of Cr-Mn-N-C systems compared to carbon-free Cr-Mn-N steels. However, both carbon and nitrogen

A very global description of the influence of alloying elements on pitting potential was published by [Pedrazzoli & Speidel, 1991]. As seen in fig. 19, nitrogen and molybdenum

Fig. 19. Influence of various alloying elements on the pitting potential. [Pedrazzoli &

has defined MARC (**M**easure of **a**lloying for **r**esistance to **c**orrosion):

must not form any precipitations but stay into solid solution.

have a significant impact on the potential shift.

Speidel, 1991]

PRE = Cr (%) + 3,3 Mo (%) + x N (%) whereas x = 13…30 (5)

resistance equivalent (PRE). It is defined as

x = 30 for steels containing Mo 4,5 – 7,0 %.

Fig. 17. Critical Crevice Temperature (CCT) as a function of PRE. [Pedrazzoli & Speidel, 1991]

Fig. 18. MARC equation to rank various alloying element in regards to the alloy pitting resistance. [ Speidel & Theng-Cui, 2003]

Critical crevice corrosion, temperature austenitic stainless steels in H2O+ 6%FeCl3

Fig. 17. Critical Crevice Temperature (CCT) as a function of PRE. [Pedrazzoli & Speidel, 1991]

 **critical crevice corrosion temperatur, T,[°C]** 

**PRE (%Cr + 3.3 x %Mo + 30 x %N)**

Fig. 18. MARC equation to rank various alloying element in regards to the alloy pitting

resistance. [ Speidel & Theng-Cui, 2003]

A commonly accepted ranking of alloys in terms of their pitting addiction is the Pitting resistance equivalent (PRE). It is defined as

$$\text{PRE} = \text{Cr}\left(\%\right) + \text{3.3 Mo}\left(\%\right) + \text{x N}\left(\%\right) \text{ whereas x} = 13...30 \tag{5}$$

It has been suggested by [Pleva, 1991] to use x = 16 for steels containing Mo < 4,5 % and x = 30 for steels containing Mo 4,5 – 7,0 %.

The PRE does not take any other elements but Cr, Mo and N into account. [Speidel & Theng-Cui, 2003] has suggested a new figure to include also C, Mn and Ni into the equation and has defined MARC (**M**easure of **a**lloying for **r**esistance to **c**orrosion):

$$\text{MARC} = \text{Cr}\left(\%\right) + \text{3.3 Mo}\left(\%\right) + \text{20 C}\left(\%\right) + \text{20 N}\left(\%\right) - \text{0.5 Mn}\left(\%\right) - \text{0.25 Ni}\left(\%\right) \tag{6}$$

The MARC-equation is the first formula that considers carbon to be beneficial against pitting. [Bernauer & Speidel, 2003] has suggested a high carbon + nitrogen alloyed steel with improved pitting resistance. This is due to the higher thermodynamic stability of Cr-Mn-N-C systems compared to carbon-free Cr-Mn-N steels. However, both carbon and nitrogen must not form any precipitations but stay into solid solution.

A very global description of the influence of alloying elements on pitting potential was published by [Pedrazzoli & Speidel, 1991]. As seen in fig. 19, nitrogen and molybdenum have a significant impact on the potential shift.

Fig. 19. Influence of various alloying elements on the pitting potential. [Pedrazzoli & Speidel, 1991]

Corrosion Resistance of High Nitrogen Steels 73

Fig. 21. Influence of cold working on current-density of X8CrMnN 18-18 (37 % cold work).

The surface enrichment theory is based on the general idea, that nitrogen is build into the lattice underneath the passive layer in solid solution. This nitrogen rich layer shall avoid dissolution of the substrate. Tentatively, there are chemical reactions with Cr and Mo who might change the local potentials as well. The formation of various N-rich phases has been reported, such as Cr2N or Ni2Mo3N [Mudali & Rai, 2004], [Pickering, 1988]. Negatively charged N-ions, i.e. N- are supposed to enrich at the metal/oxide interface. These ions will

This theory covers the formation of pit growth inhibiting species. It is basically linked with the ammonia formation theory. The NH4+-formation in the pit tip appears to happen

*NH H O NH OH H* 42 4


<sup>4</sup> []4 3 *N H e NH* (8)

(9)

42 2 *NH OH H O NO H e* 7 6 (10)

22 3 *NO H O NO H e* 2 2 (11)

42 2 *NH H O NO H e* 2 86 (12)

42 3 *NH H O NO H e* 3 10 8 (13)

UR: -250 / UL: 780 mV / U: -1030 mV / 1m H2SO4+ 0,5m NaCl [ETE-11]

lower the potential gradient of the passivation film and reject Cl-

The repassivation by NH4+ can be described as follows:

quicker than the OH—formation due to oxide reduction at the pit entrance.

**4.4 Surface enrichment theory** 

**4.5 Inhibitive nitrate formation theory** 

Cold forming is supposed to have an impact on corrosion resistance; however the role of nitrogen in this case is not fully clear. Within Literature, the following has been reported [Mudali & Rai, 2004]:


The improved pitting potential at low deformation rates i.e. below 20 % is due to the decreased tendency for twin formation. At higher deformation rates, deformation bands will appear which will be influenced by nitrogen (width and dislocation configuration). [Pleva, 1991] reports that the degree of cold working does not show any influence on the pitting corrosion. This has been also confirmed by own data on X8CrMnN 18-18 material, see fig.20 & 21 [ETE-11].

Various investigations have tried to explain the mechanism of nitrogen in terms of pitting. It has been agreed, that nitrogen stabilizes the -range in a very clear way. This is important to prevent -ferrite, esp. in Mo containing alloys. It also supports a homogenous, single-phase microstructure and avoids carbides to precipitate.

#### **4.3 Ammonium theory**

Nitrogen and Molybdenum obviously show a synergism in regards to pitting. Molybdenum seems to support the Cr2O3-formation by acting as an electron acceptor. This also leads of a depronotation of hydroxides. In addition, nitrogen reacts as follows:

$$\text{H}\_{\text{1}}\text{[N]} + 4\text{H}^{+} + \text{3}e^{-} \rightarrow \text{NH}\_{\text{4}}^{+} \tag{7}$$

The NH4 + - formation will increase the pH-value which support the repassivation. [Mudali & Rai, 2004] reports that NH4 + - ions have been confirmed by XPS within the passivation layer.

Fig. 20. Influence of cold working on current-density of X8CrMnN 18-18 (0 % cold work). UR: -250 / UL: 722 mV / U: -972 mV / 1m H2SO4+ 0,5m NaCl [ETE-11]

Cold forming is supposed to have an impact on corrosion resistance; however the role of nitrogen in this case is not fully clear. Within Literature, the following has been reported

 cold forming in nitrogen alloyed steels: a cold forming degree up to 20 % improves the critical pitting potential (CPP). A drop in CPP at higher deformation rates has been

The improved pitting potential at low deformation rates i.e. below 20 % is due to the decreased tendency for twin formation. At higher deformation rates, deformation bands will appear which will be influenced by nitrogen (width and dislocation configuration). [Pleva, 1991] reports that the degree of cold working does not show any influence on the pitting corrosion. This has been also confirmed by own data on X8CrMnN 18-18 material, see fig.20

Various investigations have tried to explain the mechanism of nitrogen in terms of pitting. It has been agreed, that nitrogen stabilizes the -range in a very clear way. This is important to prevent -ferrite, esp. in Mo containing alloys. It also supports a homogenous, single-phase

Nitrogen and Molybdenum obviously show a synergism in regards to pitting. Molybdenum seems to support the Cr2O3-formation by acting as an electron acceptor. This also leads of a

Fig. 20. Influence of cold working on current-density of X8CrMnN 18-18 (0 % cold work).

UR: -250 / UL: 722 mV / U: -972 mV / 1m H2SO4+ 0,5m NaCl [ETE-11]

+ - formation will increase the pH-value which support the repassivation. [Mudali &

<sup>4</sup> []4 3 *N H e NH* (7)

+ - ions have been confirmed by XPS within the passivation layer.

cold forming in stainless steels: no significant influence on pitting potential

[Mudali & Rai, 2004]:

reported.

& 21 [ETE-11].

The NH4

**4.3 Ammonium theory** 

Rai, 2004] reports that NH4

microstructure and avoids carbides to precipitate.

depronotation of hydroxides. In addition, nitrogen reacts as follows:

Fig. 21. Influence of cold working on current-density of X8CrMnN 18-18 (37 % cold work). UR: -250 / UL: 780 mV / U: -1030 mV / 1m H2SO4+ 0,5m NaCl [ETE-11]

#### **4.4 Surface enrichment theory**

The surface enrichment theory is based on the general idea, that nitrogen is build into the lattice underneath the passive layer in solid solution. This nitrogen rich layer shall avoid dissolution of the substrate. Tentatively, there are chemical reactions with Cr and Mo who might change the local potentials as well. The formation of various N-rich phases has been reported, such as Cr2N or Ni2Mo3N [Mudali & Rai, 2004], [Pickering, 1988]. Negatively charged N-ions, i.e. N- are supposed to enrich at the metal/oxide interface. These ions will lower the potential gradient of the passivation film and reject Cl- -ions.

#### **4.5 Inhibitive nitrate formation theory**

This theory covers the formation of pit growth inhibiting species. It is basically linked with the ammonia formation theory. The NH4+-formation in the pit tip appears to happen quicker than the OH—formation due to oxide reduction at the pit entrance.

$$\text{H}\_{\text{1}}\text{[N]} + 4\text{H}^{+} + \text{\text{\textdegree}}e^{-} \rightarrow \text{NH}\_{\text{4}}^{+} \tag{8}$$

The repassivation by NH4+ can be described as follows:

$$NH\_4^+ + H\_2O \to NH\_4OH + H^+ \tag{9}$$

$$\text{NH}\_4\text{OH} + \text{H}\_2\text{O} \to \text{NO}\_2^- + 7\text{H}^+ + 6e^- \tag{10}$$

$$2\text{ NO}\_2^- + H\_2O \rightarrow \text{NO}\_3^- + 2H^+ + 2e^- \tag{11}$$

$$\text{NH}\_4^+ + 2\text{H}\_2\text{O} \rightarrow \text{NO}\_2^- + 8\text{H}^+ + 6e^- \tag{12}$$

$$\text{NH}\_4^+ + \text{3H}\_2\text{O} \rightarrow \text{NO}\_3^- + \text{10H}^+ + 8e^- \tag{13}$$

Corrosion Resistance of High Nitrogen Steels 75

It is commonly known that high alloyed steels are generally sensitive to SCC. The role of nitrogen against SCC has been discussed over the years but it appears that the positive effect of nitrogen is to be seen in a more oblique way. As previously discussed, nitrogen delays the carbide precipitation and avoids a local Cr depletion. Additionally, the crack growth velocity does not only depend on the actual Cr content, but also on the C content. The crack growth is much higher with increased C levels. It has been reported that nitrogen doesn´t have any influence on crack growth velocity at C > 0,5 % . The impact of carbon is therefore

Fig. 23. Crack growth velocity for various alloys as a function of stress intensity. SCC for

[Pickering, 1988] reports that the role of nitrogen is somehow inconsistent. In principle, nitrogen reduces the stacking fault energy which would be diametric to corrosion resistance. Low stacking fault energy in fcc-lattice is undesired in terms of SCC. Elevated nitrogen levels above 0,3 % are reported to be beneficial as the support of passivity is obviously

stainless steels, Water, 23 °C, ventilated. [Pedrazzoli & Speidel, 1991]

**4.7 The role of nitrogen on Stress corrosion cracking (SCC)** 

higher compared to that of nitrogen. [Pedrazzoli & Speidel, 1991]

Crevice corrosion underlies basically the same principles than pitting; due to the geometry of the crevice the corrosion conditions are believed to be more challenging. [Pedrazzoli & Speidel, 1991] has reported that the critical temperature for crevice corrosion is approx. 20 ° lower compared to pitting.

#### **4.6 The role of nitrogen on Intergranular corrosion (IGC)**

IGC is mainly driven by the depletion of Cr at the grain boundaries and/or the precipitations of carbides, usually M23C6. Therefore, Carbon is supposed to be the main driver for IGC but also grain size, cold working and heat treatment have a significant influence on IGC.

Fig. 22. Precipitation of M23C6 and -phase of a given alloy (17Cr-13Ni-4,5Mo) depending on annealing times and temperatures. [Gillessen et al., 1991]

As discussed in the previous chapter (microstructure), nitrogen tends to delay the M23C6 formation as it changes the Cr activity within the carbide. It also increases the passivity (i.e. lowering the current-density) and avoids the formation of ´- martensite at grain boundaries. However, this is only valid as long as nitrogen is in solid solution. It has been reported [Truman, 1988], [Dong et al., 2003] that an excess of nitrogen can lead to Cr2N precipitations on the grain boundaries which can significantly decrease the intergranular corrosion resistance.

Crevice corrosion underlies basically the same principles than pitting; due to the geometry of the crevice the corrosion conditions are believed to be more challenging. [Pedrazzoli & Speidel, 1991] has reported that the critical temperature for crevice corrosion is approx. 20 °

IGC is mainly driven by the depletion of Cr at the grain boundaries and/or the precipitations of carbides, usually M23C6. Therefore, Carbon is supposed to be the main driver for IGC but also grain size, cold working and heat treatment have a significant

Fig. 22. Precipitation of M23C6 and -phase of a given alloy (17Cr-13Ni-4,5Mo) depending

As discussed in the previous chapter (microstructure), nitrogen tends to delay the M23C6 formation as it changes the Cr activity within the carbide. It also increases the passivity (i.e. lowering the current-density) and avoids the formation of ´- martensite at grain boundaries. However, this is only valid as long as nitrogen is in solid solution. It has been reported [Truman, 1988], [Dong et al., 2003] that an excess of nitrogen can lead to Cr2N precipitations on the grain boundaries which can significantly decrease the intergranular

on annealing times and temperatures. [Gillessen et al., 1991]

lower compared to pitting.

influence on IGC.

corrosion resistance.

**4.6 The role of nitrogen on Intergranular corrosion (IGC)** 

#### **4.7 The role of nitrogen on Stress corrosion cracking (SCC)**

It is commonly known that high alloyed steels are generally sensitive to SCC. The role of nitrogen against SCC has been discussed over the years but it appears that the positive effect of nitrogen is to be seen in a more oblique way. As previously discussed, nitrogen delays the carbide precipitation and avoids a local Cr depletion. Additionally, the crack growth velocity does not only depend on the actual Cr content, but also on the C content. The crack growth is much higher with increased C levels. It has been reported that nitrogen doesn´t have any influence on crack growth velocity at C > 0,5 % . The impact of carbon is therefore higher compared to that of nitrogen. [Pedrazzoli & Speidel, 1991]

Fig. 23. Crack growth velocity for various alloys as a function of stress intensity. SCC for stainless steels, Water, 23 °C, ventilated. [Pedrazzoli & Speidel, 1991]

[Pickering, 1988] reports that the role of nitrogen is somehow inconsistent. In principle, nitrogen reduces the stacking fault energy which would be diametric to corrosion resistance. Low stacking fault energy in fcc-lattice is undesired in terms of SCC. Elevated nitrogen levels above 0,3 % are reported to be beneficial as the support of passivity is obviously

Corrosion Resistance of High Nitrogen Steels 77

A heat resistant alloy is based on X15CrMoV12-1 and contains 0,2 % nitrogen (trade name HNS 15). It precipitates fine V(C,N) and provides a good creep resistance and heat resistance up to 650 °C. Above this temperature the appearance of Laves-phase restricts its

A Molybdenium-free version is known as HNS 28 and consists of X28Cr13 with 0,5 % nitrogen. Its purpose was the closed-die casting industry where a good polish and corrosion

A main driver for the development of HNS austenitic steel was the power generation in the 1980´s. A material for retaining rings was required that could resist the mechanical loads but also stress corrosion cracking. This has finally led to the introduction of X8CrMnN 18-18 also known as P900 or 18-18. This alloy combines superb mechanical properties, e.g. high

Further developments have also come up with Mn-stabilized austenites, e.g. X13CrMnMoN 18-14-3 (P2000). This alloy can achieve strength level (YS) beyond 2000 MPa, still with good ductility and corrosion resistance. One should consider heat treatment conditions and

Another market is powder metallurgy, i.e. thermal spraying and metal injection moulding (M.I.M.). The powders are gas atomized and very homogenous in terms of nitrogen and

ductility at elevated strength level with good corrosion resistance.

corresponding part dimensions with regards to the precipitation of Cr2N.

chemical composition. Main consumers are jewelry and general engineering.

Fig. 25. Extruder screw, Cronidur 30 [ETE-11].

usage.

resistance is required.

**5.2 Austenitic steels** 

higher than the influence on stacking fault energy. [Mudali & Raj, 2004] confirms that increased stacking fault energy does improve the SCC resistance. As nitrogen decreases the stacking fault energy it should detrimental to SCC. Carbon, due to the formation of wavy slip bands, should be theoretically beneficial. However, it appears that the role of nitrogen on SCC is fairly complex and depends on the alloy design and corrosion media.

The benefit of nitrogen alloying appears to be more oblique – the delay in M23C6 precipitation and improved pitting corrosion resistance has been recognized to be beneficial against SCC since pits are likely to initiate SCC.
