Passivity ferritic-austenitic steels

The ferritic-austenitic steels are passive in seawater due to their high chromium content and do not suffer from general corrosion. Their resistance to pitting and crevice corrosion is raised by the molybdenum and nitrogen components. The duplex steels have therefore proved their worth well in a wide range of marine engineering apphcations [131-134]. 

In addition to ferritic-austenitic steels (duplex steels), austenitic CrNiMo steels can also be used in waste water treatment plants. These steels are sufficiently passive in most waste waters so that uniform surface corrosion can be practically neglected however, under critical conditions, they can exhibit local corrosive attack in the form of pitting corrosion, crevice corrosion, or stress corrosion cracking. The decisive evaluation criteria for their possible application with respect to the media... 

Most descaling and passivation processes for steels were developed prior to the widespread use of electrochemical techniques. As a result, a variety of visual and chemical tests are widely used for determining the surface cleanliness. Chemical tests have also been established to verify the presence of a robust oxide film on austenitic and ferritic stainlesses (8). These methods are very simple to conduct in a manufacturing environment, but they are qualitative in nature and rely strongly on the judgment of the inspector. Outside of the laboratory, electrochemical methods have not been widely used to evaluate cleanliness of carbon and alloy steels after pickling. Nevertheless, they are well suited for this purpose and have been examined in considerable detail in laboratory studies. 

Austenitic stainless steels appear to have significantly greater potential for aqueous corrosion resistance than their ferritic counterparts. This is because the three most commonly used austenite stabilizers, Ni, Mn, andN, all contribute to passivity. As in the case of ferritic stainless steel. Mo, one of the most potent alloying additions for improving corrosion resistance, can also be added to austenitic stainless steels in order to improve the stability of the passive film, especially in the presence of Cl ions. The passive film formed on austenitic stainless steels is often reported to be duplex, consisting of an inner barrier oxide film and outer deposit hydroxide or salt film. 

One of the most effective elements added to austenitic stainless steel, and for that matter even ferritic stainless steel, in order to improve pitting resistance is Mo [8]. Molybdenum, however, is a highly versatile element, existing in the passive film in a number of oxidation states. In the case of the hexavalent state it has been observed in both the cationic and anionic states, namely as molybdenum trioxide and ferrous molybdate. It has most commonly been reported to exist in the quadrivalent state as molybdenum dioxide and oxyhydroxide. 

Cr, 8.73% Ni, 2.85% Mo, 0.70%Ma, 0. 55%"SL 0.24% 11, 0., 05% C, Alter quenching from U50°C fee steel contains 6 - ferrite and y - austenite. The 8 - phase is richer in Gr and poorer in Ni than fee second phase. Consequently 8 -ferrite will, be selectively etched over a wide range of potential whereas y - austenite will remain unetched (Fig. 15). Below this potential, fee two phases will be passive over a large... 

As dealt with in previous ehapters, the eorrosion resistanee of stainless steel is due to passivation by a surface film of chromium oxide. The chromium content is higher than about 11%, and the low-temperature corrosion resistance as well as the resistance to oxidation and mill scale formation at high temperature increase with increasing content of chromium. Pure chromium steels are either ferritic (low C-content, non-hardenable by heat treatment) or martensitic (traditionally higher C-content for most grades, hardenable by heat treatment). Wifli sufficient content of Ni the structure becomes austenitic, which gives increased formability, weldability. 

In ferritic and austenitic stainless steels, crevice corrosion is almost always initiated by local activation. This can be induced in a crevice by oxidant depletion, if necessary supplemented by halides. The passivity then breaks down. The access of oxidants to the material surface, and hence the passivity, may also be hindered by local deposits. 

The chromium concentration plays a dominant role in the passivation of ferritic as well as austenitic stainless steels and stainless nickel-based alloys (e.g.. Alloys 600 and 690). The reason for this is the marked enrichment of Cr " in the passive films. 

On stainless steels and on nickel-based stainless alloys, the passive film can be described by the bilayer model already presented. The concentration of Cr " in the inner oxide layer is much higher than the nominal chromium content of the alloy. The compositions of passive films formed on ferritic (Fe-Cr) and austenitic (Fe-Cr-Ni) stainless steels, and on Alloys 600 and 690... 

Table 3-2. Thickness and composition of passive films formed on ferritic and austenitic stainless steels and on some nickel-based stainless alloys (Alloys 600 and 690) in acidic solutions at 25 °C.

Figure 7-29. SEM micrographs of the etched surfaces of 5000 h aged duplex stainless steel, (a) At -268 mV(Ag/AgCl) leading to active dissolution in austenite and ferrite (b) -170 mV (Ag/AgCl) (c) -130 mV (Ag/AgCl) leading to active dissolution of ferrite, while austenite is passive at both potentials. Electrolyte 0.1 MH2SO4 + O.OI M KSCN (Jiangetal., 1992).

The passive CPS consists of absorber elements in 12 fuel assemblies of the first core zone and 36 fuel assemblies of the second core zone. The passive absorber rods are kept in the same position below the core as the active absorber rods using special triggers that are bimetallic plates made of steel of ferritic-martensitic and austenitic grades with differing coefficients of thermal expansion. The trigger is installed on top of the fuel assembly central tube. When coolant temperatures at the fuel assembly outlet exceed 900 K, the thermal deformation of a trigger reaches its critical value leading to release of the shaft and flow up of the passive absorber rod. 

Fe-Cr and Fe-Cr-Ni alloys are of high technical importance, the main benefit for ferritic and austenitic stainless steels resulting from the excellent corrosion resistance of Cr203 layers. Figure 5.31 shows the polarization curve of Fe-15 Cr in 0.5 M H2SO4 [92]. Its characteristic features are determined by the electrochemical properties of the pure alloy components. Hydrogen evolution (with cathodic currents) is observed up to E = -0.2 V followed by the potential range of active dissolution of Cr and Fe + up to OV where passivity starts due to... 

The electrochemistry of Cl-SCC in duplex (a-y) stainless steels was discussed earlier and displayed in Figure 11.17. The ferrite phase has a higher value, due to its higher Cr content, but dissolves more rapidly (due to its lower Ni content) when both phases are in the active state [124]. The crack approaches (from above, due to IR limifafions) a mixed potential where the austenite is a net cathode and the ferrite a net anode. The ferrite is thus polarized above its normal (isolated) cracking potential and cracks rapidly, while the austenite is below its isolated cracking potential and cracks slowly or not at all. Cracks propagate in the ferrite and tend to be arrested by the austenite. Another situation is possible if toe ferrite can remain passive in the crack while the austenite corrodes now cracking occurs in the austenite and is hindered by the ferrite. 

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