CrNi steels

Fig. 2-16 Relation between pit density and potential for No. 1.4301 CrNi steel (AISI 304) at 25°C.

Fatigue life can be slightly lengthened by anodic protection or by passivation. In acids even passive stainless CrNi steels suffer corrosion fatigue [104]. Resistance can occur if the passive film itself has a fatigue strength (e.g., in neutral waters [105]). 

In 1987 at the Weira River, four Kaplan turbines of 2.65 m diameter in two power stations were cathodically protected. The turbines were of mixed construction with high-alloy CrNi steels and nonalloyed ferrous materials with tar-EP coating. Considerable corrosion damage occurred prior to the introduction of cathodic protection, which was attributed to bimetallic corrosion and the river s high salt content of c(CT) = 0.4 to 20 g L... 

Figure 21 -3 shows a schematic diagram of the turbine in which the black areas are CrNi steel. The plate anodes are situated in the region of the suction manifold (A 1 to A3) in a triangular arrangement in the impeller ring (A4 to A6) and distributed in the vicinity of the segmented water inlet ring (A7 to A10) (see Fig. 21-4). These four anodes were individually connected to the transformer-rectifier with chloride-resistant pressure-resistant cable.

Four magnetite rod anodes (All to A14), each weighing 9 kg, in perforated polypropylene protection tubes were used to protect the box headers, the in-flow hood and inlet housing as well as the CrNi steel oil cooler. Protection tubes keep out solid matter carried in the river water. The oil cooler was connected to the protection equipment with its own cathode conductor. 

Materials that are corrosion resistant to the expected cathodic polarization qualify as impressed current cathodes. Austenitic CrNi steels are used with strong acids. The oleum (i.e., fuming sulfuric acid) and concentrated sulfuric acid tanks used in sulfonating alkanes and in the neutralization of sulfonic acids are anodi-cally protected using platinized brass as cathodes [15]. Lead cathodes are used to protect titanium heat exchangers in rayon spinning baths [16]. 

Anodic protection against acids has been used in a number of processes in the chemical industry, as well as during storage and transport. It is also successful in geometrically complicated containers and tubings [12], Carbon steel can be protected from nitric and sulfuric acids. In the latter case, temperature and concentration set application limits [17]. At temperatures of up to 120°C, efficient protection can only be achieved with concentrations over 90% [ 18]. At concentrations between 67 and 90%, anodic protection can be used at up to 140°C with CrNi steels [19]. 

Anodic protection allows the use of materials under unfavorable conditions if they are also passivatable in sulfuric acid. CrNi steels [material Nos. 1.4541 (AISI 321) and 1.4571 (AISI 316 Ti)] can be used in handling sulfuric acid of 93 to 99% at temperatures up to 160°C. This enables a temperature of 120 to 160°C to be reached, which is very suitable for heat recovery. 

DMTA samples were made by spreading the liquid sample on a CrNi steel substrate with a thickness of 200 /im and an area of 7x25 mm. These samples were placed in a vessel provided with a clear quartz window. Before and during irradiation the vessel was flushed with pure nitrogen (< 2 ppm O2). 

Chromium carbide is among the compounds detected as precipitating the low temperature regions of liquid metal circuits, and the system Na—Cr—C is one of the most intensively studied systems There is some evidence that the most stable chromium carbide CrjsCg is formed at temperatures between 550 and 700 °C even in stainless steels, where the chemical activity of chromium is well below unity. This reaction is the chemical process causing the carburization of austenitic CrNi steels. CrjjCs precipitates in the surface zones of the material. 

Fig. 7 Polarization curve of cathodic oxygen reduction with superimposed cathodic water dissociation at 18-8-CrNi steel in air-saturated, stirred NaOH/0.5 M NaCi solution, pH 11,25°C ( ) as well as in practically 02-free solution (o) potential referred to saturated calomel electrode [2].

Small amounts of noble metals such as Pt, Pd, and Cu increase the corrosion resistance of 18-8 CrNi steels in sulfuric acid, which is demonstrated in Fig. 19. It is assumed that the noble metals become... 

Fig. 19 Influence of small amounts of Cu, Pd, and Ptonthe corrosion rate of 18-8 CrNi steel in H2SO4 at 20 °C as a function of acid concentration [2],...

For stainless steels and Cr the transpassivity phenomenon is indicated by a branch on the anodic polarization curve as shown for an 18-8 CrNi steel in 1 N H2SO4 in Fig. 5.3. The reason for this indication of transpassivity is that the oxide is generally dissolved and Cr207 and possibly Fe " are formed at potentials below the region where oxygen is developed. 

The negative effect of increased Cl" concentration on the pitting potential Ep is of great significance. For an 18-8 CrNi steel, Ep can be expressed by [5.3] ... 

Figure 5.3 Anodic polarization curve (identical with the anodic overvoltage curve for metal dissolution in the potential range from -0.2 to +1.6 V) for an 18-8 CrNi steel in 1 N H2SO4 without oxygen at 50 C (from Kaesche [5.1] after Engell and Ramchandran).

Figure 5.4 Anodic polarization curves for an 18-8 CrNi steel in a 0.1 MNaCl solution with different amounts of Na2S04 added [5.3].

On commercially pure aluminium, depths from 0.1 to 1.6 mm have been measured after exposure for five years in four different soil types in England. For a fifth type of soil no attack was found. Similar variations have been recorded for 18-8 CrNi steel. AISI 316 (with Mo) (see Section 10.1) showed good resistance, but no general guarantee can be given. 

However, a standard 18-8 CrNi steel shows only a very shallow nitriding depth, and experience has proven that this steel can be used for years in the ammonia cycle gas at 450°C. After 10 years, the penetration depth of nitration is 1 mm at most. 

Figure 20.57 shows the corrosive effect of CO on various steels in a short-term test with pure CO-H2 mixtures at two CO partial pressures. A striking feature is the shift in the corrosion maximum of the 18-9 CrNi steel at the higher CO partial pressure. This possibly corresponds to the beginning of nickel carbonyl formation, which after a period of time leads to a chromium-enriched and more stable surface layer. 

High-aWo /ed multiphase steels - Austenitic CrNi steels 27... 

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