Iron-chromium alloys anodic polarization

Anodic Polarization Curves for Iron-Chromium Alloys... 

Anodic polarization curves for iron-chromium alloys in 1 N H2S04. Redrawn from Ref 21... 

Nickel has a very small effect on the anodic polarization behavior of iron, and hence, iron-nickel alloys are of minor significance as corrosion-resistant alloys. However, the addition of nickel to iron-chromium alloys (AISI 200 series) permits conversion of the latter as ferritic alloys to austenitic iron-chromium-nickel alloys (AISI 300 series). In... 

Figure 6.37 Anodic polarization curves for an iron-chromium alloy (Fe-13Cr) in an electrolyte consisting of 65% H3PO4, 20% H2SO4 at 70 °C, measured on a rotating disk electrode at different rotation rates 100, 400, 1600, 3600,4800 rpm [35].

The properties of the interface metal/solution. Cast iron corrodes because of exposure of its graphite to the surface (graphitic corrosion), which is cathodic to both low-alloy and mild steels. The trim of a valve must always maintain dimensional accuracy and be free of pitting and hence it should stay cathodic to the valve body. Hence, in aggressive media, valve bodies are frequently chosen of steel rather than cast iron. Because of increased anodic polarization, low-alloy steel (Cr and Ni as noble components) is cathodic to normal steel in most natural media. Accordingly, steel bolts and nuts coupled to underground mild steel pipes, or a weld rod used for steel plates on the hull of a ship, should always be of a low-nickel, low chromium steel or from a similar composition to that of the steel pipe.7... 

Most often, it is the anodic polarization behavior that is useful in understanding alloy systems in various environments. Anodic polarization tests can be conducted with relatively simple equipment and the scans themselves can be done in a short period of time. They are extremely useful in studying the active-passive behavior that many materials exhibit. As the name suggests, these materials can exhibit both a highly corrosion-resistant behavior or that of a material that corrodes actively, while in the same corrodent. Metals that commonly exhibit this type of behavior include iron, titanium, aluminum, chromium, and nickel. Alloys of these materials are also subject to this type of behavior. 

The anodic polarization of a given alloy base metal such as iron or nickel is sensitive to alloying element additions and to heat treatments if the latter influences the homogeneity of solid solutions or the kinds and distribution of phases in the alloy. The effect of chromium in iron or nickel is to decrease both EpP and icrit and hence to enhance the ease of placing the alloy in the passive state. The addition of chromium to iron is the basis for a large number of alloys broadly called stainless steels, and chromium additions to nickel lead to a series of alloys with important corrosion-resistant properties. 

Anodic Polarization of Iron-Chromium-Molybdenum Alloys... 

K. Osozawa, H.. Engel, The anodic polarization curves of iron-nickel-chromium alloys, Corros. Sci. 6 (1966) 389-393. 

Examples of metals that are passive under Definition 1, on the other hand, include chromium, nickel, molybdenum, titanium, zirconium, the stainless steels, 70%Ni-30% Cu alloys (Monel), and several other metals and alloys. Also included are metals that become passive in passivator solutions, such as iron in dissolved chromates. Metals and alloys in this category show a marked tendency to polarize anodicaUy. Pronounced anodic polarization reduces observed reaction rates, so that metals passive under Definition 1 usually conform as well to Definition 2 based on low corrosion rates. The corrosion potentials of metals passive by Definition 1 approach the open-circuit cathode potentials (e.g., the oxygen electrode) hence, as components of galvanic cells, they exhibit potentials near those of the noble metals. 

In acidic media, the metals iron, nickel and chromium have passivation current densities that increase in the order Cr < Ni < Fe. In Figure 6.11, the anodic polarization curves for the three metals in 0.5 M sulfuric acid (25 °C) are compared. Chromium has lower values of both ip and Ep than the other two metals. By alloying increasing amounts of chromium to steel one therefore improves the corrosion resistance. Experience shows that above a chromium concentration of 12 to 13%, a steel passivates spontaneously in contact with aerated water. It becomes "stainless", meaning it does not rust easily. Figure 6.12 gives the corrosion potential of different... 

The chemical composition of passive films on Fe-Cr alloys depends strongly on the corrosive environment and on the applied potential. In acid media the films become enriched in chromium oxide, while in alkaline media they contain more iron oxide. The Auger depth profiles of Figure 6.21 illustrate this observation. They were measured on a Fe-25Cr alloy after 1 hour anodic polarization in an acid sulfate... 

Anodic polarization of active/passive metals - alloys of nickel, iron, chromium, titanium, and stainless steel in weak-to-extiemely corrosive environments, where economy in consumption of protective currents is required. 

---