Stainless steels anodic protection

Introduction of electrOcheiiiicaUy active cathodes that facilitate passivation Raise potential by external e.m.f Additions of Pt. Pd and other noble metals to Ti, Cr and stainless steels Anodic protection of steel, stainless steel and Ti... 

The protection current requirement is determined mainly by the uncoated surfaces of the stainless steel whose protection potential is a few tenths of a volt more positive than that of the plain carbon steel, to avoid pitting (sec Section 2.4). The protection current requirement for the turbine section is about 10 A so that the plate anodes are only loaded to about 1 A. 

Anodic passivation and its appHcation to sulfuric acid equipment such as stainless steel acid coolers and carbon steel storage tanks has been weU studied (102—104). More recently, sheU and tube coolers made from Sandvik SX or Saramet have been installed in several acid plants. These materials do not requHe anodic protection. 

Crevice Corrosion. Crevice corrosion is intense locali2ed corrosion that occurs within a crevice or any area that is shielded from the bulk environment. Solutions within a crevice are similar to solutions within a pit in that they are highly concentrated and acidic. Because the mechanisms of corrosion in the two processes are virtually identical, conditions that promote pitting also promote crevice corrosion. Alloys that depend on oxide films for protection (eg, stainless steel and aluminum) are highly susceptible to crevice attack because the films are destroyed by high chloride ion concentrations and low pH. This is also tme of protective films induced by anodic inhibitors. 

Note that zinc anodes are often used to protect steel and other relatively noble metals cathodically. In this case, the fasteners were acting as unintentional sacrificial anodes, protecting the stainless steel. Simple solutions to the problem would be to insulate the fasteners from the stainless steel electrically or to use stainless steel fasteners. 

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]). 

The cathodic protection of plain carbon and low-alloy steels can be achieved with galvanic anodes of zinc, aluminum or magnesium. For materials with relatively more positive protection potentials (e.g., stainless steels, copper, nickel or tin alloys), galvanic anodes of iron or of activated lead can be used. 

As in the case of corrosion at the insulating connection due to different potentials caused by cathodic protection of the pipeline, there is a danger if the insulating connection is fitted between two sections of a pipeline with different materials, e.g., mild and stainless steel. The difference between the external pipe/soil potential is changed by cell currents so that the difference between the internal pipe/ medium potential has the same value, i.e., both potential differences become equal. If the latter is lower than the former for the case of free corrosion, the part of the pipe with the material that has the more positive rest potential in the soil is polarized anodically on the inner surface. The danger increases with external cathodic protection in the part of the pipeline made of mild steel. 

Magnesium anodes are frequently used as an additional protection measure at a later stage for stainless steel tanks. In this case the anodes are connected through a 5- to 10-Q resistor to the tank to avoid an unnecessarily high current for the cathodic protection of the tank and simultaneous high consumption of the anodes. 

Anodic protection is particularly suitable for stainless steels in acids. Protection potential ranges are given in Section 2.4. Besides sulfuric acid, other media such as phosphoric acid can be considered [13,21-24]. These materials are usually stable-passive in nitric acid. On the other hand, they are not passivatable in hydrochloric acid. Titanium is also a suitable material for anodic protection due to its good passivatability. 

Spiral-plate exchangers are fabricated from any material that can be cold worked and welded. Materials commonly used include carbo steel, stainless steel, nickel and nickel alloys, titanium, Hastelloys, and copper alloys. Baked phenolic-resin coatings are sometimes applied. Electrodes can also be wound into the assembly to anodically protect surfaces against corrosion. 

Imoi, H., Saito, Y., Kobayashi, M. and Fujiyama, S., Pitting-corrosion-resistant Chromium Stainless Steel , Japan Kokai 7300, 221 (1973) C.A., 79, 22569a Sato, E., Tamura, T. and Okabe, T., Aluminium Anode for Cathodic Protection. 7 Pitting and Corrosion Potentials for Gallium in Sodium Chloride Solutions , Kinzoku Hyomen Gijutsu, 24, 82 (1973) C.A., T9, 12792d... 

The metals most commonly used for water systems are iron and steel. These metals often have some sort of applied protective coating galvanised steel, for example, relies on a thin layer of zinc, which is anodic to the steel except at high temperatures. Many systems, however, contain a wide variety of other metals and the effect of various water constituents on these must be considered. The more usual are copper, brasses, bronzes, lead, aluminium, stainless steel and solder. 

In pure dry air at normal temperatures a thin protective oxide film forms on the surface of polished mild steel. Unlike that formed on stainless steels it is not protective in the presence of electrolytes and usually breaks down in air, water and soil. The anodic reaction is ... 

Contact of brass, bronze, copper or the more resistant stainless steels with the 13% Cr steels in sea-water can lead to accelerated corrosion of the latter. Galvanic contact effects on metals coupled to the austenitic types are only slight with brass, bronze and copper, but with cadmium, zinc, aluminium and magnesium alloys, insulation or protective measures are necessary to avoid serious attack on the non-ferrous material. Mild steel and the 13% chromium types are also liable to accelerated attack from contact with the chromium-nickel grades. The austenitic materials do not themselves suffer anodic attack in sea-water from contact with any of the usual materials of construction. 

Contact with steel, though less harmful, may accelerate attack on aluminium, but in some natural waters and other special cases aluminium can be protected at the expense of ferrous materials. Stainless steels may increase attack on aluminium, notably in sea-water or marine atmospheres, but the high electrical resistance of the two surface oxide films minimises bimetallic effects in less aggressive environments. Titanium appears to behave in a similar manner to steel. Aluminium-zinc alloys are used as sacrificial anodes for steel structures, usually with trace additions of tin, indium or mercury to enhance dissolution characteristics and render the operating potential more electronegative. 

To protect stainless-steel equipment from chloride stress-corrosion cracking by triggering an anodic protection system when the measured potential falls to a value close to that known to correspond to stress-corroding conditions. 

To trigger off an anodic protection system for stainless-steel coolers cooling hot concentrated sulphuric acid when the potential moves towards that of active corrosion. 

Edeleanu made use of potentiostatic curves to determine the optimum conditions for the protection of stainless steel in sulphuric acid. A pilot plant was then used to determine the practicability of anodic protection at a constant potential. He pointed out several factors necessary for proper control and indicated the spectacular results obtained. 

CV of solutions of lithium bis[ salicy-lato(2-)]borate in PC shows mainly the same oxidation behavior as with lithium bis[2,2 biphenyldiolato(2-)-0,0 ] borate, i.e., electrode (stainless steel or Au) passivation. The anodic oxidation limit is the highest of all borates investigated by us so far, namely 4.5 V versus Li. However, in contrast to lithium bis[2,2 -biphenyl-diolato(2-)-0,0 Jborate based solutions, lithium deposition and dissolution without previous protective film formation by oxidation of the anion is not possible, as the anion itself is probably reduced at potentials of 620-670 mV versus Li, where a... 

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