Steel in acids

Typical examples of inhibitors used for minimizing corrosion of iron and steel in aqueous solutions are the chromates, phosphates, and silicates. Organic sulfide and amine materials are frequently effective in minimizing corrosion of iron and steel in acid solution. 

TABLE 7.1 Effect of Dissolved Oxygen on Corrosion of Mild Steel in Acids Corrosion Rate (inyy)... 

There are no films or protective surface films on active metals, e.g., mild steel in acid or saline solutions. Passive metals are protected by dense, less readily soluble surface films (see Section 2.3.1.2). These include, for example, high-alloy Cr steels and NiCr alloys as well as A1 and Ti in neutral solutions. Selective corrosion of alloys is largely a result of local concentration differences of alloying elements which are important for corrosion resistance e.g., Cr [4],... 

Weight loss corrosion of passive metals (e.g., CrNi stainless steels in acids) (cathodic protection in acids is not practicable see Fig. 2-14) ... 

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. 

In natural waters, cold-worked commercial carbon steels of the same composition corrode at more or less the same rate as annealed steels, presumably because the corrosion rate in this case is controlled by the diffusion of oxygen. Unprotected carbon steels are sometimes exposed to natural waters, and it is this latter situation which is of greater practical importance than the behaviour of steels in acids, since steels should never be used in these environments unless they are protected. 

Table 1.13 Effect of dissolved oxygen on the corrosion rate (mm y ) of mild steel in acids ...

Streicher, M. A., Effect of Heat Treatment, Composition and Microstructure on Corrosion of 18Cr-8Ni-Ti Stainless Steel in Acids , Corrosion, 20, 57t (1964)... 

The general form of the anodic polarisation curve of the stainless steels in acid solutions as determined potentiostaticaiiy or potentiodynamically is shown in Fig. 3.14, curve ABCDE. If the cathodic curve of the system PQ intersects this curve at P between B and C only, the steel is passive and the film should heal even if damaged. This, then, represents a condition in which the steel can be used with safety. If, however, the cathodic curve P Q also intersects ED the passivity is unstable and any break in the film would lead to rapid metal solution, since the potential is now in the active region and the intersection at Q gives the stable corrosion potential and corrosion current. 

Endo, K. and Komai, K., Electrochemical investigation of the corrosion fatigue of steel in acid solution , Metalloberfldche, 22, 378-84 (1968)... 

Table 10.36 EITect of electrolyte agitation on corrosion rate and the current density to maintain passivity of mild steel in acid solutions at 27°C (after Walker and Ward )...

F. Bentiss, M. Lagrenee, and M. Traisnel. 2,5-Z>is(A -pyridyl)-l,3,4-oxadiazoles as corrosion inhibitors for mild steel in acidic media. Corrosion, 56(7) 733-742, July 2000. 

By submerging steel in acid, two main reactions take place. Both if hydrogen chloride is used both produce ferrous chloride. 

Corrosion inhibition of mild steel in acid solutions by 2-aryl-5-oxadiazolinethiones (2-hydroxyphenyl, 2-phenyl, and 2-cinnamyl) has been observed <2002MCH425>. The potentiodynamic polarization data have shown that compounds studied predominantly behave as cathodic inhibitors in acid solutions. 

Corrosion of steel in acid solution has practical importance hence, efforts to develop more efficient and environmentally compliant methods to prevent corrosion have been ongoing <1996SM(78)103>. Compounds with functional groups containing heteroatoms such as alkylimidazole <1996MI1> and triazole <1996MI2> compounds, which can donate lone pair electrons, are found to be particularly useful as inhibitors for corrosion of metals and have been used as effective inhibitors for steel in acidic media. 

As discussed above, Pt is the reference electrode material for H2 evolution since it is the most active elemental cathode. H2 is formed on Pt with a Tafel slope of 30-40 mV, the lowest ever observed for this reaction. Its cost makes this metal unsuitable for routine applications. In fact, cathode materials traditionally used in technology have long been iron or mild steel in acidic solution and Ni in (strongly) alkaline solution. Steel can also be used in moderately basic solution. 

Mechanism of zinc electrodeposition on steel in acidic solution of zinc chloride was investigated [408] as a function of pH, grain-refining additives, and current density. 

The corrosion resistance of steels in acids is represented by isocorrosion diagrams. The resistance to uniform or general corrosion in various acids and sodium hydroxide was determined by weight loss measurements over a period of 96 h and the resulting data are given in Table 4.12. 

In the range of electrode potential more positive (more anodic) than the pitting potential, the pitting corrosion occurs in the presence of chloride ions and the metal dissolution at a pit, initially hemispherical, proceeds through the mode of electropolishing, in which concentrated chloride salts in an occluded pit solution will control the pit dissolution. It is likely that the polishing mode of metal dissolution proceeds in the presence of a metal salt layer on the pit surface in the salt-saturated pit solution. It was experimentally found with stainless steels in acid solution [54] that the pit dissolution current density, pit, is an exponential function of the electrode potential, E (Tafel equation) ... 

Very often the long experience with chemicals operating as corrosion inhibitors, e. g. in the oil field, gas or petroleum industry, is taken as an example for the successful use of corrosion inhibitors for many decades. This undoubtedly is true and the overwhelming majority of literature on corrosion inhibitors deals with the effects of inhibitors on uniform corrosion, e. g. of steel in acidic or neutral solutions, where they can be classified into [2] a) adsorption inhibitors, acting specifically on the anodic or on the cathodic partial reaction of the corrosion process or on both reactions (mixed inhibitor), b) JUm-forming inhibitors, blocking the surface more or less completely, and c) passivators, favouring the passivation reaction of the steel (e. g. hydroxyl ions). 

Corrosion of Stainless Steels in Acids Stainless steels are iron-based alloys with chromium as the main alloying element. The most interesting alloys for technical applications are ferritic stainless steels, austentic stainless steels, and duplex stainless steels. The distinction between the stainless steels comes from their different crystallographic structures. Ferritic-martensitic stainless steels and martensitic stainless steels have less nickel and a higher carbon content and can be hardened by heat treatment. The corrosion behavior of these steels is mainly influenced by the formation of carbides, which generally increase the corrosion rate. 

Tab. 1 The values of Rt for galvanized steel in acid rain solutions with and without strontium chromate (100 ppm) [141]...

J. Al-Khamis, H. W. Pickering, IR mechanism of crevice corrosion for alloy T-2205 duplex stainless steel in acidic-chloride media, J. Electrochem. Soc. 148 (2001) B314-B321. 

Fluang [246] investigated the SCC of AISI 321 stainless steel in acidic chloride solutions by the SSRT technique and fracture mechanics. It was found that the cleavage fracture characterizes the fracture surface. The active dissolution mechanism controls the SCC of AISI 321 stainless steel in acidic chloride solutions and can be inhibited by using KI. The inhibition effect of KI on the SCC is due to inhibition of the anodic reaction of the corrosion process. 

Y. Huang, Stress corrosion cracking of AISI 321 stainless steel in acidic chloride solution. Bull. Mater. Sci. 25 (2002) 47-51. 

C. Arroyave, F.A. Lopez, M. MorciUo, The early atmospheric corrosion stages of carhon steel in acidic fogs, Corros. Sci. 37 (1995) 1751—1761. 

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