Etching of steel

Iron carbide (3 1), Fe C mol wt 179.56 carbon 6.69 wt % density 7.64 g/cm mp 1650°C is obtained from high carbon iron melts as a dark gray air-sensitive powder by anodic isolation with hydrochloric acid. In the microstmcture of steels, cementite appears in the form of etch-resistant grain borders, needles, or lamellae. Fe C powder cannot be sintered with binder metals to produce cemented carbides because Fe C reacts with the binder phase. The hard components in alloy steels, such as chromium steels, are double carbides of the formulas (Cr,Fe)23Cg, (Fe,Cr)2C3, or (Fe,Cr)3C2, that derive from the binary chromium carbides, and can also contain tungsten or molybdenum. These double carbides are related to Tj-carbides, ternary compounds of the general formula M M C where M = iron metal M = refractory transition metal. 

Surface cleaning/etches. As with aluminum and titanium, the most critical test for bonded steel joints is durability in hostile (i.e., humid) environments. The fact that the problem is a serious one for steel was illustrated in a study [117] that compared solvent cleaned (smooth) 1010 cold-rolled steel surfaces with FPL aluminum (microrough) substrates. Although the dry lap-shear strengths were not markedly different, stressed lap-shear joints of steel adherends that were exposed to a humid environment failed in less than 30 days, whereas the aluminum joints lasted for more than 3000 days. 

Fig. 13.1. Electrochemical etching of tungsten tips, (a) A tungsten wire, typically 0.5 mm in diameter, is vertically inserted in a solution of IN NaOH. A counterelectrode, usually a piece of platinum or stainless steel, is kept at a negative potential relative to the tungsten wire, (b) A schematic illustration of the etching mechanism, showing the "flow" of the tungstate anion down the sides of the wire in solution. (Reproduced from Ibe et al., 1990, with permission.)...

The knowledge and control of localized corrosion was also required to produce etched plates for the graphic artist with its intaglio lines. Prints made by the process first appear around 1500 AD, but there was a long prehistory in the decorative etching of metal surfaces, particularly those of steel arms and armor [279]. 

It should further be noted that the Fe—O—Si+ ion at +100 amu, which was reported in the literature as evidence for covalent bond formation [13], is not observed here. The low intensity of the ions at 56 amu (56Fe+) and the detection of iron-containing ions in the spectrum are consistent with an etching of the steel substrate by the silane solution. Such attack of metallic substrates has been reported for Zn and A1 [3], but not yet for steel. 

Hydrofluoric acid — (HF) A solution of hydrogen fluoride in water. The pure hydrogen fluoride is characterized by Mw of 20.0063 gmol-1 m.p. -83.55 °C (1 atm) b.p. 19.5 °C (latm). When concentrated, this colorless fuming liquid is extremely corrosive and can dissolve almost all inorganic oxides such as silicate compounds or oxides of metals like stainless steel, aluminum, and uranium however, it can be stored in casted iron bottles because a corrosion-resistant iron fluoride layer protects the metal. It is used for several purposes such as the preparation of titanium oxide nano tube arrays [i], silicon nanoparticles [ii] and electrochemical etching of silicon [iii], electrochemical deposition of lithium [iv], etc. 

ELECTROLYTIC ETCHING OF metals produces various results intergranular attack, attack of crystalline surfaces which is orientation dependent formation of etch pits, and anodic oxide films. The behavior of a metal or alloy depends on composition, temperature of the electrolyte, and above all on the electrode potential which varies with the metal. Applications to Al, Fe, stainless steel, Ti, 2r, U, and their alloys will be discussed. 

Etching of stainless steel represents the most typical case of the application of electrolytic etching. This technique has been used on die 18/8 stainless steels in order to obtain a reliable and rapid measure of their sensitization to intergranular corrosion after a thermal treatment at 600 to 900°C. Several organic electrolytes such as oxalic acid (1) and tartaric acid (2) as well as inorganic electrolytes such as chromic acid (3) and sodium cyanide (4) have been employed. All these electrolytes attack preferentially the grain boundaries in which a second phase has precipitated. The composition of this phase responsible for the etching is not always known. 

Edeleanu (44) applied fee technique for fee metallogra-pMc etching of stainless steels of the following composition ... 

Solutions used for electrolytic etching of stainless steels may be divided into two classes (a) Electrolytes which readily etch grain boundaries in the form of grooves regardless of the presence or absence of intergranular precipitates. Nitric acid is of this type [ J. Electrochem. Soc., 106, 161 (1959)]. ... 

Electrolytic etching of stainless steels has been used industrially to simplify and accelerate the evaluation of stainless steels for their susceptibility to intergranular attack. Be-... 

Makrides and Hackerman (20) have studied the dissolution of steel cylinders in 2M HC1 with low concentrations of ferric chloride, benzoquinone and toluquinone as depolarizers. The specific rate at a given U increased as the surface became etched, reaching a steady value in a short time. The authors suggest an equation of the form... 

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