Stainless steels electrolytic etching

The preferred method for preparing STM tips is the dc dropoff method. The basic setup is shown in Fig. 13.1. It consists of a beaker containing an electrolyte, typically 2M aqueous solution of NaOH. A piece of W wire, mounted on a micrometer, is placed near the center of the beaker. The height of the W wire relative to the surface of the electrolyte can then be adjusted. The cathode, or counterelectrode, is a piece of stainless steel or platinum placed in the beaker. The shape and location of the cathode has very little effect on the etching process, which can be chosen for convenience. A positive voltage, 4 V to 12 V, is applied to the wire, which is the anode. Etching occurs at the air-electrolyte interface. The overall electrochemical reaction is (Ibe et al., 1990) ... 

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. 

M. A. STREICHER (DuPont) A number of detailed investigations provide information on several points raised in this paper in connection with the use of electrolytic etching to detect susceptibility of stainless steels to intergranular corrosion. A brief summary of pertinent aspects is given below. 

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-... 

Fig. 1. Annealed steel. 18 Cr-8 Ni stainless steel etched electrolytically in 10% oxalic acid for 1.5 min. at 1 amp/sq. cm X500.

The electrolytic etches in oxalic acid, as described above, are carried out under standardized conditions of current density and length of etching time. Measurement of the electrode potential of the stainless steel during this etching procedure shows that this potential is also constant throughout the etching period, and, that therefore, such etches are equivalent to etching under conditions of controlled potential. 

The equipment and the experimental procedures using the C02-methanol medium have already been described in previous papers. . For the photoelectrochemical experiments, a stainless steel pressure vessel was equipped with a 2-cm thick quartz window for illumination, p-type InP and GaAs wafers were cut into ca. 0.4 cm x 0.5 cm electrodes and were mounted using epoxy resin. Ohmic contact was made with successive vapor deposition of Zn (30 mn) and Au (100 nm), which was annealed afterward at 425 C in Ar. A silver wire (0.8 mm dia) was used as a quasi-reference electrode (Ag-QRE, ca. +80 mV vs. SCE). A Pt wire (0.8 mm dia) was used as the counter electrode. The photocathode was etched in hot aqua regia for ca. 5 s before each experiment. The electrolyte solution [3 cm, 0.3 mol dm" tetrabutylammonium perchlorate (TBAP) in CH3OH] was placed in a glass cell liner in the stainless steel vessel. Gases were introduced into the pressure vessel and were left to equilibrate for one hour at the desired pressure (1 to 40 atm). 

Several ASTM standards address the susceptibility of alloys to intergranular corrosion. Stainless steels are tested for sensitization by immersion in different boiling adds according to ASTM A262 (76). Similar tests are described in ASTM G28 for Ni-rich Cr-bearing alloys (77). ASTM A262 also describes an electrolytic etch test in oxalic add, which can be used to screen prior to the more lengthy immersion tests. 

Electrolytic etching for stainless steels. Specimen polarized as anode (-t) under a cell voltage of 3 V during 10 s to color ferrite (tan or light blue) and sigma (orange) but does not affect austenite. 

Etching can be done with a solntion containing 40 g chromic acid, 20 ml. snlfnric acid (96 percent), and 2000 ml. water. This is followed by a rinse in 25 percent ammoninm hydroxide. Thin films of silver are etched in 55 percent (by weight) ferric nitrate in water or ethylene glycol. Solutions of alkaline cyanide and hydrogen peroxide will also dissolve silver. Use extreme caution. Electrolytic etching is also possible with 15 percent nitric add at 2 V and stainless-steel cathode. 

Figure 7-29. SEM micrographs of the etched surfaces of 5000 h aged duplex stainless steel, (a) At -268 mV(Ag/AgCl) leading to active dissolution in austenite and ferrite (b) -170 mV (Ag/AgCl) (c) -130 mV (Ag/AgCl) leading to active dissolution of ferrite, while austenite is passive at both potentials. Electrolyte 0.1 MH2SO4 + O.OI M KSCN (Jiangetal., 1992).

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