Electrochemical pickling

Workers in Japan have shown that ultrasound can be used in electrochemical pickling and etching and smut removal in plating a beryllium-copper alloy [98]. The plating surface of Be-Cu alloys which are used commercially as electrical connectors, need to be pretreated by anodic treatment in an acid aqueous solution. These workers have shown that the smut may be effectively removed by application of ultrasound. This leads to a more resilient finish and a thickness of 0.2—3 p coating being achieved more rapidly. 

In electrochemical pickling the objective is to remove rust and other oxides and sometimes to produce a uniform etch to produce adhesion of deposits. The surface is made anoidc in an acid medium, e.g. H2SO4 or FeCla. 

Without the addition of corrosion inhibitors, acid cleaning or pickling processes to remove oxides and scales would result in severe corrosion of exposed metal surfaces. Acid corrosion is an electrochemical or redox process, and raising cleaning temperatures or acid strength (lowering the pH) increases the hydrogen ion concentration and consequently the rate of corrosion. 

Method 3 Preparation of chloroform from Rubbing alcohol, potassium dichromate, pickling salt, and dilute sulfuric acid using an electrochemical cell... 

Summary Bleach can be made using an electrochemical process whereby pickling salt is electrolyzed. During the process, chlorine gas is evolved at the anode, and sodium hydroxide is liberated at the cathode. As the process proceeds, the chlorine reacts with the sodium hydroxide forming sodium hypochlorite (bleach). Some chlorine gas does escape, so use proper ventilation when carrying out this operation. 

Most descaling and passivation processes for steels were developed prior to the widespread use of electrochemical techniques. As a result, a variety of visual and chemical tests are widely used for determining the surface cleanliness. Chemical tests have also been established to verify the presence of a robust oxide film on austenitic and ferritic stainlesses (8). These methods are very simple to conduct in a manufacturing environment, but they are qualitative in nature and rely strongly on the judgment of the inspector. Outside of the laboratory, electrochemical methods have not been widely used to evaluate cleanliness of carbon and alloy steels after pickling. Nevertheless, they are well suited for this purpose and have been examined in considerable detail in laboratory studies. 

Removal of copper ion from plating bath liquors has been accomplished electrochemically, by plating it out, and also by extraction with tri butyl phosphate [69]. The extract brings nickel, antimony, arsenic, and copper into the organic phase, leaving a significantly cleaner acid raffinate. Tributyl phosphate has also been used to extract iron from pickling waste liquors. 

Fields of applicability. Figure 15.3 depicts the fields of applicability of pickled stainless steels in chloride-contaminated concrete exposed to temperatures of 20 °C or 40 °C. Fields have been plotted by analysing the critical chloride values obtained by different authors from exposure tests in concrete or from electrochemical tests in solution and mortar and taking into consideration the worst conditions [11-28]. Nevertheless, it should be pointed out that values are indicative only, since the critical chloride content depends on the potential of the steel, and thus it can vary when oxygen access to the reinforcement is restricted as well as when stray current or macrocells are present. For instance, the domains of applicability are enlarged when the free corrosion potential is reduced, such as in saturated concrete. Furthermore, the values of the critical chloride Hmit for stainless steel with surface finishing other than that obtained by pickling can be lower. 

Improved adhesion is obtained by galvanizing, but this is only suitable for ABS polymers. When the plastic surfaces are pickled, the rubber elastic components are anodized. This produces pores and channels in which, for example, silver can be deposited chemically. The silver then forms the adhesive base for the copper layers subsequently deposited electrochemically, and these layers are then reinforced by the galvanized coating. Here, too, it is difficult to manufacture metal layer thicknesses of more than 10 fJLm because the different thermal coefficients of expansion of plastics and metals can easily lead to stresses, and thence to bubbles or cracks. 

Aluminium and A1 alloys react with oxygen and water vapor in the air to produce a thin, conqtact surface oxide film which protects the underlying metal from further attack (Fig. 3.1-80). The surface layer contains mainly amorphous AI2O3 in several layers. The so-called barrier layer has an extremely low conductivity for electrons and ions and thus acts as an insulator in any interfacial electrochemical reactions. It thus affords effective protection against corrosion. If mechanical damage of the protective layer occurs, or if the layer is removed by pickling, it re-forms immediately. Aluminium and A1 alloys thus exhibit good corrosion resistance to chemicals, seawater, and the weather. 

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