Electroplating corrosion potentials

Sacrificial anode — is a piece of metal used as an anode in electrochemical processes where it is intended to be dissolved during the process. In -+ corrosion protection it is a piece of a non-noble metal or metal alloy (e.g., magnesium, aluminum, zinc) attached to the metal to be protected. Because of their relative -+ electrode potentials the latter is established as the -+ cathode und thus immune to corrosion. In -+ electroplating the metal used as anode may serve as a source for replenishing the electrolyte which is consumed by cathodic deposition. The sodium-lead alloy anode used in the electrochemical production of tetraethyl lead may also be considered as a sacrificial anode. 

A variety of standard electrochemical methods may be used to probe the corrosion behavior of electroplated coatings with a particular focus on assessing the effects of galvanic coupling between the coating and the substrate (ASTM G 5, G 59, G 61 , G 82 °, G 102", and G 106 ) [22], In accordance with the mixed potential treatment of galvanic corrosion, the corrosion potential and polarization resistance are expected to be a sensitive function of the anode/cathode area, i.e., p>orosity [13,14,20,23],... 

The metallic substrate, clean and rinsed, is immersed wet in the plating cell. The base metals which are usually plated present an essentially metallic surface to the electrolyte, and the slight corrosive action of the rinse water in preventing the formation of any substantial oxide film is important. A critical balance of corrosion processes in the initial stages is vital to successful electroplating, and for this reason there is a severe restriction on the composition of the electroplating bath which may be used for a particular substrate. This will be discussed later. The substrate is made the cathode of the cell it may be immersed without applied potential ( dead entry) or may be already part of a circuit which is completed as soon as the substrate touches the electrolyte ( live entry). Live entry reduces the tendency for the plating electrolyte to corrode the substrate in the period before the surface... 

Electroplating passive alloys Another application of strike baths reverses the case illustrated in the previous example. The strike is used to promote a small amount of cathode corrosion. When the passivation potential of a substrate lies below the cathode potential of a plating bath, deposition occurs onto the passive oxide film, and the coating is non-adherent. Stainless steel plated with nickel in normal baths retains its passive film and the coating is easily peeled off. A special strike bath is used with a low concentration of nickel and a high current density, so that diffusion polarisation (transport overpotential) depresses the potential into the active region. The bath has a much lower pH than normal. The low pH raises the substrate passivation potential E pa, which theoretically follows a relation... 

Electrochemical processes (e.g., electrolysis, electroplating, electromachirring, crrnetrt generation, and corrosion [Plate 8]) are distinguished by their occturence in a boundary region between an electrolyte (liqtrid or solid) and an electrode. The corrrse of these processes is strongly dependent on the potential at the electrode surface, the composition and stmcture of the electrode, the composition of the electrolyte, and the microstmcture of the electrolyte in the boundary layer near the electrode surface. In certain applications, the pore size and coimectivity of the electrode can also be important. 

Electrochemical methods of protection rest on different precepts (1) electroplating of the corroding metal with a thin protective layer of a more corrosion-resistant metal, (2) electrochemical oxidation of the surface or application of other types of surface layer, (3) control of polarization characteristics of the corroding metal (the position and shape of its polarization curves), and (4) control of potential of the corroding metal. 

Electrochemistry finds wide application. In addition to industrial electrolytic processes, electroplating, and the manufacture and use of batteries already mentioned, the principles of electrochemistry are used in chemical analysis, e.g.. polarography, and electrometric or conductometric titrations in chemical synthesis, e.g., dyestuffs, fertilizers, plastics, insecticides in biolugy and medicine, e g., electrophoretic separation of proteins, membrane potentials in metallurgy, e.g.. corrosion prevention, eleclrorefining and in electricity, e.g., electrolytic rectifiers, electrolytic capacitors. 

Chromium is a white, hard, lustrous, and brittle metal (mp 1903 10°C). It is extremely resistant to ordinary corrosive agents, which accounts for its extensive use as an electroplated protective coating. The metal dissolves fairly readily in nonoxidizing mineral acids, for example, hydrochloric and sulfuric acids, but not in cold aqua regia or nitric acid, either concentrated or dilute. The last two reagents passivate the metal in a manner that is not well understood. The electrode potentials of the metal are... 

This chapter introduced and briefly described a number of industrial surface treatments. These treatments are made to material surfaces for various reasons. One is to enhance the appearance of a finished product to attract potential buyers. Another is to improve the safety of products. Other purposes are to promote corrosion control and wear resistance, and to improve mechanical and electrical properties. The treatments discussed include electroplating, electroless plating, spray coating, galvanization, painting, anodizing, physical vapor deposition (PVD), and chemical vapor deposition (CVD). 

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