Galvanic Corrosion - Practical Study PDF
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This document explores galvanic corrosion, a type of electrochemical corrosion that occurs between two different metals. It details the experimental procedures to investigate the effect of surface area on current intensity and includes questions related to the topic. The document targets undergraduate-level students specializing in materials science or engineering.
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The galvanic corrosion (bimetallic) The galvanic corrosion appears between two different metals. The metals are in direct contact or they are connected through an electric conductor. The two metals are situated in a corrosive medium. This type of corrosion is very frequently met in soldering...
The galvanic corrosion (bimetallic) The galvanic corrosion appears between two different metals. The metals are in direct contact or they are connected through an electric conductor. The two metals are situated in a corrosive medium. This type of corrosion is very frequently met in soldering, heat exchangers contact between copper and brass, contacts between carbon steel and stainless steel etc. Fig. 1 Corrosion of electronic devices The galvanic corrosion is based on corrosion macrocells. The corrosion current is generated by a minimum potential difference of 50 mV, imposed by the two metals in contact. The anodic dissolution reaction: takes place on the metal with a more negative corrosion potential. The reduction reactions of depolarisers (hydrogen ions or oxygen dissolved in water): (cathodic reaction for acidic medium) and (cathodic reaction for neutral and alkaline medium) take place on the metal with a more positive potential. These two reactions determine the flow of the corrosion current through the contact. The electrode potential in a corrosive medium (the corrosion potential) differs from the standard electrode potential. In function of medium, pH, temperature, a relative changing of metals’ positions is observed in the metal sequence based on the corrosion potentials, compared with their order based on the standard potentials. 1 The ratio between the anodic and cathodic surface areas has an essential influence on the corrosion rate. This ratio is very important when the corrosion is under cathodic control and especially for the corrosion with oxygen as depolariser. In this case, the corrosion rate is determined by the oxygen concentration at the electrode surface. The concentration of oxygen depends on the oxygen refreshing speed of the solution. The amount of oxygen reduced is high if the cathodic surface is large and the current density will be also appreciable. At the same time, the small anodic surface generates very large current densities. The aim of this work is to emphasize the influence of the cathodic areas on the current intensity of the galvanic cell: (-) (+) Experimental The surface of zinc and copper electrodes will be polished with abrasive paper, washed with water and dried with filter paper. The electric circuit is built from: 1. anode (iron electrode) 2. cathode (cooper electrode) 3. vessel with NaCl 3% solution 2 4. miliampermeter; Figure 5.1. The schema for galvanic corrosion 1- vessel with corrosion medium (NaCl 3%); 2 - Anode (Zn); 3 - Cathode (Cu); A - miliampermeter In the first part of this experiment, the influence of the cathode surface areas on the current intensity is followed. After the circuit was closed, a large intensity due to the O 2 from the cathode surface is recorded. The current intensity is read every 1 minute, until three identical values are obtained (aprox. 10 min.). Maintaining the zinc anode in solution, the same intensity measurements are done for Cu samples of 3 different surfaces. The experimental values are shown in tables like below: Timp Current intensity [mA] [min] Couple Al-Cu or Zn-Cu 1 2 3 4 5 6 7 8 9 10 Surface 1[cm2] - - Surface 2[cm ] 2 - - Surface 3 [cm2] - - 3 Plot the dependence: - current – time corresponding to different surfaces areas of the copper electrode; - the value of current intensity after 10 minutes in function of the surface of cathode. Please comment the variation in time of the current intensity for the three different cathode surfaces. Questions: 1. What is corrosion electrochemistry and what elements are necessary for her appearance? 2. What is galvanic corrosion? 3. Write the ionisation reaction of metals, the reduction reactions and the global process of corrosion for the following galvanic couple, in sea water: Fe - Zn and Al - Sn. 4. The following corrosion cells are considered: Mg-Al; Zn-Sn; Cu-Al. Knowing the standard potential: indicat e which electrode will corrode in the presence of an acid or alkaline medium. Write the reaction from the corrosion process. 5. During the restoration of the Liberty statue from New York, between the iron work and the copper plate that cover it, teflon was used. a) explain what is the corrosion type that would take place in the absence of teflon; b) write the local galvanic cell and establish its polarity; c) write the reaction from the anode and the cathode. 4