Corrosion Resistance of Bright Nickel Coatings in the Vapor of Acetic Acid
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CORROSION RESISTANCE OF BRIGHT NICKEL COATINGS IN THE VAPOR OF ACETIC ACID D. Yu. Ushchapovskiy,1 M. V. Byk,1 O. V. Linyucheva,1 S. V. Frolenkova,1, 2 R. M. Red’ko,1 and V. V. Yakubenko1
UDC 621.357
We study the corrosion behavior of bright nickel coatings in the vapor of concentrated acetic acid. Deposits with a sulfur content of 0.25 wt.% are obtained from electrolytes intended for bright nickel plating with an admixture of saccharin (5 g/dm 3 ). It is shown that their potentials vary within the
range [(–0.02)–(– 0.01) V]. Moreover, it is demonstrated that, in the course of holding of these nickel deposits in the vapor of acetic acid, the amount of sulfur on the metal surface gradually increases and reaches 3.32 wt.%. It is established that the sulfur content of the film of black-color corrosion products formed after holding in the vapor for 24 h is close to 22 at.% and corresponds to nickel sulfide. Keywords: bright nickel deposits, saccharin, acetic acid vapor, humid tropical climate, corrosion products, cellular structure.
Introduction Bright nickel coatings are frequently used in industry in order to get decorative outward appearance and protect workpieces and products made of carbon steels and zinc-, magnesium-, and copper-based alloys against corrosion. Unlike dull coatings, they have more negative equilibrium potentials and, moreover, are less stable under the conditions of corrosion [1] due to the elevated content of sulfur deposited in the course of electrodeposition as a result of the adsorption of sulfur-containing luster-forming organic admixtures and, in particular, saccharin on the surface. According to certain data, the process of desulfurization of saccharin may run on the cathode with the formation of benzamide and nickel sulfide [2, 3]. As a characteristic feature of the tropical humid climate, we can mention the presence of organic acids and, in particular, acetic acid, in the vapor. For this reason, acetic acid is used for the standard rapid testing in chambers filled with acetic-saline or copper-acetic-acid-saline fogs for the imitation of conditions of the tropical climate. In particular, it was shown that the corrosion rate of bright nickel coatings with high contents of sulfur (0.25 wt.%) in neutral solutions of sodium chloride is almost the same as for semibright coatings with the minimum sulfur content ( Q ) (lower than 0.049 wt.%). In the presence of acetic acid, the rate of corrosion of bright coatings with high contents of sulfur increases by 1–2 orders of magnitude. A procedure for rapid testing in the vapor of concentrated acetic acid was also proposed in [4, 5]. This procedure enables one to predict the corrosion behavior of nickel coatings under the conditions simulating humid tropical climate. According to the results obtained in [3, 6, 7], the corrosion of nickel and its alloys in solutions of sulfuric acid runs mainly under 1 2
“I. Sikorsky Kyiv Polytechnic Institute,” National Technical University of Ukraine, Kyiv, Ukraine.
Corresponding author; e-mail: [email protected].
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