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Distributor's Link Magazine Winter 2020 / Vol 43 No1

26 THE DISTRIBUTOR’S

26 THE DISTRIBUTOR’S LINK Bruno Marbacher Bruno Marbacher earned his mechanical engineering degree in Switzerland, he also holds a business degree. He started out as a tool and die maker (poly-mechanic) and over the years he has held various management positions in quality and engineering. During his time in America he has developed and given numerous seminars on topics related to the proper use of mechanical fasteners and machine elements, and assists engineers in solving fastening/assembly issues. His has groomed and directed many young engineers in fastening/assembly technology. He now offers his 40 years of experience through writing and lecturing. ZINC…ZINC PLATING, THE COMMON CORROSION PROTECTION FOR FASTENERS Dear reader, have you ever wondered why we zinc plate fasteners, etc.? This article will provide some insight as to why. Fasteners almost always have some sort of finish applied to them. Unprotected plain steel parts will quickly rust under humid conditions. These finishes range from basic oxide coatings, to electro platings to organic and inorganic coatings. The finishes or platings used on any part should contribute to the engineering qualities of the final product and not merely to its aesthetic appeal. The desired finish could include: ¤ Corrosion protection ¤ Resistance to corrosive chemicals ¤ Heat resistance ¤ Electrical conductivity ¤ Wear resistance ¤ Improved lubrication qualities. In this article we address corrosion protection. The selection of a coating or plating and its thickness can lead to many design problems in terms of: ¤ Corrosion ¤ Cost ¤ Dimensional interference. In this article we address zinc plating. TECHNICAL ARTICLE Why Zinc? Zinc is relatively inexpensive; that is one explanation, however, the main reason one uses zinc is that it is less noble than steel, it has what one calls a lower electro chemical potential. What this means, zinc sacrifices itself to protect the underlying steel, it is slowly consumed rather than the underlying steel. As the zinc is being consumed it becomes white, that development is called white corrosion, which typically only occurs under high humidity or when exposed to “salty” atmosphere. If the Zinc plating gets damaged, even though steel is now exposed, it still does not corrode, however, If the exposed area becomes too large, the steel will start corroding (rusting). Electro plated fasteners do not offer long term corrosion protection if used outdoors, directly exposed to the elements. Plating Thickness Bolted joint (nuts and bolts) have to be mated, which limits the amount of coating/plating thickness one can apply. Thicker coatings require special thread tolerances and in some instances over or undercutting of threads. The plating thickness of electro plated fasteners is normally about 3 – 5 µm (0.00012 – 0.0002“) for larger size up to M24 (1”) the plating is about 8 to 12 µm (0.0003 – 0.00047“). CONTINUED ON PAGE 112

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