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Distributor's Link Magazine Summer 2019 / Vol 42 No3


62 THE DISTRIBUTOR’S LINK SPIROL INTERNATIONAL CORP. 30 Rock Avenue, Danielson, CT 06239 TEL 1-860-774-8571 FAX 1-860-774-2048 EMAIL WEB COMPARATIVE ANALYSIS OF STAINLESS AND PLATED/ COATED CARBON STEEL by Michael J. Pasko, Application Engineer The purpose of this paper is to address corrosion in general terms – not to provide a comprehensive reference in regard to all forms of corrosion or a detailed analysis of the strengths and weaknesses of specific alloys. Corrosion of stainless and carbon steel are very different and this paper will provide information regarding general attack or surface corrosion as this is often a primary consideration when selecting fastener material. Carbon and stainless steel are the most common materials from which fasteners are manufactured and each is available in hundreds of alloys or grades with a wide variety of attributes. Initial cost of a stainless steel component is generally higher than carbon steel though not always. For example - material is a smaller component of cost in a light duty Coiled Pin than it is in a Solid Pin of the same diameter and length. As a result the stainless steel Coiled Pin may be available at equal or lesser cost than the carbon steel Solid Pin. In addition, secondary processes, such as heat treating or plating, often add to the cost of carbon steel parts when they may not be incurred with stainless steel. Cost is meaningless without consideration of value. For example, the manufacturer of a high end outdoor barbecue grill may select stainless steel fasteners capable of resisting corrosion much longer than the product’s expected life span. This selection would demonstrate commitment to product integrity, cosmetic appearance, and long life. The ‘value’ of a fastener that provides maximum quality may offset any associated cost increase. The same manufacturer may opt for lower priced plated steel fasteners for a model sold under a discount brand if the anticipated consumer is more conscious of cost than quality. Both are valid reasons to select one material over the other though the choice of stainless steel requires the customer to consider value as much as cost. Designers must weigh cost, benefit, and risk when choosing the appropriate fastener material. SPIROL manufactures Coiled Pins, Slotted Pins, Solid Pins, Inserts, Tubular Products, Machined Nuts, Compression Limiters, Shims and Disc Springs to service a wide variety of industries around the globe. Although SPIROL manufactures parts from many materials, both ferrous and non-ferrous, the scope of this paper is limited to ferrous metal products, and more specifically, stainless and carbon steel. SPIROL’s ferrous product is manufactured from four primary standard material groups: ¤ Low Carbon Steel ¤ High Carbon Steel & Alloys ¤ Martensitic Chrome Stainless Steel (AISI 410 & 420, EN/DIN 1.4516 & 1.4021) ¤ Austenitic Nickel Stainless Steel (AISI 302, 304, 305 or EN/DIN 1.4319, 1.4301, 1.4303) Though carbon and stainless steel are both ferrous metals, meaning they contain iron, their response to corrosive attack is much different. By definition stainless steel must contain at least 10.5% chromium. When exposed to oxygen this alloying element creates a layer of chromium oxide at the surface that quickly stops growing thus becoming ‘passive’. This passive layer is continuous, uniform in thickness, insoluble, and nonporous. The passive layer prevents contact between oxygen in the environment and base metal and will self heal if scratched or abraded as long as oxygen remains available. The passive layer is only 10 to 100 atoms thick and as such has no dimensional impact on parts. Though stainless steel can corrode when exposed to some chemical agents under specific conditions, it will not rust by uniform or general attack as does carbon steel. As an alloying element, the chromium is part of a homogenous blend and is deemed safe as it cannot be easily liberated from the alloy. A final note regarding stainless steel is the relative ease with which it is recycled. Stainless steel is 100% recyclable and industry analysts estimate 80-90% of discarded stainless steel is captured for recycling. TECHNICAL ARTICLE CONTINUED ON PAGE 154


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