Category: Corrosion & Oxidation
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Stainless steels can be susceptible to certain localised corrosion mechanisms, namely crevice corrosion, pitting, intercrystalline corrosion, stress corrosion cracking and bimetallic (galvanic) corrosion. Localised corrosion is often associated wuth chloride ions in aqueous environments. Corrosion resistance relies on a good supply of oxygen. Higher levels of chromium, nickel, molybdenum and nitrogen increase resistance to localised corrosion.
This article describes the contents of the Architects' Guide to Stainless Steel, an online resource containing an extensive amount of architectural information concerning stainless steel. The topics covered include grade selection, product forms, durability, economics, production and fabrication, surface finish, joining, maintenance and cleaning. The computer aided learning package Stainless SteelCAL is also described.
Bimetallic corrosion can only occur when two dissimilar metals are in 'electrical' contact and are bridged by an electrically conductive liquid. The risk of corrosion and staining when stainless steels are in contact with either galvanised steel or aluminium are described. (143)
Pitting resistance equivalent numbers (PREN) are a theoretical way of comparing stainless steels, using their chemical compositions. The formulae are based mainly on chromium, molybdenum and nitrogen contents. Grades with a PREN of 40 or more are known as 'super' austenitics or duplex types, depending to which basic family they belong. A table of calculated PREN values compares some of the ferritic, austenitic and duplex steel grades.
Some chemical have both a 'scientific' and 'common' name, for example caustic soda is the common name for sodium hydroxide. Most corrosion table data uses the scientific names and so finding information can sometimes be difficult when only a common name is known. Reference is also made to alum, aqua fortis, aqua regia, bleach, caustic potash, chromic acid, ethanol and methanol (alcohol), glycol, gypsum, javelle (javel) water, Labarraque's solution, marine acid, muriatic acid, oil of vitriol, oleum, rock salt, salt acid, spirit of salt, sulphurated hydrogen and wood acid.
The carbon ranges of 'normal' and 'low' carbon 304 (304L) and 316 (316L) types are compared. The effect of carbon on intercystalline corrosion resistance and welding is also covered and why steel is often offered as a dual certified product. European grades 1.4301 1.4306 1.4307 1.4401 and 1.4404 are included in the comparisons.
Grade 316Ti is a 316 type stainless steel, stabilised with titanium to reduce the risk of intergranular corrosion (ICC). The 316L 1.4404 or 1.4432 grades can be considered as alternative choices. Under most conditions 316Ti and 316L are interchangeable, but the elevated temperature strength, corrosion resistance, machinability, cold-formability and polishing characteristics can affect the final choice of grade.
Background information on the sources of chlorides within insulation materials is mentioned. The use of paint and aluminium foil barrier methods between the steel shell and insulation layer as a method of eliminating the risk of corrosion to the steel is also outlined.
Pitting and stress corrosion can result from moist thermal insulation where chlorides are present. This information sheet provides background information on the sources of chlorides within such insulation materials and describes two corrosion prevention methods. Paints (e.g. a high temperature silicone type) or 0.06 mm thick aluminium foil can be used as barriers between insulation layers and stainless steel. BS5970, BS5422 are cited in this information sheet.
In certain aggressive environments some grades of stainless steel will be susceptible to localised attack. Six corrosion mechanisms are described in this article, namely pitting corrosion, crevice corrosion, bimetallic (galvanic) corrosion, stress corrosion cracking (SCC), general (uniform) corrosion and intergranular (IGC), sometimes known as intercrystalline or IC) or weld decay attack.