Special Topic : Testing & Inspection
Back to Basics - Testing and Inspection in Stainless Steel
Stainless steel is a robust , corrosion-resistant material , and its integrity is highly dependent upon skilled operators that work diligently to ensure the optimization of the material ’ s lifecycle . Whether it is checking for the presence of corrosion , or for imperfections that might lead to cracks , having systems in place to reliably test and inspect stainless steel will harness the material ’ s innate durability , placing reliability at the forefront while simultaneously preventing unexpected shutdowns .
This article will revisit the basics . It will provide a brief discussion of how and why stainless steel is tested or inspected , and shed light on the tools and methodologies most commonly utilized to maintain optimal results . It will also discuss the impact of effective testing and the potential of the material ’ s lifecycle , which can be an effective tool in emission abatement .
By Justin Yule
Nature and Characteristics of Stainless Steel
Stainless steel is a metal alloy of iron , chromium , and other metals like nickel , molybdenum , and carbon . Different blends with different ratios of these metals will produce different types of stainless steel . Chromium is a key element , imparting to the alloy the ability to build a passive oxide layer when exposed to oxygen making this layer is self-repairing . In addition to this , newly produced stainless steel is placed in an acid bath to enhance the passivating process . The result is a product that is both useful and aesthetically appealing .
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it resists corrosion , but it also bears a surface that makes it a reliable building material . It is , however , highly vulnerable to chlorides that destroy the passivating oxidizing film and can corrode the metal . Chlorides are detrimental to the lifecycle of stainless steel , and unfortunately are widely present in the environment . Therefore , special care must be taken to check for damage to the surface of stainless steel before cracks or corrosion develop . The good news is that the metal rewards diligent monitoring with remarkable durability .
What Variants Are There of Stainless Steel ?
Stainless steel is a category of metals with many variants . First , there are distinct families of stainless steel , each identified by a different ratio of the key metals . The primary families are : Austenitic , Ferritic , Duplex , and Martensitic . Austenitic is the most widely used . It is understood to be the most corrosion resistant and it is generally non-magnetic . Ferritic stainless steel has no – or very little – nickel , and as a result is a more cost-effective variant . It is generally a tougher and more creep-resistant form than austenitic but carries an inferior surface . Consequently , it is a favored option where visual appeal is not a consideration .
Duplex stainless steel is a combination of the Austenitic and Ferritic variants , capitalizing on the corrosion resistance of the former and the high strength of the latter . This combination is a recent innovation and is exhibited in many proprietary alloys . The Martensitic family of stainless steel is characterized by hardenability and strength , making it a common pick for valves , turbines , and other mechanical parts . Among the families of stainless steel , it is important to note the various grades that are available . These are specified by numerical identifiers ( UNS , Unified Number System ) and indicate the precise characteristics of the metal . Numerical identifiers of grade are established by system standards . The most common standards are SAE ( Society of Automotive Engineers ) and ASTM ( American Society for Testing and Materials ), but others include BS ( British Standards ) and ISO ( International Organization for Standardization ).
What Causes Corrosion Resistance in Stainless Steel ?
All metals will corrode in certain conditions . Corrosion leads to the progressive weakening and disintegration of the metal , and will jeopardize its function . As stated above , the chromium in stainless steel solves this problem by forming a passive oxide layer when exposed to oxygen . This means that chromium is reacting to oxygen just as any metal does , but the consequence is not rust but a passive , inert layer that is highly unlikely to react with most chemicals . In fact , the aspect of stainless steel that makes it corrosion resistant is the same aspect that makes it a hygienic metal . The passive oxide layer produced by chromium is understood as self-healing because , if a section of metal was cut away from its surface , the suddenly exposed metal – and the chromium therein – would react anew with the oxygen in the air , reforming the layer . With stainless steel able to fend off corrosion so well , it is important for operations teams to be familiar with potential vulnerabilities .
Why Does Stainless Steel Rust ?
Stainless steel corrosion occurs when environmental conditions cause that passive layer to break down . If the protective oxide cannot reform in a given circumstance , corrosion reactions can progress rapidly . The two most common forms of corrosion are pitting corrosion and crevice corrosion .
Pitting corrosion occurs when chlorides , a common instance being sea-water , react with and start to dissolve the protective layer on stainless steel . Pitting is visible to the naked eye as reddish-brown iron oxide deposits alongside pits on the surface of the metal . However , pitting corrosion can also lead to deeper hidden cavities that will continuously expand and rupture the metal . Common trouble spots for pitting include upward-facing surfaces where liquid like sea water can pool , or downward facing surfaces where droplets gather . In either instance , as the solvent evaporates , the chlorine gets more concentrated in the remaining liquid mak-