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Is Your Stainless Steel Rusting?

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Is Your Stainless Steel Rusting? Ornamental & Miscellaneous Shop Talk Metal Fabricator Official publication of the National Ornamental & Miscellaneous Metals Association Is your stainless steel rusting? n Iron contamination is not the only cause of rust problems. In This stainless steel this article, we explore additional bank sources of corrosion as well as depository is showing sig- solutions. nificant rust stains. See the related article on page 46 to learn how the By John Campbell author helped to restore the metal to look Like the words “cruel kindness,” stainless like new. steel appears to be an oxymoron, especial- ly when NOMMA members relate some of their experiences with staining and rusting on fabrications like stainless gates and handrails. The causes are often puzzling. Why does a stainless steel tubular handrail show a pattern of rusting like a barber- What makes an alloy stainless? For your pole? Why does a stainless steel gate information exposed to a seawater atmosphere bleed An iron base alloy with 12 percent or rust? What caused the rust on a stainless more chromium is considered a stainless steel bicycle rack? To find the answers to steel. Although there are five types of About the author: Mr. these questions would take the sleuthing stainless alloys (ie. ferritic, martensitic, Campbell was skills of our finest forensic detectives. austenitic, precipitation hardening, and formerly self- duplex) the austenitic AISI 300 series employed for 26 years with Two major alloys make up over 75 percent of all cor- Castings possibilities rosion resistant applications from Consultants The experts on stainless alloys conclude handrails to commercial kitchen appli- Inc. and is now that there are two major reasons for rust- ances. a highly regarded industry like stains on stainless steel. Surface con- Two chromium-iron base alloys with author and senior writer for Fabricator. He is based in the tamination with iron or steel dust particles chromium contents of over 12 percent are suburbs of Milwaukee, WI. is the first possibility. Fabricating steel and the ferritic and martensitic. Both are mag- What is passivation? stainless in the same area is just one of netic; and the martensitic alloys like AISI Passivation is the oxidizing many sources of this contamination. The 410 can be heat treated, hardened, and process that causes the other cause is penetration of stainless tempered to the desired balance of ductili- chromium in the stainless to steel’s passivated surface by halogen salts. ty and hardness. Knives in quality stainless combine with available oxy- These complaints occur with installations tableware are commonly AISI 410. They’re gen to form a chromium oxide film. of AISI 304 stainless near seawater or magnetic; whereas, spoons and forks around a source of halogens like winter stamped from AISI 304 are non-magnetic. Tip: A way to prevent rusting around heat-affected zones, street salt or chlorine treated swimming Adding nickel to chromium-iron alloys called sensitization, is to fab- pools. A candid explanation to a customer produces an austenitic grain structure. ricate with alloys of low car- who wants stainless steel, thinking they’re The combination of 18 percent chromi- bon content or alloys with buying the ultimate insurance against cor- um-8 percent nickel in the AISI 300 series carbon stabilizing elements rosion, may be easier before the purchase is typical of these stainless grades. AISI like titanium and niobium. than after. 302 and 303, both of which have carbon Reprinted from January/February 2005 n Fabricator 1 contents of .15 percent max., are less Although passive films form on stainless steels spontaneously corrosion resistant than AISI 304. in the presence of oxygen, some fabricating specifications mandate Additions of sulfur and phosphorous chemical passivation in an oxidizing medium like nitric acid. produce the free-machining grade AISI 303. The more popular alloys for sheet, tubing, and pipe are AISI 304 and 316 (.08 percent max carbon). um, forming chromium-carbides in Halogen salts undermine For fabrications where intergranular the heat affected zone (HAZ). That passivation corrosion is a concern, lower carbon prevents chromium from developing Chlorides are one of the most alloys (.03 max), like AISI 304L and the surface oxides that make the common elements in nature, and that AISI 316L, are available. With only 5 alloys stain resistant. whole family of halogen salts, identi- percent to 10 percent ferrite content The condition is called sensitiza- fied by the suffix ine, like chlorine, these alloys in the austenitic annealed tion. A solution annealing heat treat- are active ions, good current carrying condition will be non-magnetic. ment will restore the free chromium; electrolytes and highly corrosive to One caution: do not correlate but that’s not always practical with stainless steels. These salts will even magnetic permeability with corrosion large, rangy fabrications that will destroy Teflon. Stainless steels are not resistance. Bend an elbow in AISI 304 warp at high temperature. Another going to resist staining and rusting in stainless and the bend becomes mag- way to prevent sensitization is to fab- saltwater atmospheres without peri- netic. Cold working changes the grain ricate with alloys of low carbon con- odic cleaning and passivation. structure, but not the chemistry. To tent (.03 percent max AISI 304L) or Although passive films form on retain the former austenitic structure alloys with carbon stabilizing ele- stainless steels spontaneously in the requires a solution anneal heat treat- ments like titanium (AISI 321) and presence of oxygen, some fabricating ment (1895° F for 304 and 2003°– niobium (AISI 347). Sensitization specifications mandate chemical pas- 2048° F for 316 plus a rapid quench), explains why rusting on stainless steel sivation in an oxidizing medium like not too practical for a large fabrica- fabrications is first noticed around nitric acid. Without a steady supply tions. ASTM A-511 covers seamless the welds. Those heat affected zones of oxygen stainless steels will corrode. stainless tubing for general mechani- may be depleted of chromium. In halogen salt solutions like salt cal applications. Specifications for Normal atmospheric environ- water copper-nickel alloys such as low carbon stainless grades are cov- ments are not severe enough to Monel are more corrosion resistant ered by ASTM A-312. Any fully cause intergranular corrosion in a than stainless steels. Engineering annealed specification will be more heat affected zone; but rusting will compromises are generally a matter expensive. occur adjacent to the welds. When of cost. While on the subject of magnetic and if intergranular corrosion does permeability, be aware that stainless occur, the welds fall away from the How does finish steel casting alloys known as CF-8 corroded HAZ like wads of chewed affect staining? and CF-8M, the cast equivalents of bubblegum. Several NOMMA members men- AISI 304 and 316 respectively, may tioned that surface finishes affect the have magnetic properties. The What does resistance of stainless steels to stain- chemistries of the cast stainless alloys the nickel do? ing. David Lazarus at Polished Metals are not identical to the wrought Nickel added to the chromium in Ltd. said that 85 percent of their alloys; their chemistries are adjusted the 18-8 stainless alloys stabilizes the stainless steel sales are shipped with a to enhance fluidity. Casting grades austenitic structure and helps #8 mirror finish, which could can have a ferrite content up to 15 improve the passivity of the alloy. In increase the price 33 percent to 40 percent and be within alloy specifica- the presence of oxygen, the stainless percent over a #4 satin finish. He tion. In addition, beware of checking steels of the 300 series spontaneously believes that the smoother finish is alloy inventory with a magnet. Non- develop a chromium oxide film. The easier to maintain, which makes magnetic alloys like Monel (70 Cu-30 oxide films on metals like aluminum sense. Rust stains on a satin finish Ni) become magnetic in unheated and silver are visible. However, tend to follow the direction of the northern warehouses when the tem- chromium oxides are not visible on grinding. One tiny spot bleeds a perature drops below zero. stainless. brown streak an inch long in either For most fabrications AISI 304 is direction following the finish crevice. How welding changes the stainless alloy of economical John O’Reilly of MMF Architectural chromium content choice. To enhance the alloy’s resist- agrees, pointing out how depositions One of the problems with welding ance to pitting, molybdenum is of salt are trapped in the crevices of a stainless steel stems from the carbon’s added. Alloys with moly additions are brushed finish. In the design stages affinity for chromium. Under welding AISI 316 (2–3 percent) and AISI 317 alloy selection and finish are two temperatures carbon will absorb up (3–4 percent). Both are more resist- important considerations. Wherever to 17 times its own weight in chromi- ant to halogen salt exposure. salinity is part of the environment, 2 Reprinted from Fabricator n January/February 2005 Blasting stainless with glass beads or silica sand previously used heat into the surface that the alloy’s to blast steel will impinge surfaces with iron. Any abrasive used on grain structure changes, destroying its steel will also impart iron to stainless. natural ability to passivate. Less hazardous solutions of citric acid have successfully removed rust stains. The higher polished surfaces O’Reilly recommends AISI 316 stain- less steel surfaces. clean better than those with deeper less instead of AISI 304 along with a One time we vacationed on the grinding marks. (See article on pg. mirror finish. island of Vieques, close to Puerto Rica. 46). Although nitric acid and hydro- The villa, where we stayed, stood high fluoric may impart a more lasting Surface on the Atlantic side of the island with passivating film on stainless, it’s more contamination stainless steel handrails above the hazardous to handle.
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