مقالات آنالیز و کنترل در آبکاری

troubleshooting, testing & analysis

CORROSION AND EXPOSURES – AN

OUNCE OF TESTING IS WORTH A TON

OF RECALLS

BY BENJAMIN CARDENAS, ICP ANALYST/SALT SPRAY SPECIALIST; PHILIP

SCHMIDT, CHEMICAL ANALYST/CORROSION SPECIALIST; AND MICHAEL

PORFILIO, DIRECTOR OF OPERATIONS, ANDERSON LABORATORIES, INC.,

GREENDALE, WIS.

There are many options for surface finishes when it comes to metal products.Chemical coatings and paints are common in metal parts (Figure 1). Surfacepreparation and coating options range from polishing and blasting to pickling,passivating and plating. Application of thecoatingaddsanother layer of complexityto the finishing process. So, how do you know if your coating is going tohold up over time and exposureto the elements before going into production?The answer is simple: environmental simulation and/or accelerated corrosiontesting.

Corrosion Costs to America

The products of corrosion that are at times an aesthetic annoyance cannot becompared to the potential of failure to many corrosion modes. The numeroushidden costs of corrosion structures, equipment, bridges, proactive coatings,specialty materials, cathodes, anodes, and other manufactured parts are justsome of the items that suffer the effects of corrosion.The coatings industry is experiencing pressure from environmentalist groups,corporate sustainability programs and individuals to produce “greener” products.Environmentally friendlier coatings usually have less corrosion resistanceover the lifetime of the coating or finish. This had left the industry, in some situations,struggling to come up with new alternative formulations that performas well as their less environmentally friendly counterparts.The price of corrosion isstaggering. Some quick costs for corrosion include$108 billion in protective coatings, $7.7 billion for Corrosion Resistant Alloys(CRA), and $1.1 billion spent on corrosion inhibitors, to name a few. The totaldirect cost of corrosion in America is $276 billion in 2012. To emphasize howlarge this number is in 2012, it is projected to be 3.1% of the United States GDP.

Cabinet Salt Spray and Humidity Testing

Tests such as neutral salt spray testing (NSS) and humidity testing can help a

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Figure 1 (left): Painted power transfer assembly prior to exposure to neutral salt spray (NSS).

Figure 1a (right): Salt spray exposure IAW ASTM B117-11 in process.

manufacturer or end user determinethe efficacy of their coating/finishingprocess. By examining test specimensthroughout the duration of the test,obvious surface effects are noticeablesometimes within a single day or two,preventing expensive and time-consumingrework and recalls. This comparisonto an unfinished, or controlspecimen will help to determine thebest surface finish for the intendedapplication (Figures 1-5). The followingimages illustrate failures of coatingapplication and/or performance.There are many specifications andstandards written to helpperformthis testing and, equally as important,evaluate the results. The mostcommon specifications for performingan NSS test are ASTM B117 andISO 9227, which outline the test conditionsrequired for the neutral saltspray test. These specifications unifythe test parameters across the globein the effort to evaluate the efficacyof paints, coatings and finishes.These specifications unify the testparameters worldwide in the effortto evaluate the efficacy of paints,coatings and finishes. In this test, anatmosphere of a 5% solution of sodiumchloride is atomized and introducedat a constant rate throughouta sealed chamber. (Figure 1a) Testspecimens are inclined and placedin the cabinet so that the test surfaceis parallel to the primary directionof flow of the sodium chloridemist. The mist wets the surface andthe excess solution runs off into thecabinet.Humidity testing is another measurementof the effectiveness of thecorrosion resistance properties ofa metal part. This test can be performedis several ways. The first takesplace in an unused NSS cabinet bysimply turning off the salt solutionflow and plugging the drain. Thesecond test takes place in an environmentalsimulation cabinet specificallydesigned for this type of test. The requirements for this test can

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Figure 2. Failed painted power transfer assembly

after exposure to NSS.

Figure 4. Failed galvanized plating on a spring after

48 hours of exposure to NSS.

laid out in a manufacturer’sspecification or in an ASTM

publication such as ASTMD2247. This type of testinghelps to evaluate primarilypaints and their tendenciesto bubble and blister.These two types of cabinetcorrosions testing canalso be very effective asfreeiron detection tests.Free iron testing can be anexcellent predictor of corrosionresistance propertiesand surface finish success.Locations, on a casting or finished part, which are high in free iron, can becomeepicenters for corrosion while in service. In most cases, a free iron test can bedone via salt spray testing in about 2 hours or in ahumidity chamber in about24 hours (See Figure 6).These different combinations of coatings and hours of exposure have led toa number of different conclusions for these companies. The most important being cost effectiveness of coating and surface finishes vs. corrosion resistance.

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Figure 6. Free iron on threaded portion of powdered metal ring

Additional Free Iron Detection Tests

In addition to these two cabinet style tests, another test that can effectivelydetect free iron is a potassium ferricyanide-nitric acid (Ferroxyl) test and inaccordance with MIL-STD-753 and ASTM A380. The ferroxyl test will illuminateeven the lowest levels of iron contamination. In this test, reactions withthe chemicals in the test solution and yields a chemical reaction in the formof a bluish coloration demonstrating the presence of free iron (See Figure 7).Once the testing has begun, the challenge becomes the evaluation of theconstantly changing surfaces of interest. Sometimes it is helpful to add a pointof certain failure to the test specimen, or scribe, to see just how bad a failurebecomes if it should occur via damage or deficiency of the coating. There aremany evaluation procedures such as ASTM D610, D714 and D1654 that overseethe evaluation of test results. These specifications use numerical values to evaluate the severity of the corrosion, blistering and/or creep. Theselaboratory testshave one goal in common:to assist the surface preparationindustries evaluatethe efficacy of their coatingmaterial and/or process.Corrosion testing can helpeveryone from an epoxypowder coating companiesto chemical coating companiesto zinc/cadmium/chrome plating companiesand everything in between.The ultimate goal in applyinga coating or perform surface treatments is

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Figure 7. Free iron indications after ferroxyl test on high alloy

stainless steel casting. The blue coloration indicates the

presence of surface-free iron.

protect surfaces providingthe highest qualityproduct with the leastamount of overheadin order to yield maximumprofit.

Accelerated CorrosionTesting

Accelerated corrosiontesting allows basematerial verificationprior to final applicationof surface treatmentssuch as platingor polishing. Unwantedcorrosion in stainlesssteels and other related alloys is of significant concern in their engineering.Rough surface finishes, elevated temperatures and corrosive environmentsencountered in service can accelerate corrosion and lead to material early failure.Different forms of corrosion are commonly encountered in service, and manycan be strongly influenced by heat-treating. Fortunately there are many typesof standardized test methods, which seek to qualify materials and quantify therates corrosion that the alloy may experience.Pitting corrosion is a localized form of corrosion found in stainless steels andother related alloys, which do not typically experience uniform corrosion. Pittingcorrosion is typically caused by the breakdown of a passivating surface layer andresults in small areas of corroded “pits” to form. These pits can act as sites ofcrack initiation and are detrimental to a material’s performance.Intergranular corrosion in stainless steels is usually associated with a precipitatedinter-metallic phase found at the grain boundaries of a material. Forexample, chromium carbides precipitation at the grain boundaries can depletethe surrounding material of corrosion-inhibiting chromium. In the presence ofa corrosive environment, these grain boundaries are preferentially attacked and are subsequently weakened.

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Figure 8. Conforming and non-conforming G48C corrosion coupons.

Pitting indications correspond to dark spots on left-hand samples.

Stress corrosion cracking is another form of corrosion that can cause normallyductile materials to failin a brittle manner.When a material is bothstressed (especially atelevated temperatures)and also exposed to acorrosive environmentthe grain boundariescan be attacked andthe materials mechanicalproperties can becompromised. Stresscorrosion cracking isvery alloy specific inthat certain alloys mayonly experience SCC inthe presence of specific

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Figure 9. Intergranular corrosion of metallographic cross-section of

304 stainless steel.

chemical environments.Austenitic stainless steelsin particular are susceptibleto SCC in the presence ofchlorides.Sensitizing heat treatmentsin stainless steels canaggravate problems associatedwith intergranularcorrosion. Sensitization isa process in which a materialis heated to a temperaturein which chromiumcarbides, for example, canprecipitate out of solutionat the grain boundariesand create a chromiumdepleted region whichadversely affects corrosionresistance. Austenitic stainlesssteels are susceptible tosensitization after heatingto ~500°C - 800°C. Theaddition of carbide stabilizingelements such as niobiumor titanium can helpto ameliorate the problem;however, processes suchas welding a susceptiblematerial can still detrimentallyaffect its microstructural integrity.There are many standardized methods for testing a material’s susceptibilityto one of the three forms of corrosion addressed. A common test for pittingcorrosion is the ASTM G48 test. A sample coupon is polished, dimensioned andweighed prior to testing. It is then placed in a ferric-chloride solution for a specifiedperiod of time. The sample is reweighed after testing and visually examinedto look for signs of pitting corrosion. This test can also quantify the mass lossrate of certain materials and can very useful in ranking materials’ ability to resistthis form of corrosion.ASTM A262 Method E is a common test for intergranular corrosion. Twosamples are polished for testing, with one as a control. One of the two samplesis boiled in a copper-copper sulfate sulfuric acid solution for a specified periodof time while the other is desiccated.After testing the two samples are given a U-bend test and the apex of the bendis examined by microscope to look for evidence of intergranular fissures. Thepresence of these fissures indicates the materials’ susceptibility to intergranularcorrosion, and can be used to determine if a sensitizing heat treatment hasaffected a materials susceptibility to intergranular corrosion.The ASTM G36 test is another commonly used test to look for susceptibilityto stress corrosion cracking. Samples are polished and bolted into a U-bendconfiguration. These samples are boiled in a magnesium chloride solution fora specified period of time, and the apex of the bend is microscopically evaluatedto look for evidence of crack formation. The presence of cracks indicates suscep-tibility to stress corrosion cracking.Other common forms of testing and their applications areas of interest are Althoughcorrosion can be detrimental to the service life of many stainlesssteels and related alloys, there are, fortunately, many methods with which to helppredict a material’s expected performance. The American Society for TestingMethods has standardized many of these methods and are, therefore, part ofthe standard toolkit with which one can help to minimize the costly effects ofcorrosion in metals.

Corrosion Societies and Technical Committees.

There are a few organizations in the United States to assist manufacturers inmatters related to corrosion. The most prominent is NACE International, theNational Association of Corrosion Engineers, based out of Houston, Texas,which focuses on corrosion of numerous grades of steel and surface treatments.Most of the specified material types are austenitic stainless steel, duplex stainlesssteel, and nickel-based alloys as dependent upon the environment or exposure tosour gas and corrosive environments. NACE Int. works with individual membersas well as organizations.More support can be found for manufacturers from the ASTM world. ASTMis the American Society for Testing Materials and has hundreds of committeesand sub-committees on technical topics from testing, to material specifications,to manufacturing practices and parameters. Technical committee G01was formed in 1964, meets twice per year and has more than 350 members.This committee has many manufacturing members to drive the process for thespecifications contained in the corrosion testing volume 3.02 of the ASTM familyof standards and specifications.On a global level we have The World Corrosion Organization (WCO), whichis based out of the European Union (EU) and discusses, attends, and draftsmany NACE type material codes and testing protocols. The goals and missionof the WCO is to: alert and make industry aware of the problems and solutionsassociated with corrosion; identify best practices to deal and prevent corrosion;facilitate corrosion-related control through the industry; and normalize worldcorrosion standards. As the ISO and other EU states begin to solidify theirstances of corrosion and its by-products, the WCO is making a unified approach.

Final Thoughts

The application of corrosion tests can be a complicated subject. Once youestablish your specific goals and agenda are, the usefulness of the process canbe informative and value added. To have a solid concept of the product lifeand usefulness before elemental degradation is key for the selection of materials,coatings and/or manufacturing process. Better products can be achievedthrough proper testing selection.Anderson Laboratories, Inc., was established in 1939 in Milwaukee, Wis. They are currentlylocated in Greendale, Wisconsin. The company is an independent materials testinglaboratory specializing in chemical analysis, mechanical testing, welding procedure andperformance qualification, failure analysis, environmental and corrosion testing, as well ason-site evaluations. Anderson Laboratories, Inc. has quality and system programs accreditedor compliant to ISO/IEC 17025, 10CFR50 Appendix B, ASME NQA-1, 97/23/EC PED,RoHS, NORSOK M-650, ISO 9001, to name a few. For more information, please contactAnderson Laboratories, Inc. at (800) 950.6330, e-mail at این آدرس ایمیل توسط spambots حفاظت می شود. برای دیدن شما نیاز به جاوا اسکریپت دارید, or viatheir page www.andersonlabs.com.