Salt spray test has been recognized for ranking the performance levels of coating systems. Yet, there is little correlation between results from standard salt spray tests and real-life anti-corrosion performance. In particular, it will not be able to give accurate assessment for galvanized coating because the continual wetness during the salt spray test does not allow this passive oxide and carbonate layer to develop, and this adherent layer protects the zinc from corrosion meanwhile zinc protects the substrate steel from erosion. When coated material is evaluated by the salt spray test, but it does not have ultraviolet light which will cause deterioration for paints and primers. Therefore, using salt spray test results to guide the selection of protective coatings for steel is quite a serious problem in the engineering field. Despite the well understood limitations of the test, many project designers use the salt spray test result as a guideline to choose their favorite coatings. For decades, this test has generated misleading information about coating performance and its results still feature prominently in the marketing materials of products that yield more favorable outcomes than in practical world. This article is meant for giving some insight into the limitations of this type of accelerated testing.
What is salt spray test?
There are two types of salt spray tests in common use, neutral and acetic acid, but neither of these does what we want in a test, which is to replicate nature on an accelerated time basis. It's also called salt fog test which is standardized and popular corrosion test solution, and it's used to check corrosion resistance of materials and surface coatings. Usually, the materials to be tested are metallic and finished with a surface coating which is intended to provide a degree of corrosion protection to the underlying metal. It is an accelerated corrosion test that produces a corrosive attack to coated samples for assessing the effectiveness of the protective coating. The appearance of corrosion is evaluated after a certain period of time, and test duration depends on the corrosion resistance of the coating. Generally, the more corrosion resistant the coating is, the longer the period of testing before the appearance of corrosion or rust. The salt spray test is one of the most widespread and long-established corrosion tests, and ASTM B117 was the first internationally recognized standard which was published in 1939. The other important relevant standards are ISO 9227, JIS Z 2371 and ASTM G85.
Let's take a look at the testing process first. The salt spray procedure involves the spraying of a salt solution with 5 percent salt (sodium chloride) onto the samples, and the test is done inside a temperature-controlled chamber. The temperature within the chamber is maintained at a constant level, and the samples are constantly subject to the saltwater spray, and the testing procedure of ASTM B117 is listed below:
√ Placing wooden in the chamber
√ Muab cov qauv tso rau ntawm ib lub khib ntoo ntawm lub kaum sab xis me me
√ 5 feem pua NaCl nyob rau hauv cov kais dej twj tso kua mis los ntawm lub reservoir rau tshuaj tsuag nozzles
√ Cov tshuaj tov nrog humidified compressed cua ntawm nozzles
√ Cov huab cua compressed atomizes NaCl tov rau hauv pos huab ntawm nozzles
√ Cov cua sov tswj qhov kub ntawm 95 ° F txee
√ Kev ntsuas lub sijhawm tuaj yeem yog los ntawm 24 teev txog 1000 teev thiab cov qauv tau tig nquag kom txhua tus qauv raug pom zoo ib yam li ua tau rau cov ntsev tsuag dej.
Thaum cov tshuaj tsuag ntsev yog siv rau kev sim metallic-coated steel ntawv, qhov kev ua tau zoo corrosion yog ntsuas raws li hauv qab no:
√ Nr. ntawm cov xuab moos kom txog thaum xeb ntawm cov hlau yog thawj zaug
√ Nr. ntawm cov sij hawm kom txog rau thaum 5 feem pua ntawm thaj chaw saum npoo yog xeb
√ Nr. of hours until 10 percent of the surface area is rusted
Kev ntsuam xyuas ntawm kev ua tau zoo ntawm cov xim pleev xim, xim primers lossis topcoats yog ntsuas los ntawm cov hauv qab no:
√ Kev ntsuas qhov dav ntawm cov xim pleev xim rau hauv ib txoj kab uas sau los ntawm cov xim los yog ntawm cov npoo sheared tom qab 250, 500, 750 teev ntawm kev raug mob hauv chav kuaj
√ Ntsuas cov xim blistering uas tau tshwm sim rau ntawm qhov chaw ntawm cov xim hlau pleev xim rau hauv 250, 500, 750 teev.
Why is salt spray test misleading?
Raws li koj tuaj yeem paub tias kev ntsuas tshuaj ntsev ntsev (B117) tsis cuam tshuam nrog kev cuam tshuam rau ultraviolet lub teeb, xim fading thiab chalking. Txawm hais tias G85 muaj ntau qhov kev hloov pauv uas cuam tshuam nrog kev sib ntxiv ntawm cov kua qaub thiab SO2, tab sis nws tseem tsis tuaj yeem cuam tshuam nrog kev ua haujlwm sab nraum zoov hauv chloride cua. Muaj peb qhov laj thawj tseem ceeb tshaj plaws rau kev kuaj ntsev ntsev tsis cuam tshuam nrog rau feem ntau tiag tiag - lub ntiaj teb raug mob.
√ The surface of the test samples is constantly wet, which does not happen in a real environment.
√ The chamber temperature is constantly at 95 degree F, which increases water, oxygen and ion transport compared to the real atmosphere.
√ The chloride content is at a very high level of 5 percent , preventing zinc from forming a passive film.
Tau kawg, nws tsis ncaj ncees los hais tias cov tshuaj tsuag ntsev tsis muaj txiaj ntsig vim nws muab kev soj ntsuam ntau ntawm kev ua haujlwm tiv thaiv -corrosion, thiab yog li qhov kev sim no tau siv dav hauv cov hmoov txheej thiab pleev xim kev lag luam los xyuas cov txheej txheem. khoom zoo. Piv txwv li, yog tias qhov kev ua tau zoo ib txwm nyob hauv kev ntsuas ntsev ntsev yog 750 teev ua ntej qhov pib ntawm corrosion, nws yog ib txoj hauv kev ceev los txiav txim siab seb puas muaj teeb meem loj ntau lawm cuam tshuam rau cov khoom zoo. Qhov kev ntsuam xyuas yuav tsis xaus tias cov khoom lag luam tau txais txiaj ntsig zoo, tab sis yog tias qhov kev ua tau zoo hauv qhov kev sim no tsis zoo, kev ua haujlwm sab nraum zoov kuj yuav tsis zoo li qhov xav tau. Tsis yog qhov xav tsis thoob, ISO 9227 pom zoo tias kev ntsuas tshuaj ntsev tsuas yog tsim nyog raws li kev ntsuas kev tswj hwm zoo.
In short, salt spray test is not an accurate way of evaluation of corrosion resistance primarily because the salt spray destroys the mechanism that galvanized finish protects the steel by providing a barrier, and this barrier is also an important electrolytic protection. Due to this reason, the test result might lead to a false conclusion on how the galvanized finish will perform, which renders the test meaningless. No wonder, this test is now largely abandoned even by the automotive industry.
Hmoov tsis zoo, qhov kev sim no tseem nrov heev. Txawm hais tias ISO 9227 tau hais meej meej tias "Muaj tsis tshua muaj kev sib raug zoo ntawm kev ua haujlwm ntawm cov tshuaj ntsev ntsev thiab kev tiv thaiv corrosion hauv lwm cov xov xwm, vim tias muaj ntau yam cuam tshuam rau kev txhim kho corrosion, xws li tsim cov yeeb yaj kiab tiv thaiv, sib txawv heev nrog rau cov khoom siv. cov xwm txheej ntsib." Yog tias tus tsim qauv tsim muaj kev paub txog qhov kev ntsuas ntsev ntsev tiag tiag txhais tau li cas, nws tuaj yeem nkag siab txog nws cov kev txwv thiab siv cov txiaj ntsig hauv kev txiav txim siab.
Dab tsi ua rau corrosion hauv ib puag ncig tiag?
The most common metallic corrosion is atmospheric. If metals like steel, copper, magnesium and aluminum are exposed to the atmosphere, they react with free-flowing air and moisture to develop oxides. The atmospheric corrosion depends on five main factors:
√ Kub
√ Cov av noo
√ Los nag
√ Sulfur dioxide, nws yog tsim los ntawm cov pa phem concentration nyob rau hauv cov huab cua
√ Air salinity.
Tsis muaj ib qho ntawm cov xwm txheej no tuaj yeem raug hu ua qhov tseem ceeb rau kev txhim kho corrosion. Cov kev tshawb fawb dav dav tau ua tiav ntawm cov hlau no thiab qhov ntsuas qhov ntsuas tau zoo rau txhua tus tau kwv yees kom meej. General corrosion tus nqi ntawm carbon steel nyob rau hauv cov nram qab no micro-ib puag ncig muaj peev xwm heev tshaj cov nqi corrosion.
√ Hauv av, hlau yog raug rau ntau yam ntawm corrosive rog tsis zoo li cov uas tau ntsib nyob rau hauv atmospheric raug tej yam kev mob. Muaj ntau dua 200 hom av sib txawv hauv North America ib leeg, thiab tus nqi corrosion yog txiav txim siab los ntawm plaub yam tseem ceeb: av kub, noo noo, qib pH, thiab chlorides. Yog tias ib qho ntawm cov no tsis tuaj lawm, cov tshuaj tiv thaiv corrosion yuav tso tseg lossis maj mam mus. Hlau corrodes sai sai nyob rau hauv acidic ib puag ncig thiab maj mam los yog tsis tag nrho raws li alkalinity nce. Tus nqi corrosion ntawm cov hlau hauv av tuaj yeem nyob ntawm tsawg dua 0.2 microns hauv ib xyoos hauv cov xwm txheej zoo rau 20 microns ib xyoos lossis ntau dua hauv cov av hnyav heev.
√ Marine or water, a less common environment for metal is submerged in or exposed to water. Moisture is highly corrosive to most metals including steel, aluminum, and zinc. There are many different types of water (pure water, natural fresh water, drinking water, and seawater, and each has different mechanisms that determine the corrosion rate. The parameters that affect corrosion of metals in water include pH level, oxygen content, water temperature, agitation, the presence of inhibitors, and tide conditions. Onshore areas of marine corrosion include docks, harbors, naval yards, where sacrificial anodes providing cathodic protection may be employed, as well as protective coatings such as marine paints and galvanization. Offshore oil and gas platforms are exposed to particularly aggressive corrosion from sea salt spray and immersion.
Above ground,there are five important factors that are essential to atmospheric corrosion: moisture (dew condensation), temperature, humidity, aerosol particle deposition and pollutants. For instance, every 50 degree F (10 degree ) increase in the temperature can double corrosion activity. Another interesting phenomenon is that salt-rich environment and aerosols tend to have high levels of humidity.
Txhawm rau kom nkag siab txog huab cua corrosion yog qhov tseem ceeb vim tias hom kev puas tsuaj no feem ntau tshwm sim ntawm ntau hom kev puas tsuaj corrosion, thiab nws cuam tshuam rau sab nraum zoov thiab sab hauv tsev xws li cov khoom siv hluav taws xob,infrastructures, hnub ci zog tej yaam num, Telecom tej yaam num, ua liaj ua teb tej yaam num, tsheb,kev tsim kho chaw nres nkojthiab vaj tse nyob.
