2Why worry about corrosion? “One large chemical company spent more than $400,000 per year for corrosion maintenance in its sulfuric acid plants, even though the corrosion conditions were not considered to be particularly severe.”
3Why worry about corrosion? “A refinery employing a new process developed a serious problem after just 16 weeks of operation; some parts showed a corrosion loss of as much as 1/8 inch.”
4Why worry about corrosion? “The trend in the chemical process industries toward higher temperatures and pressures has made possible new processes or improvements in old processes. Higher temperatures and pressures usualy involve more severe corrosion conditions. Many of the present day operations would not have been possible or economical w/o the use of corrosion-resistant materials.”
5Why Worry about Corrosion? Safety/HealthEnvironmentOperabilityProfitabilityProduct QualityAppearance of Facilities/EquipmentBadly corroded and rusted equipment in a plant would leave a poor impression on the observer“A chemical plant effected an anuual savings of more than $10,000 merely by changing the bolt material on some equipment from one alloy to another more resistant to the conditions involved. The cost of this change was negligible. Slight changes in the process sometimes reduce the corrosiveness of plant liquors without affecting the process itself, thus permitting the use of less expensive materials! These changes can often be made after the plant is in operation, but original preventive measures are more desirable. Corrosion difficulties can often be ‘designed out’ of equipment, and the time to do this is in the original design of the plant! It is easier and cheaper to erase lines on a drawing than to repair or replace failed equipment in a plant.
6Examples of Corrosion Erosion-corrosion of copper water pipe (tubing) Chloride Stress corrosion cracking of stainless steelsChloride pitting of stainless steelNitric acid attack of titanium tubing
7Corrosive Environments AtmosphericIndustrialUrbanRuralMarineWaterSeawaterFreshwaterTapwaterTreated water
11Types of Corrosion 2 Main Types: General corrosion is the loss material over the entire surface, at a relatively constant rate. The metal is thinned fairly uniformly, without appreciable localized attack.Localized Attack is specific to certain areas of the material, and can take many forms.
12Forms of Localized Attack Pitting is corrosion that produces sites of localized attack that are small relative to the overall exposed surface area. It is the most common form of corrosion in aqueous environments, and the major cause of corrosion service failures inthe chemical processingindustry.
13Forms of Localized Attack cont’d CREVICE CORROSION is attack at narrow spaces or gaps between two metal surfaces, or between a metal and non-metal. It can occur at cracks or seams in a metal, or under a washer, gasket, or deposit.Crevice corrosion results from a difference between the chemistry of the bulk environment and that in or at the crevice.
14Forms of Localized Attack cont’d BIOLOGICAL ATTACK is that which caused or accelerated by organisms on the affected surface. Fouling organic deposits may cause crevice corrosion, or organisms may produce chemicals that cause corrosion.
15EROSION CORROSION is acceleration of material loss due to the combined effects of corrosion plus removal of material by the moving fluid.
16Forms of Localized Attack cont’d CAVITATION & IMPINGEMENT is material loss due to collapse of voids or cavities in the fluid, due to pressure changes (cavitation), or due to impingement of liquid droplets. This may be strictly mechanical damage, or maybe worsened by the effectsof corrosion.
18Forms of Localized Attack cont’d Fretting is a process combining wear and corrosion in removal of material from contacting solid surfaces. It typically involves very small relative movements of the components, oxidation of the surfaces, and abrasion by the oxidation products. It often occurs between a shaft and a component fitted on the shaft.
19Forms of Localized Attack cont’d INTERGRANULAR ATTACK is corrosion at the boundaries of a metal grain, with little or no attack of the grain. It results in weakening of the metal, or separation at the grain boundary. (The composition and corrosion resistance of a metal grain varies from the surface to the interior.)
20Forms of localized Attack cont’d DEALLOYING ATTACK is preferential removal of one constituent of an alloy in a corrosive environment. An example is the “dezincification” of brass, in which zinc is leached from the brass in some aqueous streams, leaving a weak structure of copper and copper oxide.
21Forms of Localized Attack cont’d GALVANIC ATTACK is attack of a metal that is in electrical contact with a more noble metal, or a non-metallic conductor, in a corrosive environment. Examples are: corrosion of copper or brass couple to steel in an aqueous environment: or corrosion of a zinc coating on steel. The latter is done intentionally to protect the steel, as in roofing nails, fencing, corrugated galvanized sheets, etc.
22Types of Corrosion Environmentally-induced cracking: Stress-corrosion cracking (SCC) is due to the combined effects of a corrodent and sustained tensile stress. SCC of AUSTENTITIC (300-series) stainless steels by chorides is a major problem in the chemical industry. SCC canbe caused in copper alloys innitrates, and in steel by caustic.To increase production, the temperature of the cooling medium in a heat-exchanger system was lowered and the time required per batch decreased. Lowering the temperature of the cooling medium resulted, however, I more severe thermal gradients across the metal wall, They in turn, induced higher stresses in the metal. Stress corrosion cracking of the vessels occurred quickly, an the plant was shut down with production delayed for some time.
23Environmentally-induced Cracking CORROSION FATIGUE occurs in a cyclically loaded part in a corrosive environment. It occursat lower stress levels orafter fewer cycles thatwould be the case in theabsence of the corrosiveenvironment.
24Environmentally-induced Cracking Hydrogen-induced cracking or Embrittlement is reduction of the ductility or toughness of a metal due to the presence of atomic hydrogen. The hydrogen can be present due to introduction into the molten metal, or through absorption by the solid metal.
25Environmentally-induced Cracking cont’d LIQUID METAL EMBRITTLEMENT is brittle failure of a normally ductile metal when coated with a thin film of liquid metal followed by stressing in tension. Examples of LME may occur when steel is brazed, soldered, welded, or plated, or dip-coated with zinc, cadmium, or tin.
26Corrosion Control Materials selection Design Method of Operation Barriers/coatingsPaint systemsLinings – e.g. rubber or plasticCladding – metal on metalGalvanizingPlatingGlass/ceramic – e.g. porcelain on steelInhibitorsCathodic/Anodic Protection
27Factors affecting choice of an engineering material StrengthAppearanceCorrosionresistanceMaterialsSelectionAvailabilityFabricabilityCost
28Corrosion-resistant Materials All materials are resistant to corrosion in some specific environments. For example, carbon steel is resistant to many process and aqueous liquids. It may corrode slowly or not at all. Steel is the main material used in chemical plant equipment.The term corrosion-resistant is used to refer to materials that resist attack in specific or unusually corrosive environments.
29Corrosion-Resistant Materials The following is a list of a few materials used in various corrosive services:Stainless steelsNickel & nickel alloysReactive & refractory metals such as tantalum, titanium, zirconium, & their alloysCopper/copper alloys (brasses, bronzes)
30Corrosive-Resistant Materials AluminumLeadChromiumPlastics, such as teflon, PVC, nylon, polypropylene, polyethylene, etc.Rubbers/elastomers such as nitrile (NBR or BUNA-N) EPDM (e.g. NORDEL), Viton, NEOPRENE, butyl, etc.COMPOSITES, e.g. fiber-reinforced plastic (FRP, fiberglass)GLASS & CERAMICS, tile, porcelain, brick
31STAINLESS STEELS Iron-based alloys, with at least 10.5% Chromium Chromium-rich oxide surface film“passive”Forms and heals in oxygenAdditives of nickel, molybdenum, copper, titanium, aluminum, silicon, niobium, nitrogen, sulphur, seleniumProcessing applications, cutlery, decorative, health and sanitary, dairy, transportation, medical
32STAINLESS STEELS FERRITIC Iron plus 10-25% chromium (BCC) Magnetic High strength, limited toughnessGood ductility & fabricabilityExamples : 405,409, 429, 430, 434, 442, monit
41ALUMINUM & IT’S ALLOYS Protected by barrier oxide film Corrodes at low and high pHResistant to nitric acid (oxidizing)1xxx – 99+% Al2xxxalloyed with copperStrong, heat-treatableLower corrosion resistance
42ALUMINUM & IT’S ALLOYS 4xxx – alloyed with Si 5xxx – Al-Mg-Mn, Al-Mg-Cr, Al-Mg-Mn-Cr
43NICKEL & IT’S ALLOYS NICKEL Alloy 200, commercially pure Plating/claddingResistant to causticMONELAlloy 400, approx. 30% copperVery good fabricability
44NICKEL & IT’S ALLOYS NICKEL-MOLY Hastelloy B, B-2 Resistant to HCI NICKEL-CHROMIUMInconel or Alloy 600(77% Ni-15%Cr-bal Fe)
45NICKEL & IT’S ALLOYS Ni-Fe-Cr Incoloy or Alloy 800 (21Cr-32Ni-bal Fe) Resistant to chloridesNi-Fe-Cr-MoIncludes Alloys 20 & 20Cb3, Incoloy 825, Hastelloy F & GIncreased resistance to sulphuric, phosphoric, and organic acids and to SCC and chloride pitting
46NICKEL & IT’S ALLOYS Ni-Cr-Mo Hastelloy C, C276, C-22 Inconel or Alloy 625Resistant to hot acid mixtures
49METALS MATERIAL Advantages Disadvantage Carbon Steel Stainless steel Low cost, readily available, resists abrasion, standard fabrication, resists alkaliPoor resistance to acids & strong alkali, often causes discolouration and contaminationStainless steelResists most acids, reduces discolouration, available with a variety of alloys, abrasion less than mild steelNot resistant to chlorides, more expensive, fabrication more difficult, alloy materials may have catalytic effectsMonel-NickelLittle discoloration, contamination, resistant to chloridesNot resistant to oxidizing environments, expensiveHasteloyImproved over Monel-NickelMore expensive than Monel-NickelOther exotic metalsImproves specific propertiesCan be very high cost
50Non-metals Material Advantages Disadvantage Glass Plastics Ceramics Useful in laboratory and batch system, low diffusion at wallsFragile, not resistant to high alkali, poor heat transfer, poor abrasion resistancePlasticsGood at low temperature, large variety to select from with various characteristics, easy to fabricate, seldom discolours, minor catalytic effects possiblePoor at high temperature, low strength, not resistant to high alkali conditions, low heat transfer, low costCeramicsWithstands high temperatures, variety of formulations available, modest costPoor abrasion properties, high diffusion at walls (in particular hydrogen), low heat transfer, may encourage catalytic reactionsFrom "Analysis, Synthesis, and Design of Chemical Processes" Turton, Bailie, Whiting, Shaeiwitz
52Data in TextIn pages of the text you can find a table recommending specific materials for specific chemicals, and descriptions of some common alloys
53The Methanol ProjectProcess streams in the methanol section are not corrosiveSulphur can cause problems in earlier stages, but that is not our problemLong term storage (days) of methanol in carbon steel can cause side reactions that generate impurities which interfere with proper operation of fuel cells
54WorkshopWhat material should we use for the majority of the equipment in the methanol reaction and purification section?What equipment requires other material?What would be some good choices for that material?