The alloys resistant to sulfuric acid like carpenter alloy, Hastelloy alloy G and Incoloy alloy 825 are employed to benefit in the dilution and reaction procedures.

1. Importance of nickel basedalloys in phosphoricacid production and hydrometallurgy
Phosphoricacid production
Phosphoricacidisproducedbyfollowingwetmethodorelectricfurnace method.
The wet methodof productionisfollowedhereasitfollowsareactionof sulfuricacidwithcalcium
phosphate rocksubsequentlyfiltrationtoseparate the productfromcalciumsulfate.Howeverpure
phosphoricacidcauseslowercorrosionascompare to mineral acidslike sulfuricacid,hydrochloric
acid etc,latterisobservedascontaminantinphosphoricacidmade byfollowingwetprocessand
causesto improve the corrosionrates.Othercontaminantslikesilicamaycomplex the fluorideand
hence decrease the corrosivenessof the phosphoricacidhence the contaminantshave significant
effectoncorrosionpotential andfordistinguishingthe corrosionratesatthe variouslocationsusing
dissimilarphosphate rockasa raw material.
Prioracidificationof phosphaterock,concentratedsulfuricacidiscombinedwithrecirculated
phosphoricacidof mediumconcentration. The datareceivedatthisstage isdescribedinthe
followingtable:
On site corrosionexperimentwithrecirculatedphosphoricacid
Alloy Corrosionrate Highestdepthof crevice corrosion
Mm/y Mpy Mm Mills
Carpenteralloy
20 Cb
0.03 1.1 - -
AlOYCO20 0.04 1.5 - -
Incoloy alloy825 0.07 2.7 0.13 5
Hastelloyalloy C 0.07 2.8 - -
IlliumG 0.08 3 - -
Inconel alloy718 0.08 (a) 3.2 - -
Worthite 0.11 4.3 0.08 3
Incoloyalloy901 0.15 6.1 0.13 5
IlliumallorR 0.18 6.9
0.36 (b) 14.3 - -
Monel alloy K500 0.79 31 C c
Monel alloy 400 0.97 38 - -
Hastelloyalloy B 1.96 78 - -
Stainlesssteel
317
Above 3.81 (s) Above 150 (s) - -
Stainlesssteel
316
Above 4.57 (s) Above 180 (s) - -
a- Pittedtoa highestdepthupto 0.13mm
b- Duplicate sampleswere attackedatthe identical corrosionrate
c- Pittingcorrosiontoa highestdepthof 0.08mm
s- Stresscorrosioncrackingaround labels

2. The reactors are oftenlinedwithrubberorbrickor both, showingmetal onlyinagitatorswhere
mechanical strengthisoverridingneed.The followingtable showsdatainslurrywithinthisreactor
howeveritmaynot completelyreflectthe erosioneffect.
On-site analysisinphosphoricacidreactor
Alloy Corrosionrate
Mm/y Mpy
Inconel alloy625
HastelloyG 0.03 1.1
Illium98 0.07 2.7
Lead,antimonial 0.08 3.2
Hastelloyalloy C 0.09 3.4
Carpenteralloy20Cb3 0.17 6.5
Worthite 0.17 6.8
Carpenteralloy 20Cb 0.17 6.8
Incoloy alloy825 0.19 7.4
Durimetalloy20 0.29 11.6
Chlormetalloy3 0.31 12.2
IlliumalloyG 0.38 14.8
Stainlesssteel 317 0.56 22
IlliumalloyP 0.69 27
Stainlesssteel 316L 1.04 41
Stainlesssteel 316 (sensitized) 1.27 50
Cast ACICF-8M Above 5.23 Above 210 (a)
HastelloyalloyB Above 1.68 Above 66 (a)
IlliumR Above 2.31 Above 91 (a)
Chlorimetalloy2 Above 7.7 Above 310 (a)
a- Damaged
The alloysresistanttosulfuricacidlike carpenteralloy, HastelloyalloyG and Incoloyalloy 825 are
employedtobenefitinthe dilutionandreactionprocedures.The mostcorrosionresistantalloysin
wetprocessphosphoricacidis Inconel 625 that isspecificallyusedinevaporationsteps.
One stepinthisprocessthat releasesphosphoruspentaoxide inabout50% contentis a
hemihydratesprocessinwhichphosphate rockreactswithconcentratedsulfuricacidtogenerate
phosphoricacidandcalciumsulfate hemihydratesslurry. The temperatureinthisprocessismore
than that of dehydrate processi.e.90to 95oC rather than75oC and the acid contentismore causing
more complicatedconditionsforthe productionmaterials.Althoughloweramountisenergyis
neededforthisprocesstherefore thisprocessiscommonlyemployed.Followingtable describesthe
outcomesof an experimentconductedinreactorof hemihydratesprocessunit:
Plantanalysisina hemihydrateprocessphosphoricacidreactor
Alloy Corrosionrate Highestdepthof pittingorcrevice corrosion
Mm/y Mpy Mm Mils
Inconel 625 0.02 0.6 Initial Initial
HastelloyG 0.02 0.6 Initial Initial

3. Stainlesssteel 904L 0.02 0.8 Initial Initial
Incoloyalloy825 0.02 0.9 0.05 2
AlleghenyLudlum6X 0.03 1 Initial Initial
IlliumalloyG 0.03 1 Initial Initial
Carpenteralloy20 Cb3 0.03 1 0.03 1
JessopalloyJS – 777 0.03 1 Initial Initial
IlliumalloyP 0.03 1.2 Initial Initial
JessopalloyJS – 700 0.03 1.2 Initial Initial
Stainless steelCastACICD4 MCu 0.04 1.4 Initial Initial
HastelloyalloyM-532 0.04 1.4 0.03 1
Stainlesssteel 317 0.04 1.6 Initial Initial
HastelloyC276 0.05 2.1 Initial Initial
Stainlesssteel 316 0.06 2.4 0.03 1
Stainlesssteel 316 sensitized B B Initial Initial
Stainlesssteel 329 0.08 3.2
Subsequentto reactionamongsulfuricacidandphosphate rock,precipitatedgypsumisseparated
fromliquor,oftenthroughusingStainlesssteel 316L or 317L filters.The corrosionratesare normally
lesserinfiltersascompare toinreactors as described inthe followingtable:
Plantanalysisinphosphoricacid-gypsumslurry
Alloy Corrosionrate
Mm/y Mpy
Stainlesssteel 309 0.02 0.6
Stainlesssteel 317 0.04 1.6
Stainlesssteel 316 0.11 4.4
Stainlesssteel 304 0.34 13.5
Ni-Cr-Fe alloyIncoloy800 0.37 14.6
Lead,chemical 0.43 17.1
Ni-CualloyMonel 400 1.14 45
Ni-CralloyInconel 600 Above 1.85 a Above 73 a
a refersto damagedalloy
Hydrometallurgy
Hydrometallurigcal methodshave beenpopularsince early17showeverthese didnotreceive much
importantuntil 20th
century.Leachingisnow usedfor about35% inNorthAmericafor combined
resultsof copper,nickel,cobalt,zincanduranium.A verycrucial proportionof leachingprocesses
employsulfuricacid.Fromthe standardfact of non-ferrousmetal refining,hydrometallurgyhas
obtainedwide popularity.
The corrosionpotential of sulfuricacidsolutionsusedin hydrometallurgychangeswidely.Unsecured
carbon steel iswidelycorrodedinevenambienttemperature,low acidcontentsthatexist inspecific
leachingprocesses.Although,austeniticstainlesssteelssuchasgrade 316L are usedto benefitin
large magnitudes.The solutionscontainingsulfuricacidor consideredforoperationin
hydrometallurgyare unlike intermsof contentfrombelow 1% to98% andchange in temperature

4. fromambientlimitto270oC or 518oF. The stainlesssteelsare notusedinhighlyaggressive
conditionshoweverwidelyemployedindiluteleachingsolutions.
There are twoessential factorsinadditionof acidtemperatureandconcentrationthathave impact
on the applicabilityof alloysthatattainactive-passive nature,particularlythe stainlesssteelsin
leachingandrefiningapplications:chloride % insolutionthataffectsalloysservice life,andoxidizing
agentsthat enhance life of alloys.Aschloridesare usuallyavailableinore andoverload,these may
decrease service life of stainlesssteel inoperationsthatincludehighacidcontentsaswell as
elevatedtemperatures.Howeverthe contentsof metal preparedorrefinedandacidwt,
temperature andotheressential factorsare oftenaccuratelyobserved,the chloride contentis
usually ignoredornotconsidered.Itmakesthe explanationof corrosionanalysesanddescriptionof
limitingconditionstough.Oxygen,bacteriaandmetal ionsinsolutionsare oxidizingagentsthat
preventthe corrosionof alloysandisocorrosionchartsorlab corrosionanalysesinpure sulfuricacid
are usuallytooconservativeforuse inhydrometallurgical operations.