Corrosion Activity of Stainless Steel SS430 and Carbon Steel B450C in a Sodium Silicate Modified Limestone-Portland Cement Extract

Ci a ion: Bon il, D.; Vele a, L.; Feliu, S., J .; Escalan e-Ga cía, J.I. Co osion Ac i i y o S ainless S eel SS430 and Ca bon S eel B450C in a Sodium Silica e Modi ied Limes one-Po land Cemen Ex ac . Ma e ials 2023,16, 5066. h ps://doi.o g/10.3390/ ma16145066 Academic Edi o s: Lihai Zhang and Kai Wu Recei ed: 1 June 2023 Re ised: 29 June 2023 Accep ed: 10 July 2023 Published: 18 July 2023 Copy igh : © 2023 by he au ho s. Licensee MDPI, Basel, Swi ze land. This a icle is an open access a icle dis ibu ed unde he e ms and condi ions o he C ea i e Commons A ibu ion (CC BY) license (h ps:// c ea i ecommons.o g/licenses/by/ 4.0/). ma e ials A icle Co osion Ac i i y o S ainless S eel SS430 and Ca bon S eel B450C in a Sodium Silica e Modi ied Limes one-Po land Cemen Ex ac Da id Bon il 1, Lucien Vele a 1,* , Sebas ian Feliu, J . 2and JoséI án Escalan e-Ga cía3 1Cen e o Resea ch and Ad ances S udy (CINVESTAV), Applied Physics Depa men , Campus Me ida, Me ida 97310, Mexico; da id.bon il@cin es a .mx 2Na ional Cen e o Me allu gical Resea ch (CENIM-CSIC), Su ace Enginee ing Co osion and Du abili y Depa men , 8040 Mad id, Spain; [email p o ec ed] 3Cen e o Resea ch and Ad anced S udy (CINVESTAV), Campus Sal illo, Ramos A izpe 25900, Mexico; i an.escalan e@cin es a .edu.mx *Co espondence: ele a@cin es a .mx Abs ac : S ainless s eel SS430 and ca bon s eel B450C we e exposed o 30 days o he aqueous ex ac o sodium silica e-modi ied limes one-Po land cemen as an al e na i e o he pa ial eplacemen o he Po land cemen clinke . The ini ial pH o 12.60 was lowe ed and main ained a an a e age o 9.60, associa ed wi h ai CO 2 dissolu ion and acidi ica ion. As a esul , he ca bon s eel los i s passi e s a e, and he co osion po en ial (OCP) eached a nega i e alue o up o 296 mV, o ming he co osion laye o FeO, and FeOOH. In he meaning ime, on he s ainless s eel SS430 su ace, a passi e laye o C 2 O 3 g ew in he p esence o FeO, Fe 2 O 3 and C (OH) 3 co osion p oduc s; hus, he OCP shi ed o mo e posi i e alues o +150 mV. I is sugges ed ha a sel - epassi a ion p ocess ook place on he SS430 su ace due o he accumula ion o alkaline sul a es on he in e ace. Because o he chlo ide a ack, SS430 p esen ed isola ed pi s, while on B450C, hei a ea was ex ended. The quan i a i e analysis o EIS Nyquis and Bode diag ams e ealed ha he Rp o he co osion p ocess o SS430 was 2500 k Ω cm 2 , ≈ 32 imes lowe in magni ude han on B450C, o which he passi e laye ended o disappea , while ha on SS430 was ≈0.82 nm. Keywo ds: co osion; ca bon s eel; s ainless s eel; alkali-ac i a ed cemen ; Po land cemen ; cemen ex ac solu ion; co osion po en ial; pH; SEM-EDS; XPS; EIS 1. In oduc ion The global Po land cemen (PC) p oduc ion was 4.1 billion ons in 2021 [ 1 ], wi h es ima ed emissions o 0.87 ons o CO 2 pe 1 on o PC [ 2 ], con ibu ing 8–10% o he an h opogenic CO 2 global emissions [ 3 ]; hese igu es could inc ease due o he g owing global u baniza ion. To mi iga e he nega i e en i onmen al impac o he abo e, se e al al e na i e ypes o cemen ha e been p oposed. The wo o he mos common al e na i e ypes o cemen include: (a) blended cemen , in which PC is pa ially eplaced by one o mo e supplemen a y cemen i ious ma e ials (limes one, blas u nace slag, ly ash, calci- na ed clays, na u al pozzolan, was e glass, e c.) [ 4 – 10 ]; and (b) alkali-ac i a ed cemen (AAC), based on he eac ion o a p ecu so (i.e., he supplemen a y cemen i ious ma e ials e e ed abo e, limes one, e c.) and alkaline ac i a o s (alkali silica es and/o hyd ox- ides) [ 11 ]. The CO 2 emission du ing he manu ac u e o hese al e na i e ypes o cemen could be up o 5–6 imes lowe ela i e o PC [ 12 ] in addi ion o some o he ea u es, such as good mechanical s eng h, a dense po e s uc u e [13], he mal esis ance [14,15], e c. Limes one (LS) and was e glass ha e been used as pa ial eplacemen s o PC wi h- ou comp omising he mechanical p ope ies [ 16 , 17 ] and as p ecu so s in AAC [ 18 , 19 ]. Limes one-based AAC ha e been epo ed o de elop comp essi e s eng h by he o - ma ion o C-S-H and N-C-S-N hyd a ion p oduc s in e mixed wi h ca bona es and silica Ma e ials 2023,16, 5066. h ps://doi.o g/10.3390/ma16145066 h ps://www.mdpi.com/jou nal/ma e ials Ma e ials 2023,16, 5066 2 o 17 gel N-S-H (15–25 MPa a e 360 days) [ 18 ], while was e glass-based AAC ha e been e- po ed o o m C-S-H in e mixed wi h a silica gel (15–29 MPa a e 28 and 180 days) [ 19 ]. In es iga ions o AAC in was e glass inco po a ing limes one as a p ecu so and as an agg ega e [ 20 ] ha e indica ed ha he limes one pa icipa ed physically and chemically in he eac ion p ocess, densi ying he mic os uc u e and enhancing i s mechanical p op- e ies and s abili y unde wa e (15–20 MPa a 90 days). On he o he hand, based on he abo e-blended ypes o cemen , PC-limes one has been modi ied wi h an alkaline ac i a o syn hesized om was e glass, esul ing in a no el al e na i e “g een” cemen , which could each 50 MPa in comp essi e s eng h a e 90 days [21]. In addi ion o he s eng h equi emen s, he applica ion o new cemen equi es he cha ac e iza ion o he elec ochemical beha io o he s eel ein o cemen due o he in e nal e olu ion o he chemical en i onmen o he conc e e. The pH o he po e solu ion gi en by he con en o alkalis in he cemen and he sa u a ion wi h Ca(OH) 2 is a key ac o o p omo ing he passi a ion o ein o cemen s eels (pH > 13), leading o he o ma ion o a hin laye o Fe-oxides/hyd oxides on he su ace o ca bon s eel known as he passi e laye , which p o ec s he s eel su ace om he ad ancemen o co osion in he p esence o agg essi e co osi e agen s [ 22 – 24 ]; o s ainless s eel, he passi e laye is p ima ily composed by a C - ich oxide laye ha ea u es sel -gene a ion [ 25 ]. I is epo ed ha he e y high alkalini y o AAC could be a o able o he co osion p o ec ion o he embedded ein o cing s eel [ 26 ]; some s udies ha e epo ed a pH ≈ 13 o he aqueous po e solu ion [27–29], which is simila o highe han ha o he PC ex ac solu ion. To ci cum en he expe imen al di icul ies o his s udy on he co osion beha io o s eel embedded in conc e e (elec ode and cell designs, posi ion o e e ence and auxilia y elec odes, d op po en ial IR in conc e e and i s compensa ion, es ic ion o oxygen di u- sion, he de elopmen o mac o-co osion cells, among o he s), he model solu ions we e p oposed o ob ain compa a i e esul s while main aining he con ol o some pa ame e s, which is di icul o accomplish in ein o ced-conc e e samples. The in luence o he ionic composi ion o he cemen po e solu ion on he s eel passi a ion beha io was in es iga ed using aqueous sa u a ed Ca(OH) 2 (pH 12–13) [ 30 – 32 ], KOH and NaOH solu ions [ 33 – 36 ] as models, and cemen ex ac (CE) solu ions wi h a a ie y o ions (Ca 2+ , Na + , K + , OH − , and SO 42− ) [ 37 – 40 ]. Fo example, i is conside ed ha he Ca 2+ ion could a ec he composi ion and he p o ec i e e iciency o he passi e laye [ 41 ], while silica es could p omo e s eel passi a ion and i s esis ance o a chlo ide a ack [ 42 ]. These esul s sugges ed ha he elec ochemical beha io o low ca bon s eel was e y simila o NaOH and sa u a ed Ca(OH) 2 model solu ions, while in a PC ex ac solu ion, he cha ge ans e esis ance o he co osion p ocess inc eased due o he adso p ion o C-S-H gel on he s eel su ace [ 43 ]. Mild s eel eba s ha e been exposed o alkali NaOH solu ions o simula e he chemis y o he AAC conc e e po e solu ion, and in he p esence o chlo ide ions, he depassi a ion o he s eel su ace was ound o be s ongly dependen on he pH po e alkalini y [ 44 ]. On he o he hand, EIS elec ochemical measu emen s ha e shown ha he ex ac o AAC based on u nace slag had a g ea capaci y o passi a e he ca bon s eel HRB400 su ace and o e ed p o ec i e e iciency agains chlo ides [ 45 ]. A e he exposu e o ca bon s eel HPD235 o an ex ac solu ion o a highly alkaline AAC based on ly ash, he passi e laye was composed o an inne laye o FeO and an ou e o FeOOH/Fe 2 O 3 . The p esence o S-species in he conc e e po e solu ion o AAC led o he o ma ion o a passi e laye ha was mo e suscep ible o he a ack o chlo ide ions, and he co osion esis ance o he passi e laye was educed [ 46 – 50 ]. Con e sely, ano he s udy epo ed ha he deposi ion o S, due o he oxida ion o sul ide anions o sul u in he po e solu ion o a slag-based AAC, p e en ed po en ial ising in o he pi ing egimen and led o he inhibi ion o he oxygen ca hode eac ion in mild s eel [51]. In ou p e ious wo k [ 52 ], he elec ochemical beha io o ca bon s eel B450C and low-ch omium e i ic s ainless s eel SS430 was cha ac e ized a e hei exposu e o up o 30 days o he CE solu ion (pH = 12.38) o a “g een” supe sul a ed cemen , which could be conside ed as an al e na i e o PC, based on 52% pumice (SiO 2 /Al 2 O 3 ), 34% Ma e ials 2023,16, 5066 3 o 17 CaSO 4·1 2 H 2 O as a sulpha ic ac i a o , 7% o Po land cemen (PC) and CaO as alkaline ac i a o s. The ini ial pH o 12.38 d opped om he i s day o 7.84, accompanied by a shi in he ee co osion po en ial (OPC) o he ca bon s eel up o − 480 mV nega i e alues, losing i s passi e s a e and leading o he o ma ion o co osion p oduc s α -, γ -FeOOH and Fe 2 O 3 ; meanwhile, he s ainless s eel SS430 p ese ed i s passi e s a e a a posi i e OCP (+182.50 mV) due o he o ma ion o a hin laye o ~0.8 nm C 2 O 3 [ 46 ]. On bo h su aces, a localized co osion a ack was obse ed, which was in luenced by he p esence o Cl-ions in he CE solu ion o igina ing om he pumice. In his s udy, ma ensi ic ca bon s eel B450C and low-ch omium e i ic s ainless s eel SS430 we e exposed o up o 30 days o he aqueous ex ac o a sodium silica e-modi ied limes one-Po land cemen named “JLSC1” [ 21 ], which was composed o 30% PC, 23.3% sodium silica e ac i a o , 26.5% LS and 0.2% supe plas icize , o simula e he en i onmen a he s eel–conc e e po e in e ace. The pH o he CE solu ion and he change in ime o he s eel- ee co osion po en ial (OCP) we e moni o ed du ing he imme sion es . Elec ochemical impedance spec oscopy (EIS) measu emen s we e pe o med o ob ain he pa ame e s o he in e ace s eel-CE po e solu ion. The s eel su aces we e cha ac e ized by he SEM-EDS, XPS echniques. To he bes o ou knowledge, no o he esea ch on his opic has been p e iously unde aken. 2. Ma e ials and Me hods 2.1. S eel Samples and Su ace Cha ac e iza ion Fla samples o low ch omium e i ic s ainless s eel SS430 (Ou okumpu, Espoo, Finland) and ma ensi ic ca bon s eel B450C (Pi ini G oup, Gemona del F iuli, I aly) we e cu (0.8 cm 2 ), ab aded wi h SiC pape o 4000 g i using e hanol as a lub ican , hen sonica ed o 10 min (B anson 1510, B anson Ul asonics Co., Danbu y, CT, USA) and d ied a oom empe a u e (294 K o 21 ◦ C). The s eel nominal composi ion (w .%) is p esen ed in Table 1, acco ding o he manu ac u e s. Table 1. Composi ions (w .%) o SS430 e i ic s eel and ma ensi ic B450C ca bon s eel, acco ding o he manu ac u e s. Elemen (w .%) C C N Cu P S Fe SS430 0.25 16.2 - - - - Balance B450C 0.22 - 0.01 0.80 0.05 0.05 Balance Acco ding o [ 40 ], SEM-EDS analysis iden i ied black do s on he ca bon s eel su ace, which con ained 5.02% C, 1.31% Mn, and 0.38% S, asc ibed o he phases o MnS and Mn3C [ 53 ]. Due o he quali y o he sc ap, Cu (0.83%) was also de ec ed. On he s ainless s eel su ace, zones wi h 24.03% C , 30.04% C, and 3.27% N we e de ec ed and associa ed wi h ch omium ni ide and ca bide (C ,Fe) 7 C 3 [ 54 , 55 ]. The p esence o 0.7% o V was asc ibed o p ecipi a es o V6C5 and VB-ni ide phases. The p esence o Si and C, associa ed wi h he SiC phases, was de ec ed on bo h s eel su aces [53]. 2.2. Sodium Silica e Modi ied Limes one-Po land Cemen “JLSC1” and I s Ex ac Solu ion The cemen used in his esea ch, labeled “JLSC1”, is ha desc ibed in [ 21 ], which de ails ha i s manu ac u e has a 50% lowe global wa ming po en ial (kg CO 2 -eq/m 3 ) compa ed o a PC which can each 50 MPa a 90 days. Table 2shows he oxide composi ion o he JLSC1 cemen . Table 2. Oxide composi ion o he JLSC1 sodium silica e modi ied limes one-PC. JLSC1 SiO2Al2O3Fe2O3CaO MgO SO3K2O Na2O LOI g136.92 10.30 6.67 285.16 3.66 6.76 2.30 19.49 128.82 w .% 22.80 1.72 1.11 47.53 0.61 1.13 0.38 3.25 21.47 Ma e ials 2023,16, 5066 4 o 17 The main oxides in JLSC1 (Table 2) a e CaO and SiO 2 ; howe e , he con en o CaO (47.53%) is lowe han ha o Po land cemen (66.84–58.4% CaO) because he JLSC1 con- ains only 30% o PC; he SiO 2 con en (22.80%) is compa able o ha o PC ( 21.35–22.30% ) and can be a ibu ed o he sodium silica e ac i a o (23.3% in JLSC1). O he oxides p esen a a low con en included 3.25% Na 2 O, 1.72% Al 2 O 3 , 1.13% SO 3 , 1.11 Fe 2 O 3 , 0.61% MgO, and 0.38% K2O. The aqueous cemen ex ac was p epa ed om a 1:1 w ./w . mix u e o JLSC1 cemen and ul apu e deionized wa e (18.2 M Ω cm). The mix u e was agi a ed and le o 24 h o hyd a e in a closed con aine . Then, he supe na an was il e ed wi h 2.5 µ m po e size il e pape (Wha man, Ken , UK) o emo e he pa icles and was kep in a sealed con aine . Table 3p esen s he chemical composi ion o he JLSC1 cemen ex ac solu ion, cha ac e ized by abso p ion spec ome y and a omic emission by plasma; he Cl − ion con en was de e mined by he ion-selec i e elec ode. Table 3. Ion chemical composi ion in mg/L o he JLSC1 ex ac solu ion ob ained by abso p ion spec ome y (AA), a omic emission by he plasma (ICP), and he ion selec i e (Cl-ion). Elemen (mg/L) Li K+Na+Al3+ Ca2+ Si SO42−S Cl−OH− JLSC1 0.099 1668.4 2700 2.555 0.505 34.96 1132.0 1.005 88 676.78 [OH] was calcula ed om he pH measu emen . The low Ca 2+ con en could be a ibu ed o he consump ion o hese ions o he o ma ion o hyd a ed calcium silica es (C-S-H). On he o he hand, he high Na + con en (2700 mg/L) came om he silica e ac i a o (powde ed sodium was e glass, PSWG) a e eac ing wi h limes one and po landi e (Ca(OH) 2 ) when he Na ac ion eac ed wi h wa e o o m a NaOH bu e o he alkalini y o he JLSC1-CE solu ion. Likewise, he K + con en (1668.4 mg/L) was a pa o K 2 O’s hyd a ion, whe e he KOH p oduc con ibu ed o he alkalini y o he JLSC1-CE solu ion (pH ≈ 12.60). Meanwhile, he 1132 mg/L SO 42− con en was a ibu ed o he hyd a ion o he SO 3 ac ion, which could eac wi h he hyd oxides esul ing in he o ma ion o sul a es [56]. 2.3. Imme sion Tes T iplica ed s eel samples (0.8 cm 2 ) we e imme sed in 10 mL o a JLSC1 cemen ex ac solu ion o a pe iod o 720 h (30 days), in sealed con aine s (wi h pa a in ape), acco ding o he s anda d ASTM-NACE/ASTM G31-12a [ 57 ]. A e 168 h (7 days) and 720 h (30 days), he samples we e wi hd awn, insed wi h deionized wa e , and d ied in he ai a oom empe a u e (294 K o 21 ◦ C). The pH o he cemen ex ac solu ion was measu ed (PH60 P emium Line, pH es e , Ape a Ins umen s, LLC, Columbus, OH, USA) a e he wi h- d awal o s eel samples a each pe iod o exposu e. The su ace o he samples a e he co osion imme sion es s we e cha ac e ized by scanning elec on mic oscopy (SEM-EDS, XL–30 ESEM-JEOL JSM-7600F, JEOL L d., Tokyo, Japan). The co osion p oduc s we e ana- lyzed wi h X- ay pho oelec on spec oscopy (XPS, K-Alpha, The mo Scien i ic, Wal ham, MA, USA) a e spu e ing he su ace o di e en imes (seconds) wi h a scanning A - ion gun. The o med laye s we e emo ed [ 58 ], and he su aces we e cha ac e ized by SEM-EDS and XPS. 2.4. Elec ochemical Measu emen s A ypical h ee-elec ode cell con igu a ion (inside a Fa aday cage), connec ed o a po en- ios a (In e ace-1000E po en ios a /gal anos a /ZRA, Gam y Ins umen s, Philadelphia , PA, USA), was used o he elec ochemical expe imen s (294 K o 21 ◦ C): he s eel pla es o B450C and SS430 we e he wo king elec odes, he P pla e was an auxilia y, and a sa u a ed calomel elec ode (SCE) was used as he e e ence elec ode. Du ing he elec ochemical expe imen s, he change in ime o he open ci cui po en ial (OCP) was moni o ed and conside ed o he ee co osion po en ial o he s udied s eels. Elec ochemical impedance Ma e ials 2023,16, 5066 5 o 17 spec oscopy (EIS) a open ci cui po en ial (OCP) was pe o med, applying an al e na ing cu en (AC) signal o ± 10 mV in ampli ude a a equency ange om 100 kHz o 10 mHz and wi h a sampling size o 10 da a poin s/decade. Nyquis and Bode EIS diag ams we e eco ded a di e en imme sion pe iods: 1, 7, 14, 21, and 30 days. The da a we e analyzed wi h Gam y Echem Analys ®( e sion 7.1, Philadelphia, PA, USA). 3. Resul s 3.1. Change in Time o pH o JLSC1 Cemen Ex ac Solu ion and Co osion Po en ial (OCP) o he S eels up o 30 Days o Imme sion Figu e 1p esen s he change in ime o he pH o he JLSC1-CE solu ion and he co osion po en ial (OCP) du ing he exposu e o up o 30 days o SS430 (Figu e 1a) and B450C (Figu e 1b) s eels. The ini ial pH was 12.60, compa ed o ha o he Po land cemen ex ac (pH = 13) [40]. Ma e ials 2023, 16, x FOR PEER REVIEW 5 o 18 2.4. Elec ochemical Measu emen s A ypical h ee-elec ode cell configu a ion (inside a Fa aday cage), connec ed o a po en ios a (In e ace-1000E po en ios a /gal anos a /ZRA, Gam y Ins umen s, Phila- delphia, PA, USA), was used o he elec ochemical expe imen s (294 K o 21 °C): he s eel pla es o B450C and SS430 we e he wo king elec odes, he P pla e was an auxilia y, and a sa u a ed calomel elec ode (SCE) was used as he e e ence elec ode. Du ing he elec- ochemical expe imen s, he change in ime o he open ci cui po en ial (OCP) was mon- i o ed and conside ed o he ee co osion po en ial o he s udied s eels. Elec ochemical impedance spec oscopy (EIS) a open ci cui po en ial (OCP) was pe o med, applying an al e na ing cu en (AC) signal o ±10 mV in ampli ude a a equency ange om 100 kHz o 10 mHz and wi h a sampling size o 10 da a poin s/decade. Nyquis and Bode EIS diag ams we e eco ded a diffe en imme sion pe iods: 1, 7, 14, 21, and 30 days. The da a we e analyzed wi h Gam y Echem Analys ® ( e sion 7.1, Philadelphia, PA, USA). 3. Resul s 3.1. Change in Time o pH o JLSC1 Cemen Ex ac Solu ion and Co osion Po en ial (OCP) o he S eels up o 30 Days o Imme sion Figu e 1 p esen s he change in ime o he pH o he JLSC1-CE solu ion and he co osion po en ial (OCP) du ing he exposu e o up o 30 days o SS430 (Figu e 1a) and B450C (Figu e 1b) s eels. The ini ial pH was 12.60, compa ed o ha o he Po land cemen ex ac (pH = 13) [40]. Figu e 1. Change in ime o pH o JLSC1-CE solu ion and co osion po en ial (OCP) s. SHE a e age alues du ing he exposu e o (a) S ainless s eel SS430 and (b) Ca bon s eel B450C. On he fi s day o exposu e (24 h) o B450C and SS430, he pH o JLSC1 ended o lowe he alues be ween 10.50 and 10.92, which eached an a e age alue o 9.68 a e 7 days and almos main ained an ending a 9.60 a e 30 days o imme sion (Figu e 1). Mean- while, he OCP o s ainless s eel SS430 (−199.96 mV) showed a endency o mo e posi i e alues, indica ing he de elopmen o a mo e p o ec i e passi e laye along he ime (≈+155 mV); a simila endency o OCP was obse ed o SS430 in he PC-ex ac solu ion (≈+217 mV) a 30 days o imme sion [40]. On he o he hand, he OCP o ca bon s eel B450C (−156.30 mV) ended o offe mo e nega i e alues om he fi s 24 h (−296.58 mV) and ended a a less nega i e OCP a ound −181.50 mV due o he o med co osion laye , which ac ed as a physical ba ie . Compa ing he beha io o he s udied s eels exposed o he JLSC1 ex ac o ha in he PC ex ac solu ion [40], he OCP alues o B450C ca bon s eel we e 1.8 imes g ea e (mo e nega i e) since he fi s 24 h, indica ing he loss o he passi e s a e. Figu e 1. Change in ime o pH o JLSC1-CE solu ion and co osion po en ial (OCP) s. SHE a e age alues du ing he exposu e o (a) S ainless s eel SS430 and (b) Ca bon s eel B450C. On he i s day o exposu e (24 h) o B450C and SS430, he pH o JLSC1 ended o lowe he alues be ween 10.50 and 10.92, which eached an a e age alue o 9.68 a e 7 days and almos main ained an ending a 9.60 a e 30 days o imme sion (Figu e 1). Meanwhile, he OCP o s ainless s eel SS430 ( − 199.96 mV) showed a endency o mo e posi i e alues, indica ing he de elopmen o a mo e p o ec i e passi e laye along he ime ( ≈ +155 mV); a simila endency o OCP was obse ed o SS430 in he PC-ex ac solu ion ( ≈ +217 mV) a 30 days o imme sion [ 40 ]. On he o he hand, he OCP o ca bon s eel B450C ( −156.30 mV ) ended o o e mo e nega i e alues om he i s 24 h ( −296.58 mV ) and ended a a less nega i e OCP a ound − 181.50 mV due o he o med co osion laye , which ac ed as a physical ba ie . Compa ing he beha io o he s udied s eels exposed o he JLSC1 ex ac o ha in he PC ex ac solu ion [ 40 ], he OCP alues o B450C ca bon s eel we e 1.8 imes g ea e (mo e nega i e) since he i s 24 h, indica ing he loss o he passi e s a e. The change in ime o he co osion po en ial alues (OCP) s ongly depended on he change in ime o he pH in he ex ac solu ion. A dec ease in pH could be associa ed wi h he dissolu ion o he CO 2 om he ai en i onmen , which, combined wi h wa e o ming on he ca bonic acid (H 2 CO 3 ), could la e dissocia e in o bica bona e ions (HCO 3− ) and hyd ogen ions (H + ), p omo ing he acidi ica ion o he 10 mL ex ac solu ion used in his s udy o he s eel imme sion es [ 59 ]. I was conside ed ha o 7 < pH < 10, he HCO 3− ions we e he p edominan species, e y co osi e o me al, and he elease o Fe 2+ occu ed, o ming i on hyd oxide II (consuming he OH − ions and lowe ing he pH) [ 60 ]. Howe e , he es ablished dynamic equilib ium be ween he o med weak ca bonic acid and he bica bona e ions (as i s conjuga ed base) o such a o med sys em could ac as a bu e o he pH change. On he o he hand, he diminishing pH o he JLSC1 ex ac Ma e ials 2023,16, 5066 6 o 17 solu ion should also be conside ed due o he lowe amoun o po landi e Ca(OH) 2 in he conc e e po e solu ion and, hus, i had a lowe abili y o main ain he pH [61]. 3.2. B450C Ca bon S eel Su ace Cha ac e iza ion a e Exposu e o JLSC1 Cemen Ex ac Solu ion Figu e 2shows SEM images o he ca bon s eel su aces, and Table 4p esen s he EDS analysis (w .%) a e 7 and 30 days o exposu e o he JLSC1 cemen ex ac solu ion. A e 7 days o exposu e (Figu e 2a) a pH o 9.77 and OCP = −259.97 mV, he s eel su ace was no comple ely co e ed by a co osion laye ; zone A p esen ed he ma ix composi ion o he s eel: Fe (88.71%), C (5.00%), Mn (0.74%) wi h some aces o elemen s, such as Na, K, and Si. Fo zones B and C, he EDS analysis epo ed a g ea e con en o Na, O, S, and K, which could be a ibu ed mainly o hei sul a es as a pa o he cemen ex ac (Table 3), and Ca-ca bona e in a lowe con en . Ma e ials 2023, 16, x FOR PEER REVIEW 6 o 18 The change in ime o he co osion po en ial alues (OCP) s ongly depended on he change in ime o he pH in he ex ac solu ion. A dec ease in pH could be associa ed wi h he dissolu ion o he CO2 om he ai en i onmen , which, combined wi h wa e o ming on he ca bonic acid (H2CO3), could la e dissocia e in o bica bona e ions (HCO3−) and hy- d ogen ions (H+), p omo ing he acidifica ion o he 10 mL ex ac solu ion used in his s udy o he s eel imme sion es [59]. I was conside ed ha o 7 < pH < 10, he HCO3− ions we e he p edominan species, e y co osi e o me al, and he elease o Fe2+ oc- cu ed, o ming i on hyd oxide II (consuming he OH− ions and lowe ing he pH) [60]. Howe e , he es ablished dynamic equilib ium be ween he o med weak ca bonic acid and he bica bona e ions (as i s conjuga ed base) o such a o med sys em could ac as a buffe o he pH change. On he o he hand, he diminishing pH o he JLSC1 ex ac solu ion should also be conside ed due o he lowe amoun o po landi e Ca(OH)2 in he conc e e po e solu ion and, hus, i had a lowe abili y o main ain he pH [61]. 3.2. B450C Ca bon S eel Su ace Cha ac e iza ion a e Exposu e o JLSC1 Cemen Ex ac Solu ion Figu e 2 shows SEM images o he ca bon s eel su aces, and Table 4 p esen s he EDS analysis (w .%) a e 7 and 30 days o exposu e o he JLSC1 cemen ex ac solu ion. A e 7 days o exposu e (Figu e 2a) a pH o 9.77 and OCP = −259.97 mV, he s eel su ace was no comple ely co e ed by a co osion laye ; zone A p esen ed he ma ix composi ion o he s eel: Fe (88.71%), C (5.00%), Mn (0.74%) wi h some aces o elemen s, such as Na, K, and Si. Fo zones B and C, he EDS analysis epo ed a g ea e con en o Na, O, S, and K, which could be a ibu ed mainly o hei sul a es as a pa o he cemen ex ac (Table 3), and Ca-ca bona e in a lowe con en . Figu e 2. SEM images o ca bon s eel B450C a e 7 (a) and 30 days (b) o exposu e o he JLSC1 cemen ex ac solu ion. Table 4. EDS su ace a e age analysis (w .%) o he B450C ca bon s eel a eas o in e es as ma ked on he SEM images o Figu e 2. Days/w .% Fe O Na Ca S K Si C Mn Cl 7 A 88.71 3.12 0.64 0.42 - - 0.76 5.00 0.74 - B 2.10 51.42 22.57 1.11 0.15 1.90 0.19 15.58 - 4.48 C 9.81 42.47 10.41 1.99 12.19 17.04 0.39 5.72 - - D 57.37 15.23 20.68 - - 0.59 0.27 4.08 - 1.78 30 1 60.28 36.58 - - 0.40 - 0.48 2.25 - - 2 55.35 40.35 - - 0.40 - - 2.86 0.44 0.47 3 51.30 42.31 - - 0.88 - - 4.49 - 0.51 A 30 days o exposu e o he ca bon s eel o he JLSC1 ex ac solu ion (Figu e 2b), he su ace was co e ed by a co osion laye en iched in Fe and O (Table 4) and was asso- cia ed wi h he o ma ion o Fe-oxides and hyd oxides o diffe en mo phology and ac- companied wi h some aces o S, Si, Mn, Cl. Figu e 2. SEM images o ca bon s eel B450C a e 7 ( a ) and 30 days ( b ) o exposu e o he JLSC1 cemen ex ac solu ion. Table 4. EDS su ace a e age analysis (w .%) o he B450C ca bon s eel a eas o in e es as ma ked on he SEM images o Figu e 2. Days/w .% Fe O Na Ca S K Si C Mn Cl 7 A 88.71 3.12 0.64 0.42 - - 0.76 5.00 0.74 - B 2.10 51.42 22.57 1.11 0.15 1.90 0.19 15.58 - 4.48 C 9.81 42.47 10.41 1.99 12.19 17.04 0.39 5.72 - - D 57.37 15.23 20.68 - - 0.59 0.27 4.08 - 1.78 30 1 60.28 36.58 - - 0.40 - 0.48 2.25 - - 2 55.35 40.35 - - 0.40 - - 2.86 0.44 0.47 3 51.30 42.31 - - 0.88 - - 4.49 - 0.51 A 30 days o exposu e o he ca bon s eel o he JLSC1 ex ac solu ion (Figu e 2b), he su ace was co e ed by a co osion laye en iched in Fe and O (Table 4) and was associa ed wi h he o ma ion o Fe-oxides and hyd oxides o di e en mo phology and accompanied wi h some aces o S, Si, Mn, Cl. The XPS spec a o he ca bon s eel B450C, a e 7 and 30 days o exposu e o he JLSC1 cemen ex ac solu ion, a e p esen in Figu es 3and 4, espec i ely. The peak o he Fe2p (Figu e 3a) spec a was decon olu ed and associa ed wi h Fe 2 O 3 /FeOOH ( 711.1–711.6 eV ) and FeO (709.8 eV), as a pa o he passi e laye , and Fe me al as a pa o he s eel ma ix (706.8 eV) [ 62 ]. The peak o O1s (Figu e 3b) we e decon olu ed in h ee peaks, which we e asc ibed o oxides O 2− (531.3 eV), OH − (532.8 eV), and C-O (532.17 eV). The in ensi y o he hyd oxide peak was 3.5 imes lowe han ha o he oxides, indica ing ha Fe 2 O 3 and FeO oxides p e ailed wi h espec o FeOOH; wi h he inc ease in he spu e ing ime, he in ensi y o O1s diminished due o he deple ion o oxides and hyd oxide species. The peaks o C1s and S2p we e a ibu ed o ca bona e and sul a e c ys als, which was sugges ed by he SEM-EDS analysis (Figu e 2, Table 4) [ 63 , 64 ]. A 30 days o exposu e (Figu e 4), when he ca bon s eel o med a hicke co osion laye (Figu e 2b), he iden i ied peaks we e Fe2p and O1s. The decon olu ion o Fe2p (Figu e 4a,b) sugges ed he p esence Ma e ials 2023,16, 5066 7 o 17 o FeOOH (711.48 eV) and FeO (710.0 eV) [ 62 ] in a simila p opo ion, acco ding o he in ensi y o O1s (Figu e 4d). Ma e ials 2023, 16, x FOR PEER REVIEW 7 o 18 The XPS spec a o he ca bon s eel B450C, a e 7 and 30 days o exposu e o he JLSC1 cemen ex ac solu ion, a e p esen in Figu es 3 and 4, espec i ely. The peak o he Fe2p (Figu e 3a) spec a was decon olu ed and associa ed wi h Fe2O3/FeOOH (711.1– 711.6 eV) and FeO (709.8 eV), as a pa o he passi e laye , and Fe me al as a pa o he s eel ma ix (706.8 eV) [62]. The peak o O1s (Figu e 3b) we e decon olu ed in h ee peaks, which we e asc ibed o oxides O2− (531.3 eV), OH− (532.8 eV), and C-O (532.17 eV). The in ensi y o he hyd oxide peak was 3.5 imes lowe han ha o he oxides, indica ing ha Fe2O3 and FeO oxides p e ailed wi h espec o FeOOH; wi h he inc ease in he spu e ing ime, he in ensi y o O1s diminished due o he deple ion o oxides and hyd oxide spe- cies. The peaks o C1s and S2p we e a ibu ed o ca bona e and sul a e c ys als, which was sugges ed by he SEM-EDS analysis (Figu e 2, Table 4) [63,64]. A 30 days o exposu e (Figu e 4), when he ca bon s eel o med a hicke co osion laye (Figu e 2b), he iden i- fied peaks we e Fe2p and O1s. The decon olu ion o Fe2p (Figu e 4a,b) sugges ed he p esence o FeOOH (711.48 eV) and FeO (710.0 eV) [62] in a simila p opo ion, acco ding o he in ensi y o O1s (Figu e 4d). Figu e 3. O e iew o X- ay pho oelec on spec oscopy (XPS) spec a, a e spu e ing o he me al su ace o diffe en imes, acqui ed om ca bon s eel B450C exposed o he JLSC1 cemen ex ac o 7 days: (a) Fe2p; (b) O1s; (c) C1s; (d) Na1s; (e) Ca2p; ( ) K2p; (g) S2p. Figu e 3. O e iew o X- ay pho oelec on spec oscopy (XPS) spec a, a e spu e ing o he me al su ace o di e en imes, acqui ed om ca bon s eel B450C exposed o he JLSC1 cemen ex ac o 7 days: (a) Fe2p; (b) O1s; (c) C1s; (d) Na1s; (e) Ca2p; ( ) K2p; (g) S2p. Ma e ials 2023, 16, x FOR PEER REVIEW 8 o 18 Figu e 4. O e iew o X- ay pho oelec on spec oscopy (XPS) spec a, a e spu e ing he me al su ace o diffe en imes, acqui ed om ca bon s eel B450C exposed o he JLSC1 cemen ex ac o 30 days: (a) Fe 2p; (b) decon olu ion o Fe2p; (c) O1s; (d) de olu ion o O1s. 3.3. SS430 S ainelss S eel Su ace Cha ac e iza ion a e Exposu e o JLSC1 Cemen Ex ac Solu ion Figu e 5 p esen s SEM images o s ainless s eel SS430 a e exposu e o he JLSC1 ex ac o 7 days (Figu e 5a) and 30 days (Figu e 5b). The EDS analysis o some zones o in e es is esumed in Table 5. Al hough he pH o he cemen ex ac solu ion diminished o 9.58, a e 7 days o exposu e, he s eel co osion po en ial o OCP was posi i e (+128 mV), as an indica ion ha he me al su ace main ained i s passi e s a e (Figu e 1). Ac- co ding o EDS analysis (Table 5), zone A p esen ed O (36.20%), S (22.87%), K (31.19%) and Na (8.28%), which could be associa ed wi h he p ecipi a ion o sul a es K, and Na (Table 3). Zone B co esponded o he s eel ma ix. In zone C, he p ecipi a ion o sul a e c ys als was obse ed and composed o Na (21.69%), O (14.62%), S (4.77%), and in he lowe con en o C (1.88%). Figu e 5. SEM images o s ainless s eel SS430 a e 7 (a) and 30 (b) days o exposu e o he JLSC1 cemen ex ac solu ion. A e 30 days o exposu e o he JLSC1 cemen ex ac (Figu e 5b), he su ace o SS430 main ained i s passi e s a e (Figu e 1, OCP o +154.73 mV), and i was no ully co e ed by a co osion laye . The high con en o O, K, S, and Na in zone 1 (Table 5) could be a ibu ed o he p ecipi a ion o K and Na sul a es (Table 3). Meanwhile, he EDS o zone 2 indica ed he highe con en s o C (15.31–17.02%) and Fe, which we e associa ed wi h he (C ,Fe)7C3 ca bide [54,55], and we e epo ed o he e e ence samples o his s eel. Figu e 4. O e iew o X- ay pho oelec on spec oscopy (XPS) spec a, a e spu e ing he me al su ace o di e en imes, acqui ed om ca bon s eel B450C exposed o he JLSC1 cemen ex ac o 30 days: (a) Fe 2p; (b) decon olu ion o Fe2p; (c) O1s; (d) de olu ion o O1s. Ma e ials 2023,16, 5066 8 o 17 3.3. SS430 S ainelss S eel Su ace Cha ac e iza ion a e Exposu e o JLSC1 Cemen Ex ac Solu ion Figu e 5p esen s SEM images o s ainless s eel SS430 a e exposu e o he JLSC1 ex ac o 7 days (Figu e 5a) and 30 days (Figu e 5b). The EDS analysis o some zones o in e es is esumed in Table 5. Al hough he pH o he cemen ex ac solu ion diminished o 9.58, a e 7 days o exposu e, he s eel co osion po en ial o OCP was posi i e (+128 mV), as an indica ion ha he me al su ace main ained i s passi e s a e (Figu e 1). Acco ding o EDS analysis (Table 5), zone A p esen ed O (36.20%), S (22.87%), K (31.19%) and Na (8.28%), which could be associa ed wi h he p ecipi a ion o sul a es K, and Na (Table 3). Zone B co esponded o he s eel ma ix. In zone C, he p ecipi a ion o sul a e c ys als was obse ed and composed o Na (21.69%), O (14.62%), S (4.77%), and in he lowe con en o C (1.88%). Ma e ials 2023, 16, x FOR PEER REVIEW 8 o 18 Figu e 4. O e iew o X- ay pho oelec on spec oscopy (XPS) spec a, a e spu e ing he me al su ace o diffe en imes, acqui ed om ca bon s eel B450C exposed o he JLSC1 cemen ex ac o 30 days: (a) Fe 2p; (b) decon olu ion o Fe2p; (c) O1s; (d) de olu ion o O1s. 3.3. SS430 S ainelss S eel Su ace Cha ac e iza ion a e Exposu e o JLSC1 Cemen Ex ac Solu ion Figu e 5 p esen s SEM images o s ainless s eel SS430 a e exposu e o he JLSC1 ex ac o 7 days (Figu e 5a) and 30 days (Figu e 5b). The EDS analysis o some zones o in e es is esumed in Table 5. Al hough he pH o he cemen ex ac solu ion diminished o 9.58, a e 7 days o exposu e, he s eel co osion po en ial o OCP was posi i e (+128 mV), as an indica ion ha he me al su ace main ained i s passi e s a e (Figu e 1). Ac- co ding o EDS analysis (Table 5), zone A p esen ed O (36.20%), S (22.87%), K (31.19%) and Na (8.28%), which could be associa ed wi h he p ecipi a ion o sul a es K, and Na (Table 3). Zone B co esponded o he s eel ma ix. In zone C, he p ecipi a ion o sul a e c ys als was obse ed and composed o Na (21.69%), O (14.62%), S (4.77%), and in he lowe con en o C (1.88%). Figu e 5. SEM images o s ainless s eel SS430 a e 7 (a) and 30 (b) days o exposu e o he JLSC1 cemen ex ac solu ion. A e 30 days o exposu e o he JLSC1 cemen ex ac (Figu e 5b), he su ace o SS430 main ained i s passi e s a e (Figu e 1, OCP o +154.73 mV), and i was no ully co e ed by a co osion laye . The high con en o O, K, S, and Na in zone 1 (Table 5) could be a ibu ed o he p ecipi a ion o K and Na sul a es (Table 3). Meanwhile, he EDS o zone 2 indica ed he highe con en s o C (15.31–17.02%) and Fe, which we e associa ed wi h he (C ,Fe)7C3 ca bide [54,55], and we e epo ed o he e e ence samples o his s eel. Figu e 5. SEM images o s ainless s eel SS430 a e 7 ( a ) and 30 ( b ) days o exposu e o he JLSC1 cemen ex ac solu ion. Table 5. EDS su ace a e age analysis (w .%) o SS430 a eas o in e es as ma ked on he SEM images o Figu e 5. Days/ w .% Fe C O Na S K Si C Cl 7 A 1.21 - 36.20 8.28 22.87 31.19 0.24 - - B 64.28 14.00 6.63 3.53 - 0.24 0.67 10.74 - C 46.93 9.77 14.62 21.69 4.77 0.33 - 1.88 - 30 1 4.08 1.23 52.72 22.77 0.33 0.43 1.06 17.02 0.35 2 32.57 7.58 25.22 5.78 - 1.47 9.10 15.31 0.28 A e 30 days o exposu e o he JLSC1 cemen ex ac (Figu e 5b), he su ace o SS430 main ained i s passi e s a e (Figu e 1, OCP o +154.73 mV), and i was no ully co e ed by a co osion laye . The high con en o O, K, S, and Na in zone 1 (Table 5) could be a ibu ed o he p ecipi a ion o K and Na sul a es (Table 3). Meanwhile, he EDS o zone 2 indica ed he highe con en s o C (15.31–17.02%) and Fe, which we e associa ed wi h he (C ,Fe) 7 C 3 ca bide [54,55], and we e epo ed o he e e ence samples o his s eel. I has been sugges ed ha anions wi h a highe cha ge (SO 42− , 1132.0 mg/L, Table 3) may accumula e a he me al–solu ion in e ace inside he pi s and lead o a educ ion e ec on he elec ical po en ial and, hus, o he sel - epassi a ion o he s eel su ace [65]. The XPS spec a o s ainless s eel SS430, a e exposu e o 30 days o he JLSC1 cemen ex ac solu ion, a e p esen ed in Figu e 6. Based on he decon olu ion o Fe2p (Figu e 6a), C 2p (Figu e 6b), and O1s (Figu e 6c) peaks, i could sugges ha he passi e laye o med on he s ainless s eel su ace was composed o FeO (709.1 eV), Fe 2 O 3 (711.1 eV), C 2 O 3 (576.5 eV), and he co osion p oduc s o C (OH) 3 [ 62 , 66 ]. The decon olu ion o he O1s con i med he peaks o he oxides (531.3 eV) and oxide-hyd oxide OH − (533.1 eV), and in a low in ensi y, he peak o C-O (535.8 eV) could be associa ed wi h he p esence o Na (Na1s) and K (K2p) ca bona es o bica bona es; he peaks o Cl2p, C1s, S2p a e also indica ed. Ma e ials 2023,16, 5066 9 o 17 Ma e ials 2023, 16, x FOR PEER REVIEW 9 o 18 I has been sugges ed ha anions wi h a highe cha ge (SO42−, 1132.0 mg/L, Table 3) may accumula e a he me al–solu ion in e ace inside he pi s and lead o a educ ion effec on he elec ical po en ial and, hus, o he sel - epassi a ion o he s eel su ace [65]. Table 5. EDS su ace a e age analysis (w .%) o SS430 a eas o in e es as ma ked on he SEM images o Figu e 5. Days/ w .% Fe C O Na S K Si C Cl 7 A 1.21 - 36.20 8.28 22.87 31.19 0.24 - - B 64.28 14.00 6.63 3.53 - 0.24 0.67 10.74 - C 46.93 9.77 14.62 21.69 4.77 0.33 - 1.88 - 30 1 4.08 1.23 52.72 22.77 0.33 0.43 1.06 17.02 0.35 2 32.57 7.58 25.22 5.78 - 1.47 9.10 15.31 0.28 The XPS spec a o s ainless s eel SS430, a e exposu e o 30 days o he JLSC1 ce- men ex ac solu ion, a e p esen ed in Figu e 6. Based on he decon olu ion o Fe2p (Fig- u e 6a), C 2p (Figu e 6b), and O1s (Figu e 6c) peaks, i could sugges ha he passi e laye o med on he s ainless s eel su ace was composed o FeO (709.1 eV), Fe2O3 (711.1 eV), C 2O3 (576.5 eV), and he co osion p oduc s o C (OH)3 [62,66]. The decon olu ion o he O1s confi med he peaks o he oxides (531.3 eV) and oxide-hyd oxide OH− (533.1 eV), and in a low in ensi y, he peak o C-O (535.8 eV) could be associa ed wi h he p esence o Na (Na1s) and K (K2p) ca bona es o bica bona es; he peaks o Cl2p, C1s, S2p a e also indica ed. Figu e 6. O e iew o X- ay pho oelec on spec oscopy (XPS) spec a, a e spu e ing he me al su ace o di e en imes, acqui ed om SS430 s eel exposed o he JLSC1 cemen ex ac o 30 days: (a) Fe2p; (b) O1s; (c) C 2p; (d) C1s; (e) Na1s; ( ) K2p; (g) Cl2p; (h) S2p. 3.4. S eel Su ace Damage a e he Exposu e o JLSC1 Cemen Ex ac Solu ion Figu e 7compa es SEM images o s ainless s eel SS430 and ca bon s eel B450C su aces a e he emo al o laye s o med du ing hei exposu e o he JLSC1 cemen ex ac o 30 days. On he s ainless-s eel su ace (Figu e 7a), isola ed pi s ( ed-colo ed ci cles) a e isible wi h a diame e lowe han 1–3 µ m due o he p esence o chlo ide ions in he cemen ex ac (88 mg/L, Table 3); hese ions pene a ed he oxide passi e ilm. These localized a acks could appea in he icini y o pa icles wi h ca hodic ac i i y. Such pa icles (named A, B, and C) we e hose sugges ed by EDS analysis (Table 6), p esen ing high con en s o Fe, C , and C and a lowe con en o V, which could be asc ibed mainly o C -Fe ca bides (C ,Fe) 7 C 3 [ 54 , 55 ] and he p ecipi a es o V 6 C 5 phase, all cha ac e is ic o he SS430 ma ix ( e e ence samples). On he ca bon s eel su ace (Figu e 7b), he a acks we e mo e uni o m because his s eel los i s passi e s a e due o a shi in he JLSC1 cemen ex ac pH o lowe alkaline alues. Table 7 esumes he EDS analysis o he zones o in e es . The con en o Mn could be asc ibed o he phases o MnS and Mn3C [53]. Table 6. EDS su ace a e age analysis (w .%) o he SS430 a eas o in e es as ma ked on he SEM image o Figu e 7a. w .% Fe C V Si C SS430 A 58.12 40.09 1.79 - - B 50.26 43.51 2.10 0.46 3.67 C 74.29 15.61 - - 10.11 Ma e ials 2023,16, 5066 16 o 17 26. P o is, L. Alkali-ac i a ed ma e ials. Cemen Conc . Res. 2018,114, 40–48. [C ossRe ] 27. Pue as, F.; Fe nández-Jimenez, A.; Blanco-Va ela, M.T. Po e solu ion in alkali.ac i a ed slag cemen pas es. Rela ion o he composi ion and s ucu e o calcium silica e hyd a e. Cemen Conc . Res. 2004,34, 139–148. [C ossRe ] 28. Lloyd, R.; P o is, J.L.; an De en e , J.S.J. Po e solu ion composi ion and alkali di usión in ino ganic polyme cemen . Cemen Conc . 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