Raws li kev sib xyaw tshwj xeeb ntawm Applied Cathodic Protection Potential thiab Sulfide Concentration puas 316L rhaub Sheath hauv Seawater ntawm 25℃Hloov ntawm Hydrogen Embrittlement mus rau Ductile Cwj Pwm Tom qab 10,000 Teev

Kev txwv ntawm Hydrogen Kev puas tsuaj hauv Marine Cathodic Protection

The risk of hydrogen embrittlement (HE) in 316L stainless steel sheathed electric heating tubes in seawater with cathodic protection (CP) such as on offshore platforms, marine vessels or coastal structures is a function of the potential (vs. Ag/AgCl) applied to the tubes. At larger negative potentials (more cathodic) hydrogen evolution (2H2O + 2e- -> H2 + 2OH-) occurs and atomic hydrogen can infiltrate into the 316L sheath producing HE cracking. The presence of sulphides (H2S or HS- ) increases the hydrogen entrance, since sulphide is a toxin for the hydrogen recombination reaction. The ductile-to-HE-dominated transition is established above a critical combination of applied voltage and sulphide content. 316L is still ductile ( no HE ) at potentials exceeding -1000 mV vs . Ag / AgCl in saltwater at 25 C , no sulphides . HE can be seen at -1,050 mV during 5,000-10,000 hours. HE occurs at -1,100 mV within 1,000 hours. In the presence of 10-50 ppm sulphide, the safe potential moves to -900 mV. 316L is prone to HE at >100 ppm sulfide txawm ntawm -800mV. Kab lus no ntsuas qhov pib ntawm CP peev xwm (sulphide concentration) rau HE zam tom qab 10,000 teev ntawm kev raug.

Mechanism ntawm Hydrogen Embrittlement nyob rau hauv Cathodic Protection

Cathodic tiv thaiv ntawm 316L hauv dej hiav txwv feem ntau yog ua los ntawm kev txi anodes (zinc los yog txhuas) los yog impressed tam sim no (ICCP). Hydrogen evolution pib ntawm qhov muaj peev xwm tsis zoo dua li -800 mV vs. Ag/AgCl. Cov hydrogen nkag mus rau hauv 316L nce nrog qhov muaj peev xwm txo qis. Cov sulphide hom (H2S, HS-, thiab S2-) yog adsorbed ntawm cov hlau nto thiab lom cov txheej txheem hydrogen recombination (2H → H2). Qhov no boosts atomic hydrogen khov kho-lub xeev concentration nyob rau hauv cov hlau los ntawm yam ntawm 10 mus rau 1,000. Qhov pib hydrogen concentration rau HE hauv 316L yog li 5-10 ppm (qhov hnyav). Saum toj no cov ntaub ntawv ua nkig thiab ua tsis tiav ntawm intergranular los yog transgranular cracking nyob rau hauv residual los yog thov tensile stresses.

Quantifying CP Muaj Peev Xwm -Sulfide Thresholds rau HE Mitigation

Cov nram qab no HE pib pib tau txais los ntawm kev tswj cathodic polarization ntawm 316L U- khoov cov qauv (nruam tensile strain) hauv cov dej hiav txwv hluavtaws (pH 8.0, 25 degree) ntawm ntau yam sulfide concentrations (raws li Na2S) rau 10,000 teev.

Applied CP Potential (mV vs. Ag/AgCl) Sulphide Concentration (ppm as S 2- )Hydrogen Entry Rate (relative) Time to HE Cracking (hours) Failure Mode Recommended for 10,000h Seawater Service > -800 0 No H evolution >20,000Yes None -800 to -900 0 Trace >20,000 Tsis Muaj
-900 to -1,000 0 Low >15,000 Tsis Yog
-1,000 rau -1,050 0 Nruab nrab 8,000-15,000 Mixed (qee HE) Txais
-1,050 rau -1,100 0 Siab 3,000-8,000 HE Tsis pom zoo (ntev) < -1,100 0 Siab heev<3,000 HE No > -800 10-50 Low (sulphide limited) >15,000 Tsis Tau Txais
-800 rau -850 10-50 Nruab nrab 8,000-15,000 Mixed Marginal
-850 rau -900 10-50 Siab 4,000-8,000 HE Tsis pom zoo < -900 10-50siab heev<4,000 HE No > -750 50-100 Sulphide increased) Moderate 8,000-15,000 Mixed Marginal -750 to -800 50-100 High 3,000-8,000 HE Not recommended < -800 50-100Very high <3,000 HE No 
Any >100 siab mus heev<5,000 (≤ -700 mV) HE No (316L not suited)
Susceptibility rau HE raws li ib tug Function ntawm kub

Qhov kub siab dua ua rau tus nqi ntawm hydrogen diffusion thiab embrittlement.

Kub (degree) Ntau tshaj CP Muaj Peev Xwm rau 10,000h HE-Kev Pab Dawb (mV vs. Ag/AgCl, 0 sulphide) Qhov siab tshaj plaws CP Muaj peev xwm rau 10,000h HE-Kev Pab Dawb (10 ppm sulphide)
5 -1,050 -900 15 -1,030 -880 25 -1,000 -850 35 -970 -820 50 -920 -780 
Cov lus pom zoo rau kev tiv thaiv Cathodic ntawm 316L Stainless Hlau hauv Seawater

Rau 316L encased cua sov hauv dej hiav txwv nrog kev tiv thaiv cathodic, cov kev txwv CP hauv qab no yuav siv tau nyob ntawm qhov kwv yees ntawm sulfide concentration.

Seawater Hom Nruab Nrab Sulphide (ppm) CP Muaj Peev Xwm Ntau Yam (mV vs. Ag/AgCl) Qhia Txog Lub Neej Muaj Peev Xwm Ntau Tshaj 10,000h (mV) Lwm Cov Khoom Siv
Huv si, aerated<1 -800 to -950 -1,000 316L acceptable Polluted (harbour) 1-10 -800 to -900 -950 316L with monitoring Polluted (high organic) 10-50 -800 to -850 -880 Duplex 2205
Anoxic/sulfidic 50-100 -750 rau -800 -820 Duplex 2205 los yog alloy 825
Severe sulfidic >100 Tsis Pom Zoo N / A Titanium lossis Inconel 625
Field Identification ntawm CP Induced HE

Lub tshuab ua tsis tiav 316L hauv dej ntsev hauv qab kev tiv thaiv cathodic feem ntau qhia tau tias transgranular (los ntawm cov nplej) nws tawg nrog me ntsis ceg thiab feem ntau ib qho chaw tiaj tus pob txha tawg. Qhov ua rau muaj kev lees paub los ntawm qhov muaj sulfide (tseem hu ua H2S, dub hlau sulfide deposits) thiab CP muaj peev xwm tsis zoo dua li -900 txog -1,000 mV. Cov lus teb yog ua kom CP muaj peev xwm (tsis tshua muaj qhov tsis zoo) rau qhov kev nyab xeeb, lossis hloov mus rau ntau dua HE resistant alloy (duplex 2205, lossis titanium).

Xaus: Kev tswj ntawm CP Muaj Peev Xwm thiab Sulphides los tiv thaiv HE

For 316L stainless steel heater sheaths in saltwater at 25°C with cathodic protection, the change from ductile to hydrogen embrittlement occurs at applied potentials more negative than -1,000 to -1,050 mV (sulphide free) or -850 to -900 mV (10-50 ppm sulphide). Maximum safe CP potential during 10,000 hour service is -1000 mV (0 sulphide), -850 mV (10-50 ppm sulphide). 316L is not advised for use at >100 ppm sulfide tsis hais txog ntawm CP muaj peev xwm. Cov kws tsim qauv qhia txog cathodically shielded 316L sheaths rau kev pabcuam hauv hiav txwv yuav tsum tswj CP lub peev xwm kom nyob hauv lub qhov rais nyab xeeb thiab saib xyuas cov tshuaj sulfide. Cov qauv tsim ntawm no txuas cov CP muaj peev xwm, sulphide concentration thiab HE pib lub sijhawm ntawm 25℃hauv dej ntsev thiab muab lub hauv paus rau cov neeg yuav khoom los txhais cov ntsiab lus CP uas yuav zam kom tsis txhob muaj hydrogen embrittlement ntawm 316L lub rhaub dej kub thoob plaws 10,000 teev tsim lub neej.