Kako pri izbiri korozijsko{0}}odpornega grelnika iz titana za uporabo v morski vodi stabilnost površinskega oksidnega sloja spremeni dolgoročno-delovanje?

Metallic immersion heaters are one of the hardest electrolyte medium in sea water. In a chloride rich environment (about 19,000 ppm) in the presence of dissolved oxygen, different pH and biofouling potential most engineering alloys will rapidly degrade. The remarkable corrosion resistance of titanium in seawater is related to the spontaneously produced and thermodynamically stable TiO 2 passive layer, which is usually 2-5 nm thick . But the long term performance of a titanium heating tube in seawater depends not only on the inherent qualities of the base metal but also on the stability of this oxide layer under cyclic thermal and electrochemical stress. If the oxide is stable, it will give you decades of trouble free service. If it is unstable, localized pitting, hydrogen absorption and untimely failure will result. Formation and Destruction Mechanisms of Oxide Films in Marine Service Titanium spontaneously develops a passive layer in contact with oxygenated sea water. Stability is defined by three parameters, the electrochemical potential at the tube surface, the local pH at the metal-fluid interface and the temperature gradient in the sheath. Ideally, if the oxide is mechanically damaged, it will heal itself in seconds. But the difference in thermal expansion between the oxide and the metal substrate leads to micro-strains when a titanium heater is cycled between ambient and severe temperatures (e.g. 25°C to 80°C in a seawater tank). Repeated cycling can generate nano-scale cracks, exposing fresh titanium. If the surrounding seawater has a low level of dissolved oxygen (such as in stagnant areas, or underneath biofilms) repassivation is very slow, and the exposed metal starts to corrode locally. The activity of sulfate reducing bacteria reduces the local pH further to 4-5 and enhances oxide solubility. How heater performance deteriorates with time due to oxide instability The deterioration oxide layer is not leading to rapid failure but it is leading to detectable changes in the behavior of the heater. First, the corrosion potential of the titanium tube (E corr ) shifts from the passive area (typically +0.1 to +0.3 V vs. SCE) to active values (less than -0.3 V vs. SCE). This evolution can be monitored periodically using electrochemical measurements. Second, localised breakdown of the oxide allows access of chloride ions to the metal substrate and induces metastable pitting. Each metastable pit event gives rise to a small current transient which can be measured but has not yet pierced the wall. Third, after the stable pits are produced, the heat transfer area is reduced, thus increasing the local heat flow at the remaining intact oxide. The higher flux raises the temperature of the metal and further deteriorates the oxide in a positive feedback loop. Data from seawater cooled heat exchangers suggests that a 20% drop in the oxide stability (i.e., an increase in the passive current density) corresponds to a 35% loss in the remaining tube life. Oxide Management with a Scenario Based Strategy The following table is a decision guide for selection and maintenance of titanium heaters according to service circumstances in seawater and priorities for oxide stability. Scenario & Main Concern of Seawater UsageProposed Improvement on Oxide Stability Basic Rationale & Change in Performance High flow (>1 m/s), neprekinjeno prezračevano odprto zanko za ogrevanje morske vode. Titan stopnje 2, kot je narisano brez uporabljene površinske obdelave. Ostanki naravnega oksida. Kisik je zelo dostopen. Gibanje preprečuje stagnacijo. Nič več ni potrebno izboljšati. Rezervoarji z nizkim pretokom ali stacionarni rezervoarji morske vode (doki, ogrevanje balastne vode) Anodna pasivacija ali pred-oksidirana cev (toplotna niansa pri 400 C) Debel umetni oksid (do 50 nm) lahko preživi razgradnjo v pogojih nizke vsebnosti kisika. Sprejema nekoliko večjo toplotno odpornost. Periodično biološko obraščanje in šok kloriranja Vbrizgavanje morske vode Titan, prevlečen z žlahtno kovino (paladij, 0,5 µm). Pd pospeši ponovno pasivacijo oksidov po izpostavljenosti Cl. Spremenite način okvare iz luknjičastega v enakomerno raztapljanje, hitra stopnja izhlapevanja, slana voda (70-90 stopinj) Katodna zaščita grelnika (-0,6 V v primerjavi z Ag/AgCl) Stabilnost oksida na račun zunanje napetosti nad kritično temperaturo. Zaščitni tok je treba občasno preveriti. Konstrukcijski dejavniki za dolgoročno zanesljivost (dodatno) Debelina stene je manj pomembna od stabilnosti oksida v morski vodi. Ne glede na to, kako debela je cev, bo vsaka cev z nestabilnim oksidom v nekaj mesecih prešla skozi. Nasprotno pa lahko cev z dobro oksidno stabilnostjo in tankimi stenami (0,6 mm) obstane leta v čisti tekoči morski vodi. Nujno je redno čiščenje za odstranjevanje biofilmov, vzdrževanje ravni raztopljenega kisika nad 5 ppm in preprečevanje prekomerne katodne zaščite (ki lahko spodbuja absorpcijo vodika). Če določate titanov grelnik za uporabo v morski vodi, prosite ponudnika za podatke elektrokemijske impedančne spektroskopije (EIS), ki prikazuje odpornost pasivnega filma v sintetični slani vodi pri določeni delovni temperaturi. Na podlagi-specifikacij Pri izbiri titanovega grelnika za morsko vodo, odpornega proti koroziji, se je treba preusmeriti od lastnosti kovine v razsutem stanju k lastnostim površinskega oksida. Cevi iz naravnega oksida standardne stopnje 2 so učinkovite v aplikacijah z visokim pretokom in prezračevanjem. Prevleke s predoksidacijo ali prevleke iz plemenitih kovin spremenijo način odpovedi od hitrega luknjanja v stagnirajočih, vročih ali biološko onesnaženih pogojih do stalnega, predvidljivega redčenja. Kombinirajte nadzor stabilnosti oksida z ustreznimi intervali čiščenja in spremljanjem raztopljenega kisika. Ta pristop spremeni titanov grelnik iz pasivnega elementa v predvidljivo komponento z dolgo življenjsko dobo v morskem okolju.