Pri odabiru korozijsko{0}}grijača od titana za aplikacije u morskoj vodi, kako stabilnost površinskog oksidnog sloja utječe na dugoročne-izvedbe?

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), kontinuirano aerirano grijanje morske vode u otvorenoj petlji Titan stupnja 2 kao što je nacrtano bez primijenjene površinske obrade Ostaci prirodnog oksida. Kisik je vrlo dostupan. Kretanje sprječava stagnaciju. Nije potrebno više poboljšanja. Niski protok ili stacionarni tankovi morske vode (dokovi, grijanje balastne vode) Anodna pasivizacija ili pred-oksidirana cijev (toplinska boja na 400 C) Debeli umjetni oksid (do 50 nm) može preživjeti kvar u uvjetima niske razine kisika. Prihvaća nešto veću otpornost na toplinu. Periodično biološko obraštanje i šok kloriranja Ubrizgavanje morske vode Titan presvučen plemenitim metalom (paladij, 0,5 µm). Pd ubrzava repasivaciju oksida nakon izlaganja Cl. Promijenite način kvara s jamičaste na ravnomjerno otapanje, brza stopa isparavanja slana voda (70-90 stupnjeva )Katodna zaštita grijača (-0,6 V u odnosu na Ag/AgCl) Stabilnost oksida po cijenu vanjskog napona iznad kritične temperature. Zaštitnu struju potrebno je povremeno provjeravati. Čimbenici dizajna za dugoročnu pouzdanost (dodatno) Debljina stijenke manje je važna od stabilnosti oksida u morskoj vodi. Bez obzira koliko je cijev debela, svaka cijev s nestabilnim oksidom probit će se kroz nju za nekoliko mjeseci. Naprotiv, cijev s dobrom oksidnom stabilnošću i tankim stijenkama (0,6 mm) može izdržati godinama u čistoj tekućoj morskoj vodi. Neophodno je redovito čišćenje radi uklanjanja biofilmova, održavanja razine otopljenog kisika iznad 5 ppm i sprječavanja prekomjerne katodne zaštite (koja bi mogla pospješiti apsorpciju vodika). Ako specificirate grijač od titana za korištenje u morskoj vodi, zatražite od dobavljača podatke spektroskopije elektrokemijske impedancije (EIS) koji pokazuju otpornost pasivnog filma u sintetičkoj slanoj vodi na navedenoj radnoj temperaturi. Usmjeren-na specifikacije Pri odabiru grijača od titana za morsku vodu otpornog na koroziju, potrebno je prijeći sa svojstava metala u rasutom stanju na svojstva površinskog oksida. Cijevi od prirodnog oksida Standard Grade 2 učinkovite su u primjenama s visokim protokom i prozračivanjem. Prethodno oksidirane prevlake ili prevlake od plemenitih metala modificiraju način kvara od brzog pitinga u stagnirajućim, vrućim ili biološkim uvjetima do stalnog, predvidljivog stanjivanja. Kombinirajte kontrolu stabilnosti oksida s pravilnim intervalima čišćenja i praćenjem otopljenog kisika. Ovaj pristup pretvara titanski grijač iz pasivnog elementa u predvidljivu dugovječnu komponentu u morskom okruženju.