Agus Téitheoir Tíotáiniam Frithsheasmhach á Roghnú-Téitheoir Tíotáiniam Frithsheasmhach in aghaidh Feidhmchláir Uisce Farraige, Conas a Athraíonn Cobhsaíocht an Chiseal Ocsaíd Dromchla-Feidhmíocht Téarma Fada?

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), téamh uisce farraige lúb oscailte aeraithe go leanúnach Tíotáiniam Grád 2 mar a tarraingíodh gan aon chóireáil dromchla curtha i bhfeidhm Iarsmaí ocsaíd nádúrtha. Tá ocsaigin ar fáil go mór. Coscann gluaiseacht marbhántacht. Níl gá le feabhsú níos mó. Umair uisce farraige atá ag sreabhadh íseal nó ina stad (duganna, téamh uisce ballasta)Pasivation anodic nó feadán réamh-ocsaídithe (tint teasa ag 400 C) Is féidir le hocsaíd shaorga tiubh (suas le 50 nm) maireachtáil faoi mhiondealú faoi choinníollacha íseal ocsaigine. Glacann friotaíocht teasa beagán níos airde. Bith-shalú Tréimhsiúil agus Instealladh Shock Clóiríniúcháin Uisce FarraigeTíotáiniam brataithe le miotal uasal (Pallaidiam, 0.5 µm).Pd luasanna suas ath-aisghabháil ocsaídí tar éis nochtadh do Cl. Athraigh an modh teipe ó piteáil go dtí tuaslagadh aonfhoirmeach ráta tapa galú uisce sáile (70-90 céim )Cosaint cathodach don téitheoir (-0.6 V vs. Ag/AgCl)Cobhsaíocht ocsaíd ag costas voltais sheachtraigh thar theocht chriticiúil. Ní mór an sruth cosanta a sheiceáil ó am go chéile. Fachtóirí Dearaidh d'Iontaofacht Fadtéarmach (Breise) Níl tiús bhalla chomh tábhachtach ná cobhsaíocht uisce farraige an ocsaíd. Is cuma cé chomh tiubh is atá an feadán, rachaidh aon fheadán a bhfuil ocsaíd éagobhsaí air i gceann cúpla mí. Os a choinne sin, féadfaidh feadán le dea-chobhsaíocht ocsaíd agus ballaí tanaí (0.6 mm) seasamh in uisce farraige glan reatha ar feadh na mblianta. Tá sé riachtanach glanadh go rialta chun bithscannáin a bhaint, chun leibhéil ocsaigine tuaslagtha a choinneáil os cionn 5 ppm agus chun ró-chosaint cathodach a chosc (a d'fhéadfadh ionsú hidrigine a chur chun cinn). Má tá téitheoir tíotáiniam á shonrú agat le húsáid in uisce farraige, iarr ar an soláthraí sonraí speictreascópachta impedance leictriceimiceach (EIS) a thaispeánann friotaíocht scannán éighníomhach i sáile sintéiseacha ag an teocht oibriúcháin sonraithe. Sonraíocht-Tiomáinte Agus téitheoir tíotáiniam resistant creimeadh a roghnú le haghaidh uisce farraige, ní mór duine a aistriú ó airíonna an mhiotail bhulc go dtí airíonna an ocsaíd dromchla. Feidhmíonn feadánra ocsaíd nádúrtha Grád 2 go héifeachtach i bhfeidhmchláir aeraithe ard-sreafa. Athraíonn bratuithe réamh-ocsaídiúcháin nó miotail uasal an modh teipe ó tholladh gasta i ndálaí stagnant, te nó bithshalaithe go tanúchán seasmhach intuartha. Comhcheangail rialú cobhsaíochta ocsaíd le tréimhsí glantacháin cuí agus monatóireacht ar ocsaigin tuaslagtha. Déanann an cur chuige seo an téitheoir tíotáiniam a thiontú ó eilimint éighníomhach go comhpháirt fadsaoil intuartha faoi thimpeallacht mhuirí.