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Corrosion Behavior of Galvanic Corrosion in Multi-material System in Marine Environment

Author: WangChunLi
Tutor: WuJianHua
School: Harbin Engineering University
Course: Materials Science
Keywords: galvanic corrosion oxygen content temperature area ratio of cathode to anode united protection
CLC: TG172
Type: PhD thesis
Year: 2013
Downloads: 18
Quote: 0
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Abstract

The structures of ship and ocean engineering are part of marine structures composed bymany kinds of metal alloy materials that is easy to form galvanic corrosion. Therefore, exceptthe relevance to corrosion resistance (corrosion) of metal materials, the durability of marinestructures is closely related to galvanic corrosion between metal alloy materials. As galvaniccorrosion is great dangerous to safety of marine structures, the galvanic corrosion research onmetal materials adopted by marine structures can provide data base and technical foundationfor the design of material selection and corrosion protection, that has important theoreticalsignificance and economic value. This paper selected the typical marine environmentalengineering material: commercially pure titanium (TA2), high strength low alloy steel (1#,2#,921A), copper nickel alloy (B10). The systematic study was conducted to obtain the changerule of single metal corrosion, and galvanic corrosion of bimetal coupling of TA2/921A,B10/921A,1#/921A,1#/2#and2#/921A and typical multi-material system of TA2/B10/921Aand1#/2#/921A. The control effect on galvanic corrosion of coating and sacrificial anodesprotection were also studied.For TA2and B10(the passive metal), increasing oxygen concentration is beneficial toformation and repair of passivation film with dissolved oxygen content at0~6mg/L, and thecorrosion rate decreases linearly with dissolved oxygen; the compact and stable passive filmwas formed with oxygen content≥6mg/L, and then the corrosion resistance trended tostability. For high strength low alloy steel (the non passivation metal), the oxygen contentincrease promotes the diffusion rate of oxygen molecules, resulting in the acceleration ofcathode oxygen depolarization, and the corrosion rate increases linearly with dissolvedoxygen. The effect of temperature raises on the diffusion rate of oxygen is greater than on thedecreases of dissolved oxygen content, and the corrosion resistance of metal materialsdecreased with the increase of temperature.The anode in bimetal coupling system had dissolution reaction, corrosion was aggravated,the corrosion process was the same as self-corrosion; cathodic reduction of dissolved oxygenoccurred, the protective effect of galvanic corrosion made the cathode of high potentialdifference (≥500mV) pair TA2/921A, B10/921A was almost no corrosion, and the cathode oflow potential difference (≤75mV) pair1#/921A,1#/2#,2#/921A was still self-corrosion due to small galvanic effect without complete protection, but the corrosion obviously slowed down.galvanic corrosion rate of high potential difference pair showed linear growth with the arearatio of cathode to anode (Sc/Sa), but had extreme because the galvanic potential shift anddriving voltage decreases; galvanic corrosion rate of low potential difference pair reachedextreme under small Sc/Sa without linear relationship, due to low potential difference andrapid decrease of driving voltage with area ratio increase. Density of pair galvanic currentincreased linearly with oxygen content. Galvanic current density increased with thetemperature accord with Arrhenius equation.The anode in multi-material system had dissolution reaction, corrosion process wasdifferent from corrosion in that the galvanic effect accelerates the transition step of corrosionreaction; reduction of dissolved oxygen occurred on catholic surface, high potential differencemulti-material system of TA2/B10/921A had strong polarization, anodic921A strongly wascorroded strongly, catholic TA2and B10were almost no corrosion due to completelyprotected by galvanic effect; low potential difference multi-material system of1#/2#/921A hadweak polarization, accelerated corrosion on anode921A and slowed down corrosionobviously on cathodic1#and2#. Galvanic corrosion rate of high potential differencemulti-material system showed linear growth with Sc/Sc/Sa, but had extreme because that theincreased area ratio enhanced anodic polarization, galvanic potential shifted positively anddriving voltage decreased gradually; galvanic potential of low potential differencemulti-material system of1#/2#/921A shifted positively with Sc/Sc/Sa increase, the2#corrosion potential reversed polarity as it was lower than galvanic potential under condition ofSc/Sc/Sa was3:3:1, from cathode to anode. The corrosion current density of anode couplingsystem increased linearly with oxygen content. Galvanic current density increased with thetemperature accord with Arrhenius equation.Bimetal and multi-material system are both accord with the mixed potential theory, andresults of polarization curve analysis with the galvanic corrosion behavior performed greatconsistence with the actual test. It can be applied to predict the galvanic corrosion behavior ofvarious materials by using polarization curve rapidly and accurately.Compared to the coating protection, the united protection of coating and sacrificial anodeshowed better resistance to the galvanic corrosion. Under united protection, the case ofaluminum anode provided complete protection current and stable protection potential, which effectively delayed the failure process of coating system as cathode. During120-daysexperimental period, the coating resistance has been maintained above7.3×10~7·cm~2. Theincrease of dissolved oxygen content and temperature in seawater promoted the coatingdegradation and galvanic corrosion, and galvanic corrosion further promote the failureprocess of anodic coating system with single coating. In united protection of coating andsacrificial anode, the enhanced galvanic effect slowed degradation speed of cathode coatingsystem, the coating resistance was maintained at1.1×10~7·cm~2under the worst condition of65℃.

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