GALVANIC CORROSIONGALVANIC CORROSION Corrosion as a result of the electrical contact of different met Corrosion as a result of the electrical contact of different metals. als. Galvanic series: understand and use it properly! Galvanic series: understand and use it properly! Insulate anode from cathode if possible. Insulate anode from cathode if possible. Area ratio of anode and cathode is important. Area ratio of anode and cathode is important.Avoid unfavorable ratio of small anode to large cathode. Avoid unfavorable ratio of small anode to large cathode.Weld metal, bolts & nuts, rivets should be more noble than the s Weld metal, bolts & nuts, rivets should be more noble than the substrate ubstrate UnderstandUnderstand noble metal coating noble metal coating concept. concept.Never just specify coat the anode only. Never just specify coat the anode only. Careful in using copper tube, Hg Careful in using copper tube, Hg- -containing manometers. containing manometers.Galvanic corrosion Galvanic corrosionGalvanic corrosionGalvanic corrosionSTS/carbon steel STS/carbon steel STS STS- -Cu CuCORROSION of Al LAMELLAECORROSION of Al LAMELLAEAl Al- -Cu CuAl Al- -STS STSGALVANIC SERIESGALVANIC SERIESChange in environment?Change in environment?A new series should be established.A new series should be established.--ComplexingComplexingions, pH, temperature, velocity ions, pH, temperature, velocity ....--Potential (SCE) of highPotential (SCE) of high--alloy STS in sea water alloy STS in sea water (SW)(SW)Deaerated SW:Deaerated SW:- -450 mV 450 mVSW with 200 ppb O SW with 200 ppb O2 2: : - -100 mV 100 mVHot SW: Hot SW: 100 mV 100 mVNatural, aerated SW: Natural, aerated SW: 300 mV 300 mVChlorinated SW: Chlorinated SW: 600 mV 600 mVCh i l l i hGalvanic Corrosion by Potential ReversalGalvanic Corrosion by Potential Reversalbicarbonate, nitrate bicarbonate, nitratesulfate sulfatephosphate phosphateGalvanic Corrosion by Copper DepositionGalvanic Corrosion by Copper DepositionCooling water systems: corrosion of steel after deposition of co Cooling water systems: corrosion of steel after deposition of copper.pper. FORMS OF GALVANIC CORROSIONFORMS OF GALVANIC CORROSION Contact corrosion Contact corrosion Deposition corrosion Deposition corrosionCathodic contamination of anode via deposition of more nobleCathodic contamination of anode via deposition of more noble metals or metal oxides metals or metal oxides Reversal of potential Reversal of potential Thermogalvanic corrosion Thermogalvanic corrosionPreferential attack at the hot area when metal is subjected to a Preferential attack at the hot area when metal is subjected to athermal gradient. thermal gradient.GALVANIC CORROSION CELLGALVANIC CORROSION CELLTypical conductivity values: Typical conductivity values:Fresh distilled waterFresh distilled water0.5 0.5- -22 S/cm S/cmStored distilled water Stored distilled water 2 2- -44 S/cm S/cmSupply water Supply water 50 50- -15001500 S/cm S/cmSea water Sea water 40,00040,000 S/cm S/cmSat.Sat. NaCl NaCl solution solution 250,000250,000 S/cm S/cmSulfuric acid Sulfuric acid