Shipwrecks, Corrosion and Conservation Summary Slides PART 3 – Jack Dengate.
SS Corrosion Slides
description
Transcript of SS Corrosion Slides
Galvanic andStray Current Corrosion
OverviewGalvanic Corrosion
Understanding Galvanic CorrosionControlling Galvanic Corrosion
Stray Current CorrosionUnderstanding Stray Current CorrosionPreventing Stray Current CorrosionTesting for Stray Current
Galvanic Corrosion
Understanding Galvanic Corrosion
CausesResultsGalvanic Series of MetalsAdditional Notes
CausesRequires
Two different metals (electrodes)Immersed in current-carrying solution (electrolyte)Interconnected by a current-carrying conductor
Results
New Zinc(for 1” diameter
shaft)
of Galvanic Corrosion
Old Zinc after 8 months
(for 1” diameter shaft)
Galvanic Scale of Metals
What is the voltage difference between Zinc (Zn) and Copper (Cu)?
What is more noble than Stainless Steel (Passive)?
An. 0.67v
An. Graphite
Additional NotesExpect corrosion with 0.25 V difference
Most negative electrodes will decomposeMagnesium @ - 1.50 V for freshwaterZinc @ - 1.03 V for saltwaterAluminum @ - 0.75 V will decompose if neither
magnesium or zinc are present
Zinc (or magnesium) will protectStainless steel shaftBronze propellerAluminum outdrive
Signs of Galvanic CorrosionBlistering of paint
1st Warning SignFormation of powdery substance
2nd Warning SignPitting of metal
Too lateSevere Galvanic Corrosion
Don’t treat the symptom, fix the problem
Galvanic Corrosion
Controlling Galvanic Corrosion
Types of MetalArea of MetalsSelf-Destroying MetalsUse of Sacrificial AnodesIndirect Cathodic ProtectionResistance of an Electrical PathBetween boats
Types of MetalCopper, bronze and copper-nickel are
compatible
Avoid bronze propeller on plain steel shaftStainless steel shaft with bronze prop may be
usedNeed zinc washer and/or zinc prop nutAvoid graphite grease
Area of MetalGood – applying a less noble metal to a large
areaBronze through-hull on steel hull
Bad – applying a more noble metal to a larger areaSteel screws / bolts on large bronze or monel
plate
Self-Destroying Metals
Brass (an alloy of copper and zinc)Zinc will corrode away in sea water, leaving a
copper sponge
Stainless steel hose clamps with different metal take-up screws
Stainless steel should be non-magneticIf magnetic, it will corrode
Use of Sacrificial AnodesMade from active metals
Magnesium, zinc or aluminumCorrosive action occurs on the expendable
metal anodeBolted to the metal they are to protectNever paintedReplaced when half-corroded or annually
Shaft Prop Nut Rudder
Powerboat Zincs
6 Zincs
Trim Tab
Indirect Cathodic ProtectionUsed when direct contact not possible
Zinc bolted to outside of hullInside boat connect with insulated AWG#8 to
Rudder Post Shaft (requires shaft brush)
Resistance of Electrical PathFresh water is less conductive than salt water
Less galvanic currentUse magnesium sacrificial anodes
Salt water is more conductive than fresh waterMore galvanic currentUse zinc sacrificial anodes
Magnesium sacrificial anodes will not last
Graphite grease is an excellent conductor, but is a cathodeDo NOT use in stuffing boxesDo NOT use on shaft bearings
Between BoatsTwo different metals
Aluminum vs steel (or other metal)Immersed in current-carrying solution
Sea waterInterconnected by current-carrying conductor
AC ground (green) wire
Galvanic Isolator
Stops DC current in AC ground wire
or Isolation Transformer
Isolation TransformerGalvanic Isolator
Stray Current Corrosion
Understanding Stray Current Corrosion
CausesResultsAdditional Notes
Stray Current CorrosionRequires
External source of electricityFrom wetted metal surface (electrodes)To return circuit of lower potential
(electrolyte)
Stray vs Galvanic Current
Stray current corrosion is more destructiveHundreds of times strongerGalvanic potential difference 0.25 to 1.5 voltsStray current from 12 volt battery
Sources of stray currentInternal from boat’s 12 volt battery and
defective wiringExternal to boat from another source of DC
Resultsof Stray Current
Corrosion
Additional NotesStronger than Galvanic current
100 times more destructive
Metals can be similar or dissimilarCurrent flow from positive through electrolytePositive DC terminal will corrodeBoth AC terminals will corrode
Electrolyte is any moist surfaceBilge waterWet woodWet or moist surface
Stray Current Corrosion
Preventing Stray CurrentWiringBondingBattery chargerGalvanic isolatorsIsolation transformers
WiringDefective wiring is the most common cause
Deteriorated insulation on hot wireAlways use marine grade wires
Run wires above water lineMoist or wetted surfaces conduct currentMoisture in loose connections will cause
corrosionWires in bilge
Waterproof terminals and butt spicesHeat shrink tubing is 2nd choiceLiquid electrical tape is also an optionElectrical tape is inadequate
BondingMaintain adequate bonding system
All metallic bodies and surfaces at DC negativeChapter 2 (Wiring) covered bonding
Propeller shaft bondingRecommend by some authoritiesWill also reduce propeller “hash” (Chapter 7)Requires a shaft brush
AC Ground Isolation•If your boat has the better ground… and a nearby boat has stray current
Your boat will be damaged, unless…Stop DC current in AC ground wire
Galvanic Isolators & Isolation Transformers but
•Stray current may flow through your boatIn one underwater fittingThrough bonding systemOut another underwater fitting (remember corroded prop and shaft pictures)
Corrosion FactsNot all corrosion is electrical
Seawater deteriorates all metalsCavitation also erodes props
Stray current corrosion can be eliminated
Galvanic corrosion can be reduced and controlled
DC current is 100 times worse than AC current
Testing for Stray CurrentMeasuring Stray Current Corrosion Source and Mitigation
Measuring Stray CurrentNormally AC ground and DC negative
connectedTo measure current, insert ammeter in series
To
Battery
Negative
DC Neg
AC Gnd
To
Shore
Power
Bus
Bar
Bus
Bar
ABYC Req
Temporary
break wire
to insert
Ammeter
A
AC Stray Current TestingAC main circuit breaker “On”
All branch circuit breakers “Off”
Set multimeter to read AC currentCurrent should be less than 1 milliampere
Then selectively turn on each AC circuit
If AC current exceeds 1 mAYou have stray current in that circuit
After testingReconnect AC ground & DC negative bus bars
DC Stray Current TestingDC main circuit breaker “On”
All branch circuit breakers “Off”
Set multimeter to read DC current
Current should be less than 0.01 milliampere
Then selectively turn on each DC circuit
If DC current exceeds 0.01 mAYou have stray current in that circuit
After testingReconnect AC ground and DC negative bus bars
Testing with Mitigation
Galvanic Isolators & Isolation TransformersStop DC current
To check for stray current with isolatorPlace ammeter between DC negative bus and
green shore power wire to isolator
To check for stray current with transformerPlace ammeter between DC negative bus and
green shore power wire to transformer
Internal DC Current Testing
Turn off DC main and all branch breakers
Insert ammeter in battery negative cable
Hold down bilge pump float switchSo pump will not turn on
Turn on DC main and bilge pump breaker
Measure stray current, if anyDefective wiring or pump switch
Test other wiring with DC devices turned off €
Summary 1Types of electronic corrosion
Galvanic caused by dissimilar metalsStray current requires external current
Galvanic current
Requires Different metals Immersed in current carrying solution Connect together by current carrying conductor
Brass will disintegrate in sea waterZincs are used to protect other metal
components
Summary 2Stray current
Requires an external source of currentNormally is caused by defective wiring
Especially in / through bilge Make sure any connections are waterproof
DC is 100 times more destructive than AC Over 1 mA AC Over 0.01 mA DC