Sciencemadness Discussion Board

sea water , fresh water corrosion

aeacfm - 5-11-2010 at 22:58

my friend working as corrosion engineer , I was amazed when he told me that distilled water more corrosive than fresh water and fresh water is more more corrosive than sea water ?is there any thing right in that?


also he told me that when boat or ship travel in fresh water it would corroded faster than when it travel in sea water ?

bquirky - 6-11-2010 at 12:11

sounds odd..

I chould understand a MOVING boat coroding slightly faster than a stationary one due to more oxygen incontact with the metal & agitation etc.


Perhaps the sacrificial anodes (zinc or magnisum) work better with a decent electrolyte (thats an outright guess)

chould it be due to the speed the boat travels at ? do boats go slower in rivers than in the ocean ?

it kind of reminds me of sompthing my grandfather told me once (he used to own a WWII metal working factory) that he used to store some of his metal working tools in POWDERD SALT!!!! to stop them rusting.. I allways thought he was pulling my leg or that dry salt made a remarkably good dessicant. but ive never understood it but frakly its hard to argue with old dudes that have lived 5x as long as you..

im not much help sorry. But its a interesting observation.


i


crazyboy - 6-11-2010 at 12:46

Quote:

Seawater is normally more corrosive than fresh water because of the higher conductivity and the penetrating power of the chloride ion through surface films on a metal.


http://www.corrosion-doctors.org/Corrosion-by-Water/Types-of...

food - 6-11-2010 at 19:03

I'm located next to the Pacific Ocean and I've worked on outboard motors for the last three summers. What you've written above doesn't bear out what I've been seeing. Salt water is quite aggressive.

aeacfm - 7-11-2010 at 02:36

no no, he told me that salt water would make uniform corrsoion or galvanic corrosion but fresh water would make pitting corrosion

i was amazed , as sea water contain Cl- ion which is strong aggressive so pitting would be favoured in sea water

i am now confused

[Edited on 7-11-2010 by aeacfm]

aonomus - 7-11-2010 at 12:32

Deionized water is amazingly corrosive because metals exist in equilibria with trace amounts in the water itself. Tap water has enough trace metals to prevent further dissolution, however a metal pipe with large amounts of flowing DI/RO water can and will corrode. I know a old prof of mine who always made his espressos with DI water claiming it tasted better, but he kept corroding parts on his espresso machine and not understanding why...

Salt water is corrosive because of galvanic action between dissimilar metals and acts across the entirety of the object due to the conductivity. The steel hull of a ship acts as one electrode and another part (say a propeller, etc) act as an electrochemical cell, and it evenly corrodes across the hull.

Fresh water is corrosive because it can start the oxidization of iron/steel, and the pitting occurs because a small electrochemical cell forms between the rust-metal interface. That is why rust is 'infectious' on pieces of iron/steel.

aeacfm - 7-11-2010 at 13:38

Quote: Originally posted by aonomus  
Deionized water is amazingly corrosive because metals exist in equilibria with trace amounts in the water itself. Tap water has enough trace metals to prevent further dissolution, however a metal pipe with large amounts of flowing DI/RO water can and will corrode. I know a old prof of mine who always made his espressos with DI water claiming it tasted better, but he kept corroding parts on his espresso machine and not understanding why...

Salt water is corrosive because of galvanic action between dissimilar metals and acts across the entirety of the object due to the conductivity. The steel hull of a ship acts as one electrode and another part (say a propeller, etc) act as an electrochemical cell, and it evenly corrodes across the hull.

Fresh water is corrosive because it can start the oxidization of iron/steel, and the pitting occurs because a small electrochemical cell forms between the rust-metal interface. That is why rust is 'infectious' on pieces of iron/steel.


so u want to say that pittig will occure in fresh water more than in sea water !!!so cool