Oskip & BC Mike:
is NOT generally recommended for use underwater*.
* As a General Rule
- May be used for fully exposed components, frequently washed with fresh water
- May be used for all hull
fitting applications above the waterline
Duplex S/S: These are stainless steels containing relatively high chromium levels (between 18 and 28%) and moderate amounts of nickel (between 4.5 and 8%). Super duplex grades have enhanced pitting and crevice corrosion
resistance compared with the ordinary austenitic or duplex types. This is due the further additions of chromium (over 25% cr) , molybdenum and nitrogen to these grades.
- Offers higher strength and lighter weight for the same applications as 316 and can be additionally used up to 60 degrees C in wet exhaust
systems and in all submersed applications.
- Offers higher strength and weight savings and can handle all marine
applications with no risk of corrosion in tropical waters (above & below the waterline) and hot, wet exhaust
From “Flow-Induced Corrosion" - By: Dr. Harvey Hack
UnderWater Magazine Article reprint: May/June 2001
”I would also not recommend using stainless steel for continuous immersion in seawater. Most common stainless steels, including type 316, can suffer crevice corrosion in seawater which can lead to large leaks which could sink your boat at the dock. Although the stainless steels can be protected from corrosion by electrical coupling with aluminum, the aluminum will suffer galvanic corrosion in the process and the electrical contact between the stainless steel and the aluminum is difficult to maintain in seawater.” ... Either 316 or 316L are usually poor choices for long term immersion in seawater or more concentrated brackish water unless they are supplied with cathodic protection. Without cathodic protection they will suffer rapid crevice corrosion.”
BASIC GALVANIC SERIES
NOBLE (Least Corroded)
( Passive) **
COPPER / NICKEL ALLOYS
STAINLESS STEEL (Active) **
ACTIVE (Most Corroded)
** The two positions of Stainless Steel illustrates how oxygen depletion affects its resistance to corrosion. The normal passive state is more noble than most other metals. However when the metal is in an oxygen deprived environment
(underwater, sealed off, or in a crack) the less noble / active condition applies.
Forespar makes “Marelon” Thru-Hulls, Seacocks, and Ball Valves. Marelon® is a proprietary formulation of a reinforced, marine grade, engineered polymer (plastic).
Silicon bronzes are alloyed with 1-3% of silicon; about 1% of iron, nickel, manganese or tin can also be present. Although silicon bronze presents some fabrication difficulties, it has excellent chemical resistance and becomes stronger when worked, e.g. by rolling. Silicon bronzes are used for pumps, boilers, marine hardware
. If bronze fittings appear bright and shiny, this is an indication that galvanic corrosion may be occurring. If bronze fittings are pink, this a indicates a dangerous condition known as dezincification - replace immediately!
Sacrificial Anodes etc:
Very rapid zinc loss that results in bright, shiny metal being exposed is a clear indication of electrical activity (stray current
or galvanic), usually the former . Bright zinc in association with heavily corroded bottom paint
means you have a problem that needs to be addressed immediately.
If you are getting white and/or green, or red/brown halos around your zincs or underwater metals, you have a stray current
leak. Copper-based metals like brass and bronze leave green oxides, white for aluminum, and reddish-brown for stainless steels.
Underwater Stainless fittings will usually end up with rusty looking oxides around them when crevice corrosion is involved, but no telltale oxides when stray current is the culprit.