304L /316L Welded in Salt Water Environment

[dave4458]

I have been involved with manufacture of a subsea umbilical termination unit and the vendor has used 316L S.S and welded it to 304L S.S They should have only used 316L but decided off their own back to use the 304L. My question is will one be anodic in repect to the other? i know that 304L is prone to pitting in saltwater. should sacrificial C.P. be used?

 

[Rich2001]

FROM THE GALVANIC CHART BELOW YOU WILL NOTICE THAT 316 IS SLIGHTLY MORE NOBLE THAN 304. I WOULD ASSUME THAT THE LOW CARBON GRADE WOULD BEHAVE THE SAME.

COMPARE THE AREAS OF THE TWO METALS THE CORROSION RATE OF THE 304 IN THIS COMBINATION WOULD BE

R304_COMBINE = R304*(1 + AREA316/AREA304) WHERE R304 IS THE CORROSION RATE OF 304 IN SEA WATER
AREA316 IS THE EXPOSED ARE OF THE 316 SS
AREA IS THE EXPOSED AREA OF THE 304 SS



IF YOU DECIDE NOT TO USE CP MAKE SURE YOU PASSIVATE THE STAINLESS STEEL.

HOPE THIS HELPS,

RICH


GALVANIC SERIES OF COMMONLY USED METALS WHEN EXPOSED TO SEA WATER

ACTIVE OR Magnesium
LEAST NOBLE Magnesium Alloys
Zinc
Galvanized Steel

Aluminum 1100

Aluminum 6053
Alcad

Cadmium

Aluminum 2024 (4.5 Cu, 1.5 Mg 0.6 Mn)

Mild Steel
Wrought Iron
Cast Iron

13% Chromium Stainless Steel
Type 410 (Active)
18-8 Stainless Steel
Type 304 (Active)
18-12-3 Stainless Steel
Type 316 (Active)

Lead-Tin Solders
Lead
Tin

Maganese Bronze
Naval Brass

Nickel (Active)
76 Ni - 16 Cr - 7 Fe Alloy (Active)

60 Ni - 30 Mo - 6 Fe - 1 Mn

Yellow Brass
Admiralty Brass
Aluminum Brass
Red Brass
Copper
Silicon Bronze

70:30 Cupro Nickel
G-Bronze
M-Bronze
Silver Solder
Nickel (Passive)
76 Ni - 16 Cr - 7 Fe Alloy (Passive)
67 Ni - 33 Cu Alloy (Monel)

13% Chromium Stainless Steel
Type 410 (Passive)
Titanium

18-8 Stainless Steel
Type 304 (Passive)
18-12-3 Stainless Steel
Type 316 (Passive)

Silver

Graphite
PASSIVE OR Gold
MORE NOBLE Platinum

 

[rustbuster1]

There will be no measurable galvanic corrosion between these two alloys. The problem is that the 304L will pit more readily - depends on temperature and available oxygen for reduction to support metal oxidation. Once the pitting starts in the 304L it will be accelerated by the non-pitting 316 adding to the area available for oxygen reduction. Cathodic protection is one way to prevent pitting. Many coatings can also protect the stainless steel - immersion grade amine-cured epoxies or 100% solids epoxy novolacs should provide durable protection.