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

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.