Backstay Chainplate Discussions [pg 152 in Manual]

[Scott Galloway]

Hmmm...The problem is that some galvanic series charts, or electrolytic tables (as some sources refer to them) show different relationships between the various metals and alloys, or at least it appears to me that they do.

For instance, one of my printed source books, a publication of the USCG Auxiliary, lists the most cathodic metals in that order as Mercury, Monel, nickel, and bronze (silicon) in that order, whereas stainlesss steel is way down on that list behind, copper, brass (red), bronze (aluminium), gun metal, brass (yellow), bronze (phosphor) and lead. Trailing stainless steel are iron, mild steel, aluminium , cadmium, galvanized iron and steel, zinc, and magnesium.

I don't have a copy of the 64th edition of Chapman, but I took a brief look at a similar table in that edition the other day, and I recall that it showed different relationships than the above, or at least it appeared to me that it did.

Then when I consulting Dan Spurr's “Upgrading the Cruising Sailboat” I was perplexed by the relationships shown in the "Galvanic Series of Metals in Seawater" table. If someone has that book, check out Appendix B and explain the table. It has two columns that appear to run from anodic or least noble to cathodic or most noble, in otherwords from magnesium on the anodic end to graphite (above platinium) on the cathodic end. Silicon bronze is listed once on that table, but (18-8 Stainless Steel, Type 304) and (18-8 3% Mo Stainless Steel Type 316) are listed twice, once as being more cathodic than silicon bronze and once as being more anodic.

At Ebb's suggestion, I consulted the "Corrosion Doctors website at: http://www.corrosion-doctors.org/Def...es_in_Seawater

In their "Galvanic Table", the Corrosion Dotors do indeed list passive 304 and passive 316 stainless steel right next to silicon bronze. Their source appears to be: the galvanic series of metals in sea water from Army Missile Command Report RS-TR-67-11, "Practical Galvanic Series."

So: That is a long list, but the relevant part of that list reads:

#70 Silicone Bronze 655
#71 Stainless steel 304 (passive)
#72 Stainless steel 301 (passive)
#73 Stainless steel 321 (passive)
#74 Stainless steel 201 (passive)
#75 Stainless steel 286 (passive)
#76 Stainless steel 316L (passive)

With the higher numbers like 304 stainless steel (passive) (#71) and Stainless steel 316L (passive) (#76) above being more cathodic than the lower numbers like silicon bronze (#70). So that would be a good thing if you were a piece of passive #316 stainless steel sitting next to a piece of silicon bronze, and not quite so good for the bronze.

But Stainless steel 304 (active) also appears on that list in position #40, some thirty places away from silicon bronze, (and thus much more anodic). Stainless steel 316 (active) appears in position #67, three places more anodic than silicon bronze. So that would be a bad thing if you were a piece of passive #316 stainless steel sitting next to a piece of silicon bronze, but a good thing for the bronze.

On that same web page and from a different source, the Corrosion Doctors provide a table titled "Galvanic Series in Seawater" about which they say,

"A galvanic series has been drawn up for metals and alloys in seawater, which shows their relative nobility. The series is based on corrosion potential measurements in seawater. The relative position of the materials can change in other environments. The further apart the materials are in this series, the higher the risk of galvanic corrosion."

The Corrosion Doctors are using "The Handbook of Corrosion Enginnering" by Pierre Roberge as a source.

This how that table reads:

Begin Quote from that source

"Most cathodic, noble, or resistant to corrosion

Platinum
Gold
Graphite
Titanium
Silver
æ Chlorimet 3
è Hastelloy C
æ 18-8 Mo stainless steel (passive)
ç 18-8 stainless steel (passive)
è Chromium steel >11 % Cr (passive)
æ Inconel (passive)
è Nickel (passive)
æ Silver solder
ç Monel
ç Bronzes
ç Copper
è Brasses
æ Chlorimet 2
è Hastelloy B
æ Inconel (active)
è Nickel (active)
Tin
Lead
Lead-tin solders
æ 18-8 Mo stainless steel (active)
è 18-8 stainless steel (active)
Ni-resist
Chromium steel >11 % Cr (active)
æ Cast iron
è Steel or iron
2024 aluminum
Cadmium
Commercially pure aluminium
Zinc
Magnesium and its alloys

Most anodic or easy to corrode"

End Quote from that source

In this list you also see stainless showing up twice with active stainless steel scoring worse than silicon bronze and passive stainless steel scoring better in the degree of nobility or cathodity.

So does the key to understanding the relationship between silicon bronze and stainless steel in these charts depend on the definition of the words "active" and "passive" related to 304 and 316 stainless steel?

I was feeling much too passive to look this definition up for myself, so I asked my chainplate, but it didn't answer. Does that mean that it is passive?

Anyone know the definition?