Richard Kollman, the Boat Refrigeration
expert, posted the following interesting and vexatious query on the SSCA bulletin board. Perhaps some of our more educated or experienced members can help explain the corrosion
problems Richard describes, and hopefully propose solutions.
Richard poses the following:
Gord, I could not find a working Email to send this to you direct. I have followed some of your posts about corrosion and maybe you can shed some light on a problem I am trying to chase down.
Water cooled refrigeration has been around for many years on large boat refrigerators. In the last few years several companies are cooling small Danfoss 12/24 Volt refrigeration units with seawater.
Some systems use a pump to circulate water through a condenser others use a keel cooler or through hull with a coil inside, in several installation corrosion pin holes develop allowing seawater into the system destroying it.
I talked with one manufacturer about three of their failures on two year old tubular condenser and they won’t admit they have a problem. After cutting these units apart I found that the inner copra nickel tube had a hole eaten through it where it contacted the outer copper tube. Keel cooler systems seem to have the same type of failures when the refrigerant lines are connected to a Danfoss compressor. One keel cooler manufacturer has recognized the problem and is on their third generation cooler. Their newest keel cooler has two zincs on its exterior face. A ground lug from keel cooler is to be connected to the battery. The zincs on a local boat with the new coolers lasted two weeks. The only thing that seems to be common to the failures is the boat’s refrigerators are used full time in warm salt water.
In reply to Richard
Let me start by telling everyone that Richard is one of the most helpful experts (in any field) that I’ve come across. He shares his Boat Refrigeration expertise here, on other boards, and at his EXCELLENT website: http://kollmann-marine.com
and on his Forum: http://kollmann-marine.com/phpBB/index.php
There are three conditions that must exist for galvanic corrosion to occur.
1. There must be two electrochemically dissimilar metals present.
2. There must be an electrically conductive path between the two metals.
3. There must be a conductive path for the metal ions to move from the more anodic metal to the more cathodic metal.
If we can eliminate any one of these three, then galvanic corrosion cannot occur.
Here’s a few practical ‘rules of thumb’ should be considered:
1. Select combinations of metals which will be in electrical
contact (coupled) from groups as close together as possible in the galvanic series (compatible metals).
A 0.15 V (150 mV) difference in the "Galvanic Series (Anodic Index) is generally considered”compatible”, up to 0.25 V generally “acceptable”, and anything over 0.50 V is generally considered “incompatible”.
2. Electrically insulate metals from different groups from each other, as far as practical. If complete insulation
cannot be achieved, paint
or plastic coating at joints will help.
3. If you must use dissimilar materials (far apart in the series), avoid joining them by threaded connections, as the threads will probably deteriorate excessively. Brazed or thermal joints are preferred, using a brazing alloy more noble (cathodic & more corrosion resistant) than at least one of the metals to be joined.
4. Avoid making combinations where the area of the less noble (anodic) metal is relatively small, compared with the area of the more noble metal (cathodic). The more anodic metal should always present a higher mass.
5. Apply coatings with care. Example: Do not paint
the less noble metal without also painting the more noble; otherwise, greatly accelerated attack may be concentrated at imperfections in coatings on the less noble metal. Keep such coatings in good repair.
6. Only finally - consider use of cathodic protection (Zinc or other Anode, or Impressed Current
Cupronickel (Cu-Ni) to Copper (Cu) Couples
Referring to the Galvanic Series ( http://www.uscg.mil/hq/g-m/nvic/7_95/n7-95.htm
), We see that the two Cu-Ni alloys listed (70/30 & 80/20) are fairly close to Copper in the series, and unprotected couples should’nt present undo problems.
Depending upon the exact alloys, Cu-Ni (70/30 Alloy @ about -0.18V, or 80/20 Alloy @ about 0.26V) and Cu (0.31V) have a potential difference of between 0.05V to 0.13 V.
However, Richard has observed “pin-hole” corrosion at internal Cu-Ni to Cu tube contact points.
Richard also notes that one Keel-Cooler manufacturer has added Zinc Anode to the external Surface of the keel cooler. What metal is the external surface? Could this be a third metal in the couple?
Richard also notes typical problems where refrigeration tubing (Cu?) connects to the Compressor
(pump?) fitting (Bz?). Dissimilar threaded fitting are always problematic. What metals are the fittings & tubing? Copper & Bronze should usually
be compatible. (Cu @ 0.31V vs Bz @ between 0.25-0.29V).
Richard notes a significant similarity between all failures, as “operation in warm seawater
If we presume that a significant portion of this time is in Polluted Harbours, then Bio-fouling could present a problem. Many accelerated corrosion problems and premature failures of Cu-Ni have been related to the activity of sulfate reducing bacteria (SRB’s), and the presence of hydrogen sulfide (originating from biochemical reactions). Low-level chlorination may be effective in preventing bio-fouling, although I’ve never encountered the practice aboard boats (I’ve seen it used in Industrial Closed-Loop systems, particularly those with intermittent operation).
Stagnation, such as when the unit is not running (circulating), can also present problems.
Seeing as the (presumably ‘expert & interested’) Manufacturers don’t appear to have solved
these problems (some not even admitting to it) - it may be unrealistuic for us to expect to come up with practical & effective solutuon(s)
I’m certain I can speak for Richard in asking for your help - we’d welcome any input.