Refer & A/C Corrosion

[GordMay]

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, etc).

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)
but
I’m certain I can speak for Richard in asking for your help - we’d welcome any input.

Regards,
Gord May

[Rick]

I have seen Richard's problem elsewhere as well. In at least one case there were actually TWO factors contributing towards the premature demise of the tubing integrity. The first factor Gord has already covered regarding couples and the resulting galvanic potential differences which occur AND are proportional to temperature AND salinity (a seemingly minor detail often overlooked because it is easier to present the galvanic series data for one temperature and assumed salinity of the electrolyte).

The second factor is almost NEVER considered and that is the potential differences in electric circuits having multiple paths of conduction shared by load current flow, often referred to as "ground-loops".

The reason that a proper dc negative bus and bonding bus have one and only one connnection between the two is to prevent a ground loop that some dc load might take using some bonding wire INSTEAD of ONLY the dc negative bus wire. Should any load take a bonding wire path it WILL create a difference of potential between two points of the bonding system and, therefore, one cannot be assured that an anode is protecting all elements of the system. The only question is just how MUCH of a potential difference is created.

Now in Richard's case I have noted that most compressor/heat exchanger installations have built-in ground-loops comprising the compressor electrics (including water pump current), the salt water (connected to the heat exchanger), the bonding system, and the battery negative bus. Because often the compressor electrics have their negative internally connected to the copper of the heat exchanger one cannot separate out a ground-loop from the rest of the vessel. What happens is that every time the compressor is running some of that current passes through the copper, through the copper-nickel tubing (where they touch), through the WARM (translate: lower resistance than cold) salt water, through the bonding system, through the battery negative bus and to the battery negative. MOST of the current passes, of course through a wire from the compressor negative power lead back to the negative bus, but not all.

What is important here is that any ground loop compressor current ADDS a voltage drop to the metal couple between copper and copper-nickel accelerating the degradation of the copper-nickle which, without the copper, would otherwise be fine.

Adding zincs to the heat exchanger does not solve the problem it merely delays the time of ultimate destruction. The REAL solution is to isolate the compressor electric negative from the copper heat exchanger making the two "come together" only at the dc negative bus and bonding system single wire which is often far away from the compressor thereby eliminating the ground loop. Obviously any salty or dusty-dirty accumulation on the compressor which contacts the power leads will exacerbate the problem. If it is not feasable to isolate the compressor power negative then the next best thing is to rewire the power negative lead from the compressor so that it goes DIRECTLY to the dc negative bus AT or near the point where the bonding system connects. If there is not discrete bonding system connection then it ACTUALLY exists where the engine starter negative cable connects to the engine block (the engine's salt water cooling connects to the starter negative in most installations).