Zinc rot - Nothing to be proud of.....

The "original" Mills .75 diesels had the same problem.
That is why they were painted black to try to stop the corrosion (to no avail).

Back to zinc rot, Bob, do you think that's what's going on here? Just
happened upon this Cox Chopper photo on the bay.
Bob

Hi Bob,  I bought one of those for my son years ago.

But model railroad manufacturers and buyers were looking for weight, while Cox was looking for lightness.  Zinc is heavier than aluminum and I think that's just aluminum alloyed with something else and is just common corrosion.  But, I'm guessing here, perhaps someone more knowledgeable will chime in.

Severe zinc rot will crumble like a cracker when handled.

In the Galvanic Series of metals, Zinc and aluminum are considered sacrificial metals. They will sacrifice themselves when electrically connected to other metals higher on the scale like steel and iron, to protect them from corroding when placed in water (table developed from sea water). This is why you may see aluminum or zinc bars being attached to bottoms of hulls of ships.

Your water heater has a zinc or magnesium rod inserted that can be easily replaced, through a screw plug fitting usually from the top although can be in other locations. It protects the inside of the tank, usually for up to around 6 years (why water tank warranties are that long). Although the interior of the steel tank has a corrosion resistant coating on it, it is not perfect. The rod protects the surface imperfections such as pin holes by sacrificing itself over time.

http://www.eaa1000.av.org/technicl/corrosion/galvanic.htm

It was why my K&B .35 Stallion was heavily pitted on the outside, whereas the internal steel parts had only mild surface rust.



This is why usually the aluminum or base metal zinc alloys that show heavy corrosion versus the steel parts, which may only show surface rust that usually cleans up pretty good.

Very true George, but I have the Plummer cut off that sacrificial rod in my water heater because if left in place my well water reacts with it and smells like sulfur, partly because of that I have had to replace my water heater twice in the last 11 years.

rsv1cox wrote:Very true George, but I have the Plummer cut off that sacrificial rod in my water heater because if left in place my well water reacts with it and smells like sulfur, partly because of that I have had to replace my water heater twice in the last 11 years.  

My Rheem (gas) hot water heater has a mfg. date of 1994.
The first time removing the magnesium rod was a bear. It was really tight and I thought I wouldn't be able to remove it.
Like anything else -- heat did the job.
I've been replacing that sacrificial rod every 5 years and that Rheem tank is still being use to this date. (27 years later).
The only other thing I do is to drain the tank once a year to get the sediments out from the bottom of the tank as well.
That is where the magnesium ( from the sacrificial rod) will be when it gets eaten up by the water.

A little info on zinc rot, sometimes zinc pest, it is not caused by any external factors, but by an internal one, namely, lead impurities in the alloy, The lead eventually crystallizes, expands, and causes the casting to break apart. There is no stopping it, and even fairly recent zinc castings are not immune. The only good news is, if you have an old casting that isn't affected, it won't start now. And yes, I do have a some old locomotives suffering from this ailment. Plastic aircraft modelers have a similar problem, the lead weights they use for nose weight, begin to crystallize, and split their aircraft's noses apart.

Outboard motors also have sacrificial anodes installed in the lower units. To stave off galvanic corrosion.

OhBee wrote:Outboard motors also have sacrificial anodes installed in the lower units. To stave off  galvanic corrosion.

I've seen them on anchors and anchor chains.
Bob

This is interesting to me I don't think I have every hear of this sacrificial rod inhibitors But I do know rot

About 15 years ago, I got certified as a NACE (National Association of Corrosion Engineers) Cathodic Technician II for work, so, although I am by no means an expert have a little understanding through training. Since retiring have let that lapse. I don't know if I'd want to be lowered into a 150,000 gallon elevated tank using a rubber lift raft to take current readings to determine if the cathodic protection system has current levels properly set.

What you have with the zinc / magnesium / aluminum anodes is basically components of a battery when including the tank / vessel and electrolyte which is the water or other medium of fluid tank / vessel is carrying. One tests various locations in the water to determine if there is sufficient current produced to protect the defects in the tank / vessel internal coating.

Usually on tanks larger than say, 5,000 - 10,000 gallons, they install a transformer rectifier set and a wired anode system. The transformer steps down the voltage to the required level, the bridge rectifier converts A.C. to D.C. There are taps off the transformer to step down the voltage to the required range, then a rheostat adjusts the current level. Readings are taken at various locations in the water to determine the current levels.

Just sosam117 indicated, properly maintaining the cathodic protection system can provide a very long life for the tank. Theoretically, those major municipal water storage tanks can have a life of a 100+ years or so I understand if the cathodic and internal coating (and exterior weather coating) are maintained. There is also exterior cathodic protection for above ground and in ground tanks, to protect their surfaces exposed to soil and water.

Anyway, it is a good field to get involved with if one wants a stable income.