Quick depletion of anodes 5

[gcomyn]

We have an Air Conditioning Chiller condenser that is cooled by seawater. We operate in the middle east so seawater inlet temperatures are routinely 35C and so the flow is pretty high (200 m3/hr). The problem we are having is that the zinc anodes inside are depleting really quickly and we've had to increase the number and size of the anodes.

I was wondering if the cause of the rapid depletion was normal for the temperature and water flow or if there might be something else going on that we should be addressing.

 

[EdStainless]

Can you monitor the electrical current? Warm water and high flows will devour anodes. You may need to look at some of the alloys that will be more durable.

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Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[gcomyn]

I'm not sure how I would measure the current. The zinc anode is attached to the inside of the condenser so I don't think there is a potential difference to measure, but if I'm wrong I'd appreciate any education you could offer.

What are the factors involving anode depletion?
Temperature, galvanic potential...?

 

[strider6]

gcomyn

It would be necessary to look at the design of the cathodic protection. To define the number and size of the anodes is necessary to account for a number of different factors and maybe during the design stage they had made wrong assumption on total surface to be protected, design life...
The material of your vessel is Carbon Steel? Is the surface exposed to seawater painted?

S.

http://www.corrosionist.com

 

[unclesyd]

The anodes in our refrigeration system are stud mounted and isolated from the equipment. The stud provides the electrical connection and resistance washers are used to control the anode dissolution rate. If the anode was bright and shinny when inspected one or two resistance washers were added to slow the rate. If the heads or tubesheets showed any sign of corrosion a washer was removed. I do recall some measurements being taken but since 1953 it has been up to the inspection group to add or remove a washer.

The water varies from fresh to sea water up to approximately 90F in the summer with low flows.

 

[mshimko]

The primary factor is the dissolution rate of sacrificial anodes is DEMAND.

If there is a lot of bare metal to be protected, DEMAND will be high and so will the dissolution rate.

If you need to insert resistance washers to reduce the dissolution rate of the anodes, then something is NOT being cathodically protected; I would never recommend this practice.

You need to look at what metal surfaces are in the "electrical circuit". Something is there causing a high demand that needs to be addressed.

 

[EdStainless]

In some cases the anodes are installed isolated with a lead that can be disconnected externally so that you can check voltage and current.
I have seen resistance washers used when the space/geometry required that anodes be placed too close to some areas in effect causing 'over protection'.
Unless you can take measurements you don't know if you have any protection to give up.

Just remember as Mshimko says, the anodes don't do anything, the process is related strictly to demand and electrolyte condctivity.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[stevenal]

gcomy,

The potential is usually measured from the protected metal to a half cell reference electrode placed in the water.

http://www.westmarine.com/webapp/wc...e&storeNum=null&subdeptNum=null&classNum=null

 

[gcomyn]

That's very interesting. I did a bit some research on these silver half-cells to try to understand the principle.

On building a corrosion meter:

http://www.oceannavigator.com/ME2/d...5F&tier=4&id=73EE01040E0041D58AB93E47DD02F2C4

Galvanic series in seawater:

http://www.ocean.udel.edu/seagrant/publications/corrosion.html

I'm trying to understand the voltage measurements that the first article talks about - and the current that EdStainless mentions.

I'm not clear what the voltage is indicating. If the case of the heat exchanger there is a steel casing and 90-10 copper-nickel tubes. If you put the silver anode into the seawater and connect the other lead to steel it should read 540 mV. If you connect it to the cu-ni tubes it should read 130 mV. If you connect it to the zinc anode it should read 880 mV. By substracting the numbers you can see the potential between steel and cu-ni or steel and zinc, etc.

Is this right so far? It doesn't seem to match up with the numbers in the first article.

I'm not sure how this helps. I guess if the voltage between the tubes and the steel is larger than the tubes and the zinc then the zincs are not preferentially corroding. How would the voltage tell me if I need more zinc?

 

[mshimko]

When analyzing/designing a sacrificial anode system, there are two factors to be considered (unless, of course, I'm merely taking minimum requirements off a spec tabble):
(a) the max expected DEMAND, which dictates the effective surface area of the sacrificial anodes installed, and
(b) the expected service life of the anodes, which dictates the MASS of the anodes to be installed.

Both (a) and (b) above are in turn determined by the number of anodes (assuming you've already selected the anode type and dimensions); one of the above will be limiting, the other will not.

Regarding taking measurements in the field; you cannot measure the potential difference between electricall connected metals in the same system (such as the tubes and casing mentioned above). IOT measure this potential difference, you first need to electricall isolated the parts, then connect them via an appropriate meter, which is not typically practical in the field.

Bottom line is: you may be trying to dig into the science too deep at this point in time. If your anodes are being depleted too rapidly, you simply need to add more anodes, OR determine and address the cause of the high current demand.

 

[gcomyn]

I did add anodes. And I made them larger.

I guess it's probably due to the temperature and flowrate but now I'm curious how these things work.

 

[strider6]

The flowrate has a huge effect on corrosion of Carbon Steel in seawater, approx the corrosion rate double with water velocity, but in your case ad more anodes without a deep review of the design of the cathodic protection can't solve the problem. It could be also a galvanic coupling that increase the corrosion demand of your system, but without a clear idea of how your system is made is not possible understand the problem. I've asked it before, quite often the cathodic protection is used in with a coating, is you surface painted?

S.

http://www.corrosionist.com

 

[gcomyn]

Sorry for the delay.

The inside surface of the chiller is not painted.

Also, the bolts holding the anodes on are brass.

 

[stevenal]

gcomyn,

Your anode and the steel are bonded together, so you will read the same voltage wherever you place the lead. Per the article you linked, you should add zinc until the potential is 750 to 950 mV in order to protect the steel (the most active metal you need to protect).

Your galvanic series article used a different type of reference electrode, and shows the values for unprotected metals alone.

 

[unclesyd]

All our condenser anodes are attached with a Silicon Bronze stud. The anode is isolated from contact with other metals with a sleeve on the stud and sitting on red rubber gasket. If you used an insulating washer on the stud there would be no potential between the anode and the other materials. By using resistance washers it is possible to control the dissolution of the anode and still protect the other materials.

 

[eminentfoundrypro]

Interesting how the depletion is so great at the flows stated...could it be that the sea H2O is aerated somewhere in the system and we are also dealing with an aggressive environment where the normal effects related to the difference of potential are enhanced by the excess oxidating agent presence...Also, what exactly is the temperature change of the H2) night to day - is it more than 35 degrees F...if so, it will behoove you to repeat & the demand testing for day & night ( using two consecutive installations of course ).

 

[mshimko]

Not sure if the depletion of the anodes descruibed is "rapid", or "expected", without additional information.

1. What is the number and dimensions of the anodes installed?
2. What is the total surface area of bare metal electrically connected to the anodes?

I suspect (but obviously don't know) that you simply have a larger surface area to protect than you realize.

 

[gcomyn]

eminent,

Do you mean temperature change from day to night of the seawater inlet or across the chiller?

I don't think the seawater is aerated. We are getting cavitation on our pumps at times but there shouldn't be too much air in there. We have some trouble with marine growth in the sea inlet but it doesn't get as far as the chiller piping.

Mshimko,
You're right, it could be expected - it is just annoying to change them out so often and wanted to see if there might be another problem I should be looking into.
I'll try to get back to you on surface area, etc.

 

[stevenal]

http://assets.fluke.com/appnotes/electricalpower/B0269b_u.pdf

 

[gcomyn]

The chiller shell (steel) is 500mm I.D. and 4100mm long, giving approximately a surface area of 6.83 square meters.

The chiller tubes (cu-ni) are 16mm diam. and 3400mm long. There are 120. Approximate surface area of 20.5 square meters.

There are 18 zinc anodes fitted to one end. They are 65mm diameter and 45mm long.