Aluminium anode in Seawater 3

[rachba]

Hi,
For a seawater storage tank i am using a sacrificial Aluminium anodes system to protect the internal submerged surface. Is it normal that when i calculate anode life the result is about 2 years (depending on the dimension of the chosen anodes).
according to the calculation formulas and due to the low resistivity of the seawater the short life of the anodes seems to be normal, but in this case when and why aluminium are recommanded for seawater usage if this type of anode doesn't last for a long periode?

Thank you

 

[SJones]

The output of your calculations is entirely dependent upon the inputs. Nonsensical inputs = nonsensical outputs. The tank should be coated, and you should be working with some realistic coating breakdown factor. Selection of temperature will also impact heavily on the calculation results. If sacrificial anode life is not acceptable, switch to impressed current.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[EdStainless]

And the choice of anodes is important as well. Alloy anodes are often used to control both potential and fouling, a fouled anode does nothing for you.
2 years sounds optimistic to me.....

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[rachba]

Hi,
@ SJones: My calculation of the design life of the anode is based on the following formulas:
L= (one anode weight*utilization factor* effeciency) / (one anode current output * consumption rate of anode)

Anode current output depend only on the dimension of the anode, its weight and the their resistance with depend on the water resistivity.
according to these formlas the required current to protect all the surface ( current density, coating and Temperature) affect the number of required anode but not their design life.
water resistivity low ⇒ one anode resistance is low ⇒ one anode current output is high ⇒ anode life is low
According to this i think that my calculation is not Nonsensical and all Al anode logically does not last a long unless their individual weight have to be high.

@ EdStainless: true, in reality my calculation give me less than two years of life for 34kg Al anode.

I am perplexed about this result but Unfortunately where I'am working, there is no senior CP engineer , so I have no one to validate my calculation.

 

[SJones]

Current output is also dependent upon anode temperature, i.e. the efficiency is affected. Which standard are you using to derive your input values, DNV RP-B401? What coating breakdown factor is being used - the anodes only have to protect the steel exposed by coating breakdown?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[rachba]

@ SJones: I am working with our customer standard which specify a current density equal to 0.5 mA/m² for internal coated tank. This current density seems to be very low (not realistic for me) and may require a frequent maintenance of the coating , i already send a clarification about that to the customer and i am wainting

I think that the temperature affect the resistivity of the environment and when the T° rise the ions motion is bigger which involve a more current output (depolarisation)

What i am trying to explain in my last post is: for a given T° the lifetime of the anode depend on: the dimension of the anode, its weight and the their resistance with depend on the water resistivity.
the current density and the breakdown factor affect the anode required number and not their lifetime

L= (one anode weight*utilization factor* effeciency) / (one anode current output * consumption rate of anode)

If my understanding is right, alumium anode shall be used in seawater either for a short periode of life or with a very high weigh

 

[CoCoE]

@rachba

I could not understand this "the current density and the breakdown factor affect the anode required number and not their lifetime". So do you mean there is no effect of coating break down factor on the life time of an SACP?

It should be current demand based on area to be protected is what determines the quantity of anode required. This current demand is a function of area to be protected, protection current density, and COATING BREAKDOWN FACTOR which you have considered for your entire design life. So, if your coating breakdown factor varies, your anode quantity will also vary. If there is higher coating breakdown factors than considered in the design, obviously it will affect the life of your anode since the anode will consume faster.

So, basically coating breakdown factor will affect the actual life of your anode! this is my understanding.

What do you say on this?

Coco