Corrosion rate of steel in saltwater 4

[DouglasD]

The ballast tanks onboard a ship experienced much more corrosion than expected when opened for inspection. Consequently, insufficient funding was in place to accomplish the repairs.

I have been asked to come up with a formula to predict the corrosion rate of steel that is exposed to saltwater. This is not my area of expertise. I am hoping that someone who is more familiar with corrosion engineering/control can provide me with a reference where I can go and find answers.

Thank you.

DouglasD

 

[MEKANIKEREN]

Hi DouglasD,

Corrosion in seawater is at approx 0,1 to 0,2 mm/year with a good access to oxygen as if the ballast water is changed out frequently. (This is also dependent on temperature).

If there is a lot of dirt at the bottom and no working cathodic protection system (sacrificial anodes), it may cause microbiologically induced corrosion (MIC) then the rate can be as high as 2 mm/year!

Severe corrosion in ballast tanks are often the main reason for scrapping ships, thus this problem should be addressed with care.

Instead of calculating corrosion rates, I would recommend you to paint repair and install sacrificial anodes inside the tank.

 

[DouglasD]

Hello MEKANIKEREN,

Thank you for your post. The information is well received. Let me give you a bit more information concerning the ballast tanks.

These ballast tanks are on a US Navy amphibious ship. The tank is *sometimes* filled completely with saltwater, in which case zincs provide protection against galvanic corrosion. Most of the time, however, the tank is only partially filled with saltwater. In this instance, the zincs do absolutely nothing to protect the upper portion of the tank not submerged in saltwater.

The upper tank is subject to being wetted by saltwater and when not actually wet it is in an environment of "salty air". The upper tank is the portion of the tank with which we are concerned. That is where I have been asked to "predict" a corrosion rate.

Does this additional information change rates with which you responded?

PS. I have been advised to look at "Handbook of Corrosion Data" published by ASM International and that pages 679 to 706 cover seawater corrosion. I don't have a copy of this book but will need to check our local libraries. Do you possibly have access to this book?

 

[MEKANIKEREN]

DouglasD,

It sounds like you have a condition equal to a "splash zone" for an offshore structure where the surface is constantly wetted without the anodes working. In such cases the corrosion rate may increase to typical 0,25 to 0,4 mm/year /Ref. Fontana and N.D.Greene: Corrosion engineering, McGraw Hill, 1967/

For splash zones, you need to rely totally on a high quality coating system.

(I have the 1989 edition of the ASM International book you mention, it doesn,t give any specific answers.)

I would recommend you to talk to a coating supplier for specifying a good coating system:

http://www.jotun.com

http://www.akzonobel.com

Regards,
Mekanikeren

 

[DouglasD]

Hello MEKANIKEREN,

Thank you again for your additional comments. I concur that the conditions in the tank are probably very similar to an offshore oil platform or similar structure. The interior of the tank is indeed coated; however, as the coating breaks down over time, the bare steel is subjected directly to saltwater and the corrosion begins. When the design thickness is reduced by 25% structural repair is required.

I do have one question on how to interpret the data you have provided. I am unfamiliar with your usage of the comma when suggesting corrosion rates. Am I to take the meaning of the comma to be a decimal point? Specifically, is "...0,25 to 0,4 mm/year" the same as 0.25 to 0.4 mm/year?

Very respectfully,

DouglasD

 

[lauri]

Wouldn't it be good shipping practice to just specify frequent surveys of ballast tanks? I don't mean special class surveys but just visual inspections by the crew.
Remember Erika and Prestige!
Best regards,
Lauri

 

[MEKANIKEREN]

DouglasD,
YES 0,25 to 0,4 mm/year means 0.25 to 0.4 mm/year (These are ball park fiures)

Note that I agree with lauri to keep an eye on the development as corrosion rates may be much higher, all dependent on temperature and the amount of seawater changeout.

Also note at some point of coating breakdown the amount of work you would have to put in on grit blasting cleaning and re-painting the whole internal will be much higher than if you re-cote when there is only a few paint flaws. If you are calculating economy, you probably are aware of this already.

Regards,
Mekanikeren

 

[DouglasD]

Good morning lauri and MEKANIKEREN,

Thank you for your input. There is indeed an inspection program for these tanks; however, some bad decisions were made. Suffice it to say that if the inspectors recommendations had been acted upon, I wouldn't be asking these questions...

Have a great day!

DouglasD

PS. Who are Erika and Prestige?

 

[lpm46]

Hi DouglasD

The rate of corrosion is goven by the aviability of oxygin British Steel has a handbook in respect to sheet pile wall and give guidens in respect to the corrosion rate. The hifghest rate is for the atmospheric zone which hasa raTE OF .075mm/year. This rates are base on cold climate condition and if the water temperature is higher the rate would also be higher . You are able to get British Steel Piling handbook on the net.

Regards
lpm46

 

[cod]

Ipm-46;

Can you advise the address (URL) where the British Steel Piling handbook is located on the net? I would like to see a copy of it.

Thanks

C. Daniell

 

[subhuman]

Hi,
there is a regular article at

http://www.diveweb.com/commdive/features/mayjune2001.01.htm

on corrosion from Dr. Harvey Hack. I think they link an email address for him, he would probably have some good information for you.
Regards,
Tim

 

[boo1]

Consider using MIL-P-23236 specification for epoxy polyamide coatings for your ballast tanks. "Proline" or "Tankguard" are products we have used.

 

[grampi1]

If air movement and volume is relatively low, vapor phase inhibitors might slow down the problem. We have bouyancy cans offshore and use this in the still-air vapor space.

See

http://www.cortecvci.com/index2.php

 

[chiefeng]

I am a technical manager for a small shipping company and have just experienced a similar problem. We have encountered Sulphate Reducing Bacteria and the corrossion rate for this is 10mm - 25mm per year! It is distinguished by sharp edged pits with an oxide coloured ring around them.
All the replies have valid input but corrossion may also depend on the stresses put on the vessel, the working climate, etc. The only real way to monitor is for regular inspectios, say 6 monthly or even less if you have some doubt about the conditions.

 

[robertprince]

This may be stating the obvious but if you have corrosion and know something about the operating history of the vessel then you should be able to assess the corrosion rate by taking measurements in the tanks. Do this at several locations and this will give you an idea about the 'general' corrosion rate and any localised effects caused by stress cycling, galvanic effects, MIC whatever. The corrosion engineering handbook will give you some idea of how to model and predict rates and your measurements will improve confidence. It's also worth noting that the corrosivity is regional since O2 and bacteria food levels in seawater vary around the world.

As I recall a lot of the data on seawater corrosion origninates from the US and UK navy so you should be talking with in-house experts.

As for regular inspections it should always be recognised that this may entail risks to personnel so try to limit the number of tank entries using a risk based approach.

I should point out that I'm a civil/oil ind engineer not a corrosion engineer

 

[rrao3047]

Hello DouglasD,

Corrosion rates inside ballast tanks especially in the splash zone is typically 0.6 to 0.75mm/year in case of uncoated tanks.

Present rules for classification do not permit uncoated tanks or those coated with soft coating.
You have to opt for a hard coating, typically spray applied, in order to protect surfaces.

The wear rate on coated tanks is nil. I have noticed this during periodic surveys that I have carried out on ships over a 3 year period.

Break in coating (local damage) is to be treated immediately and the best way to do this is to inspect tanks after a rough voyage (you could expect sloshing damage).

Surface preparation standards for local repair could be
a. clean out grease & dust from surface
b. prepare surface to ST 2 (Swedish Standard)
c. minimum 250 micron DFT of hard coating

regards & good luck
rrao3047

 

[gramya]

I had worked in a process plant which used sea water extensively as cooling water and then the process plant was salt based plant producing Soda Ash ( Sodium Bicarbonate).
We had measured the corrosion rates on Cast Iron and Steels. The sea water had around +28000 ppm chlorides. The atmospheric temperature in the area goes as high as 40C in summer. The rate as recorded by thickness surveys of equipments and piping leads to a figure of 0.7- 1.1 mm per year.

Narendra K. Roy
Gramya Research Analysis Institute,
PO box 4016, Vadodara 390015, India
Website:

http://www.gramya.com

;

http://www.charismaglobal.com

 

[kenvlach]

Found this US Navy site: National Surface Treatment Center.

http://www.nstcenter.com/

Click on ‘Navy Community,’ then ‘Interior Approved Coatings,’ then ‘Ballast Tanks’ to reach
“Seawater Ballast Tank And Floodable Void Coating Systems.”

This page has or links to all necessary cleaning and application procedures and materials. I suggest not deviating from official U.S. Navy Guidelines and Instructions. Photos after 3 years exposure demonstrate that the given procedure is superior to the previous coating procedure.

One link, NAVSEA STANDARD ITEM 009-32 "Cleaning and Painting Requirements" (30 AUG 2001)

http://www.supship.navy.mil/ssrac4/nsi/03nsi/00932.pdf

seems to require abrasive blast cleaning.

 

[moshref]

we have a small dive vehicle that have a long and thin ballast tank.
we paint the inner wall for corrosion protection. but when we wled the tank ,the paints in welded area burned and we can't repair it and can't monitor the corrosion rate
do you have a sujestion for me?
tahnk you