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Technical Discussions
Kevin Tsai
Kevin Tsai
04:42 Feb-21-2009
Water-based Couplant causing Rusting on Ductile Iron castings

Our campany have been dealing with a problem that have recently arose in our foundry.

Because of our poor ground level management, castings that have undergone UT did not have their couplant scraped off afterwards. Anti-rusting agents were supposed to be added
to the couplant, but some workers have neglected to do so. Secondly,because of limited shot-blasting capacity, some castings sit there for 2-3 days before shot-blasting.

One of our customers found an un-related defect and decided to shot-blast the paint off
one of our castings and found rust underneath. This occured several months after the casting
has been painted.

Another incidence reflected by our customer has been paint falling off due to rust undeath.

We have decided to change our production protocol to make sure the couplants and scraped
off after UT and that anti-rusting agents are added to the couplants before use. The result of
this change will not be noticeable until a few months have passed.

We have also considered changing our couplant to one that have anti-rusting properties.

Our german consultants (with 40 years of experience at Siempelkamp) said they have never heard of such problems and suggested that we scrap off the couplant after UT and also switch
our couplant.

I am wondering if any of you have encountered similar problems and can perhaps offer your
opinion on this matter.

thank you for your consideration.

Kevin Tsai

Technical Department
Ningbo Yeong-Shang Casting Iron Co.,Ltd
No.95 Huang Hai Rd Beilun District,
Ningbo city, China P.R 315803
Tel: +86 574 8622 8866 Ext:8520
Mobile/Handy: +86 137 7722 8948
Email: kevin@nbys.com.cn

    
 
 
Michel Couture
NDT Inspector,
consultant, Canada, Joined Sep 2006, 827

Michel Couture

NDT Inspector,
consultant,
Canada,
Joined Sep 2006
827
05:32 Feb-21-2009
Re: Water-based Couplant causing Rusting on Ductile Iron castings
In Reply to Kevin Tsai at 04:42 Feb-21-2009 (Opening).

Kevin,

I can't say that I have encountered this situation before. Than again, the weather in China and Canada may be somewhat different. Since I've been doing UT, I've used numerous kind of couplant. Hamekleer in the nuclear industry and presently we are using Sonoglide on the forging we are inspecting. The material for these forging range from 4030 steel to 403 Stainless and even Inconel. We clean the couplant off with water and are not experiencing the problems you are citing.

Since you are mentioning that it may take many days from the time of the UT inspection to painting, may I suggest using a light oil as a couplant? This would illiminate the oxidation until the casting is painted. It would be up to the paint department to ensure of proper cleaning prior to painting. As for the proper viscosity, you should use an oil of SAE 10 for a surface finish between 5 - 100 ìin., SAE 20 for 50 - 200 ìin., SAE 30 for 100 - 400 ìin. and SAE 40 for 250 - 700 ìin.

    
 
 
Kevin Tsai
Kevin Tsai
09:10 Feb-21-2009
Re: Water-based Couplant causing Rusting on Ductile Iron castings
In Reply to Kevin Tsai at 04:42 Feb-21-2009 (Opening).




I apologize for not proof-reading my first message before posting it.
I cringed when I read it the second time.

I wish to provide additional detail about our situation so everyone
can have a stronger basis on which to make judgement on.

We are currently using Krautkramer USM 35X from GE Inspection
Technology as our primary UT equipment.

Our couplant is from a local manufacturer who claims it meets
the associated Chinese national standard GB1904-89 (which has
been replaced by GB1904-2005) and is composed primarily of
CMC (Carboxymethyl Cellulose).

Currently our short term solution is to create a more detailed
manufacturing protocol that includes scraping off the couplant
using either hard-cardboard or used newspaper, and adding
anti-rusting agents to the couplants before UT. The responsibility
of enforcing the new protocol falls largely on the shoulders of
foundry management and the foremen.

I hope the root of this problem is poor management and poor
foundry practice instead of something more technical.

However I do think we should start using a higher quality
couplant with anti-corrosion and anti-rust properties, in order
to speed up the production cycle. The reason we have not
switched to imported couplant has been the cost. The couplants
recommended to us by our European customers are simply
too expensive, considering we are performing 100% UT on
24 tons and 36 tons wind turbine hubs/frames, 100% UT on gas
turbine components.

Does anyone know of a supplier that have a subsidiary in China
that can offer competitive prices on quality UT couplants? Or a
Chinese supplier perhaps?

To Mr. Couture,

Thank you for your speedy reply and we will consider using
water to clean the surface of the casting after UT. We have to
consider the disposal of waste water however, as per ISO
requirements. I will also let my superiors know of the
possibilities o using oil-based couplants.

I have uploaded a picture of an example of paint falling off
due to rust.

regards,

Kevin Tsai

Technical Department
Ningbo Yeong-Shang Casting Iron Co.,Ltd
No.95 Huang Hai Rd Beilun District,
Ningbo city, China P.R 315803
Tel: +86 574 8622 8866 Ext:8520
Mobile/Handy: +86 137 7722 8948
Email: kevin@nbys.com.cn
    
 
 
Joe Buckley
Consultant, ASNT L-III, Honorary Secretary of BINDT
Level X NDT, BINDT, United Kingdom, Joined Oct 1999, 519

Joe Buckley

Consultant, ASNT L-III, Honorary Secretary of BINDT
Level X NDT, BINDT,
United Kingdom,
Joined Oct 1999
519
11:21 Feb-21-2009
Re: Water-based Couplant causing Rusting on Ductile Iron castings
In Reply to Kevin Tsai at 09:10 Feb-21-2009 .

Kevin
I hate to bring bad news, but i think you will find the only solution is effective cleaning. The constituents of water-based couplants are hydroscopic - if left on the surface they will retain and absorb water. (especially if humidity is high) Anti-rust agents will help to some extent but are not a complete solution. Most corrosion inhibitors are not hand-friendly so even good quality couplants dont always contain them. In any case couplant left on the surface will likely prevent paint from adhering properly, allowing moisture ingress and rusting

Oil based couplants should eliminate the rusting problem, but are likely to require even more thorough removal before painting.

Many training schools insist on oil based couplants for this reason - They dont like their test blocks and samples rusting away after repeated use by careless students

    
 
 
bob sudharmin
Engineering, Reliability and Integrity Eng
Shell Malaysia Trading, Malaysia, Joined Jan 2008, 54

bob sudharmin

Engineering, Reliability and Integrity Eng
Shell Malaysia Trading,
Malaysia,
Joined Jan 2008
54
13:53 Feb-21-2009
Re: Water-based Couplant causing Rusting on Ductile Iron castings
In Reply to Michel Couture at 05:32 Feb-21-2009 .

I presume SAE 10 to 40 is lubricating oil. If I remember it correctly, UT operators are reluctant to use oil as a couplant, reason being it penetrates and ingress into the probes causing premature defects in a short span of time. The Panametrics model normally comes with glycerine as a recommended couplant.

    
 
 
Michel Couture
NDT Inspector,
consultant, Canada, Joined Sep 2006, 827

Michel Couture

NDT Inspector,
consultant,
Canada,
Joined Sep 2006
827
22:24 Feb-21-2009
Re: Water-based Couplant causing Rusting on Ductile Iron castings
In Reply to bob sudharmin at 13:53 Feb-21-2009 .

Yes Bob,

I my reply to kevin, I was suggesting the use of lubricating oil, but don't forget that SAE 10 - to 40 refer to the viscosity of a liquid. Sonoglide for exemple come in those grade as well.

As joe mentionned in his reply, my suggestion for lubricating oil relate to the corrosion problem. I've never been in Asia, but hearing from my frined who have been there or were born there, it is very humid. I taught this could be a solution to Kevin's problem. And as Joe added and Kevin as well proper post cleaning would help lots.

Like we say back here: "Cleanliness is next to Godliness"

Cheerio's

    
 
 
Ryan Burns
Ryan Burns
10:53 Feb-23-2009
Re: Water-based Couplant causing Rusting on Ductile Iron castings
In Reply to Michel Couture at 22:24 Feb-21-2009 .

Michel,

As as technical comment, I believe SAE grades do not refer to the viscosity of "a" liquid, but only apply to the viscosity of lubricating oils as per the Society of Automotive Engineers. There is no correlation between a SAE 40 weight (250-500 Centipoise) and Sonotech's SonoGlide Gr 40 (80,000 cps).

Kevin,

I would contact Sonotech to locate a local rep or to make an international order. They have a few different couplants and water treatments that might suit your application. As far as cost is concerned, what is expensive? Returned parts and components resulting in re-runs, re-painting, re-shipping, potential cancelation of future orders, or a few thousand dollars or yuan on quality couplants and post cleaning processes. I noticed that you are located very close to the sea. This type of marine environment may also be causing you some grief depending on your storage practices.

    
 
 
Kevin Tsai
Kevin Tsai
15:00 Feb-23-2009
Re: Water-based Couplant causing Rusting on Ductile Iron castings
In Reply to Ryan Burns at 10:53 Feb-23-2009 .

To Mr. Burns, Mr. Buckley, Mr. Sudharmin, and Mr. Couture:

Thank you for your advice. To learn of a definitive approach to our problem that's agreed
upon by a number of seasoned NDT professionals with different backgrounds is quite reassuring.

-a thorough inspection of casting surfaces prior to painting is a solution that can be implemented quite readily.

-the extent of post-cleaning we can achieve at the moment is limited. Without an area
with which the castings can be washed with high-pressured water and a waste management
system, we can only wiped the excess couplant off with towels/paper/cardboard, and rinse
the surface with buckets of water/wet towels.

The foundrymen are thinking that because the surface of ductile iron castings are very porous. Couplants left too long on the surface will seep into the casting. Considering Mr.Buckley's assessment that couplants are usually hydroscopic. It's possible that they will retain moisture from the atmosphere. Shot-blasting might even trap some of the excess
couplant on the surface, by altering the surface's micro-structure. By this train of thought,
we can speculate that after the primer and the final coat of paint has been applied, reactions
are occuring under the primer. Rust is allowed to build and the primer/paint's adhesion becomes compromised.

Another possible explanation is insufficient cleaning of the surface before the primer application. Or, the primer application is inadequate. Only by following the production flow step by step can we find the problem.

Removing the couplants from the surface before they have a chance to retain water sounds like an integral part of our solution, regardless of which couplants we use.

Mr. Diederichs have suggested that I should post images with a higher resolution so that
we can get a better picture (excuse the pun) of the situation, and I should have some soon.

regards,

Kevin

    
 
 
Peter Bruckner
NDT Inspector, NDT Manager
United Kingdom, Joined Jul 2008, 21

Peter Bruckner

NDT Inspector, NDT Manager
United Kingdom,
Joined Jul 2008
21
00:26 Feb-24-2009
Re: Water-based Couplant causing Rusting on Ductile Iron castings
In Reply to Kevin Tsai at 15:00 Feb-23-2009 .

Hi Kevin,
I have followed this thread and wonder, that considering all the problems listed, whether you would not be better off carrying out the UT after the primer stage. Considering the size of your castings any attenuation due to to the primer would be minimal. With careful planning shot blasting and priming could take place while the castings are still above ambient temperature to keep them dry in a high humidity environment.

Regards
Peter

    
 
 
Michel Couture
NDT Inspector,
consultant, Canada, Joined Sep 2006, 827

Michel Couture

NDT Inspector,
consultant,
Canada,
Joined Sep 2006
827
06:31 Feb-24-2009
Re: Water-based Couplant causing Rusting on Ductile Iron castings
In Reply to Ryan Burns at 10:53 Feb-23-2009 .

Hi Kevin,

I've done some digging as well and I got to say mea culpa!!! In this case, I assumed since a third party requested SAE 10 and that a client of mine procured Sonoglide Gr. 10 the viscosity was similar. Again, I'm not involved with the purchases of consumable and believed that those who are involved are knowledgeable in what they are doing. It does give me something to think about.

Thanks for setting me straight.

    
 
 
Kevin Tsai
Kevin Tsai
06:30 Feb-26-2009
Re: Water-based Couplant causing Rusting on Ductile Iron castings
In Reply to Peter Bruckner at 00:26 Feb-24-2009 .

zoom image
To Mr. Bruckner,

Thank you for your suggestion. It will be logistically complicated in order to
try to apply the primer first before UT.

I will give a brief outline of casting handling procedures from leaving the flask
to painting.

1. Casting must be cooled after it leaves the flask. Use tools to remove core sand
and a pneumatic shake-out to remove sand on the surface.

2. Casting goes through 1st Shot-blasting. If surface is not acceptable by our
standards, a 2nd shot-blasting will be done.

3. Rough Fettling, removes risers, gating/runner systems, burned on sand.

4. Fine Fettling, certain areas on the casting will need finer fettling. For example,
areas with a higher requirement in terms of VT. Gas holes, sand inclusions should
all be fettled away.

5. MT if required. In the areas indicated by the customer.

6. UT if required. In the areas indicated by the customer.

7. Any surface or close to surface defects that can be removed by fettling will
be removed.

8. Castings will sit for 2-3 days before they can be painted (this is due to the limited
capacity of our painting facilities). If the surface does not meet the requirements of the customer's painting specifications, or rust have accumulated, shot-blasting will be done until the surface is suitable for primer application.

I have included a few pictures showing rust under the primer and paint. These pictures
were taken when our customer needed to do additional machining and sand-blasted
the casting. This casting is a wind turbine hub weighing around 12 tonnes.

regards,

Kevin
    
 
 
Nigel Armstrong
Engineering, - Specialist services
United Kingdom, Joined Oct 2000, 1096

Nigel Armstrong

Engineering, - Specialist services
United Kingdom,
Joined Oct 2000
1096
04:26 Feb-28-2009
Re: Water-based Couplant causing Rusting on Ductile Iron castings
In Reply to Kevin Tsai at 06:30 Feb-26-2009 .

Kevin

Just about every UT procedure I have read requires post-test removal of couplant. Simply wipe with paper or cloth wipes to remove excess and properly dispose of them, then a final clean with a cloth dampened with a low sulphur and halogen penetrant remover. If its done with as much care as should be applied to the original inspection then that should be the end of your problem. Its all part of the inspection process, like properly disposing of waste paper, used aerosols, etc.

    
 
 
Tony McMurtry
Tony McMurtry
20:15 Mar-01-2009
Re: Water-based Couplant causing Rusting on Ductile Iron castings
In Reply to Nigel Armstrong at 04:26 Feb-28-2009 .

When inspecting components where water cannot be used as a couplant, we have often used cooking oil (soybean or corn). When the residue must be removed, as before painting, a
fast once over with a torch burns off the remains.

    
 
 
S.V.Swamy
Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex , India, Joined Feb 2001, 786

S.V.Swamy

Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex ,
India,
Joined Feb 2001
786
10:31 Mar-02-2009
Re: Water-based Couplant causing Rusting on Ductile Iron castings
In Reply to Tony McMurtry at 20:15 Mar-01-2009 .

Burning off oil? Not advisable since some carbon residue may get trapped in the surface of the metal, or some of the surface may get oxidized. Thorough cleaning (with solvents as already suggested by other experts here) is the better solution.

S.V.Swamy
Quality Management Expert

    
 
 
Staf Henderieckx
Staf Henderieckx
00:56 Jul-24-2009
Re: Water-based Couplant causing Rusting on Ductile Iron castings
In Reply to S.V.Swamy at 10:31 Mar-02-2009 .

I saw the pictures and I get the impression that the casting may be suffer from dross inclusions (upper part outside surface, inner part lower inside surface) and this dross can absorb water or liquids...

Solution: avoid dross in ductile iron

    
 
 

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