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JBK
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Help - Cause of Corrosion on Mixer

04/28/2013 2:08 PM

During commissioning of waste water treatment plant, we've observed that the corrosion has started on the mixer installed at biological tank.


Please refer to the following.


1. Fluid in biological tank

- Treated Water of Sewage Treatment Plant

- TDS 2,000 ppm - Chlorine 1,339 ppm

- NaoCl 14% dosing

2. Operation Period: 2 month

3. Mixer casing: Cast iron with painting


Q) Please advise the cause of corrosion on mixer.
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Pathfinder Tags: Corrosion mixer
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SolarEagle
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#1

Re: [Help~] Cause of Corrosion on mixer

04/28/2013 2:25 PM

Clearly your motor mounting assembly has not been properly coated for this application....

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lyn
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#2

Re: [Help~] Cause of Corrosion on mixer

04/28/2013 2:56 PM

You're not really serious, are you?

"the cause of corrosion on mixer" is material incompatibility.

Obviously the motor either wasn't specified properly for the application , built properly for the application or inspected properly for the application .

You can't protect against incompetence.

Order a new motor that is suitable for the application.
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Tornado
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#3

Re: Help - Cause of Corrosion on Mixer

04/28/2013 3:42 PM

Most of the assembly appears corrosion-free, but the NDE (non drive end) bell seems to be of unsuitable material. The chlorine could certainly be problematical; I don't know about the NaOCl (is there a misprint there?)

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kramarat
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#4

Re: Help - Cause of Corrosion on Mixer

04/28/2013 4:53 PM

Assuming this is the correct mixer for the application, you may want to look into connecting a sacrificial anode. They prevent the type of corrosion you are seeing. I believe there should be one on that unit.

http://chemwiki.ucdavis.edu/Analytical_Chemistry/Electrochemistry/Case_Studies/Corrosion/Sacrificial_Anode

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Just an Engineer
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#5

Re: Help - Cause of Corrosion on Mixer

04/28/2013 5:38 PM

The casue would seem to be inadequate or inappropriate protective coating as previously suggested.

There may be some "galvanic" contribution due to your guide rail material. Notice the localised corrosion near the rivet in the nameplate for a possible example.

You mentioned NaOCl at 14%. Surely that's way too high for that location in the process!! We use 14% as our dosing source solution.

Your coating should maybe have been the usual 2 part epoxy that is common for stop valves and such.

I would also suggest that similar issues might be evident on multiple other components in your plant if this is typical of what has been used.

A FULL evaluation of all items is recommended.

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PWSlack
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#6
In reply to #5

Re: Help - Cause of Corrosion on Mixer

04/28/2013 6:09 PM

The 14% NaOCl probably has some carrier water with it to reduce its concentration, particularly as the Total Dissolved Solids is given as only 2000ppm. Were the assembly immersed in 14% NaOCl, then the corrosion would probably be much more severe than that illustrated!

Agreed that the cause of corrosion is incompatibility between the process fluids and the wetted materials and their coatings.

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JBK
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#7
In reply to #5

Re: Help - Cause of Corrosion on Mixer

04/28/2013 11:41 PM

We also use 14% NaOCl as our dosing source solution. Residual Chlorine = 0.5 ppm.

Thanks for reply.
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bwilko
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#8
In reply to #5

Re: Help - Cause of Corrosion on Mixer

04/29/2013 2:38 AM

My guess is that the 14% number is actually the strength of the Liquid Chlorine before it hits the mix and not the concentration in solution as 14% is about the standard available Chlorine at which it comes from the manufacturers.

By contrast, Granular Chlorine (Calcium Hypochlorite - CaOcl2) is around 65% available Chlorine.

Liquid chlorine reverts to common salt as it degenerates, and this could add to corrosion.

I also wonder if there are any corrosion mitigation measures in place.
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Just an Engineer
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#9
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Re: Help - Cause of Corrosion on Mixer

04/29/2013 3:20 AM

Yes, we get 14% as our starter for dosing.

Looking at the pictures supplied, there is something else "strange". The second picture shows very clear water with what look like air diffuser bars at the bottom of the tank. This does not look like an operational sewage treatment facility yet and is probably still in proving without actual process material flowing yet.

That water does not have the suspended solids proportions mentioned in the original post.

It may be that the water is highly oxygenated and with no bacteria present (and nothing to digest) the corrosion is enhanced.

I'm also thinking that if that is the coating specified for the motor housing, then not only pipework, but also the grades of concrete used might not be suitable for high sulfur issues common in sewer.

I can only see more and more problems there.

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JBK
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#11
In reply to #9

Re: Help - Cause of Corrosion on Mixer

04/29/2013 4:04 AM

Now we are in pre-commissioning stage. We've filled the biological tank for new waste water treatment plant with the treated water receiving from another sewage treatment plant 1.5 months ago. And then the diffuser & blower test were carried out for around 1 month. During this test period, the mixer has been operated intermittently. We've found the corrosion recently. For more investigation, the diffuser/blower has been stopped and I've took the picture of the lifted mixer.
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PWSlack
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#10
In reply to #8

Re: Help - Cause of Corrosion on Mixer

04/29/2013 3:21 AM

Clearly they are inadequate as corrosion is taking place.

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I.L.M.E.
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#12

Re: Help - Cause of Corrosion on Mixer

04/29/2013 7:37 AM

By any chance do you have copper glands at the cable entrance to the motor casing?

If so, you would require installation of di-electric couplings.
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Anonymous Poster #1
#13

Re: Help - Cause of Corrosion on Mixer

04/29/2013 9:44 AM

Twelve comments , and nobody's chuckling over the stainless ID plate with SS screws/rivets holding it to the cast steel/iron frame-piece...?!

Just BEGGING for the frame (anodic, with respect to the SS) to begin corroding aggressively, to protect the nameplate...

SO MANY things such as this, everywhere we look.

All that profound, in-depth understanding of the (corrosion) mechanisms provided to us SO many years ago...(and so much more...)

"Aw, fuhgeddaboudit"...! Tsk tsk...
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PWSlack
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#14
In reply to #13

Re: Help - Cause of Corrosion on Mixer

04/29/2013 11:07 AM

It came from the mixer manufacturer like that. One presumes that location is its return shipping address.

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Anonymous Poster #1
#18
In reply to #14

Re: Help - Cause of Corrosion on Mixer

04/29/2013 6:01 PM

"Shame on ME_!" and , apologies to J_an_E .

I scrolled up-and-down (even did "Find on This Page") looking for "SS", "stainless", and "screw" ... (forgetting to do "rivet", even tho' I know most all such nameplates are affixed using the hammer-in / "more-or-less-permanent" rivet form of attachment...!)

Oooble-de-gooble-dees!!!
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Just an Engineer
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#17
In reply to #13

Re: Help - Cause of Corrosion on Mixer

04/29/2013 5:21 PM

Have a look at line 2 in post 5.

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rashavarek
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#15

Re: Help - Cause of Corrosion on Mixer

04/29/2013 1:36 PM

Ok, now let me think.....

Hmmmm.....

What would possibly cause a metal object submerged in water to corrode?

Man that's a tuffy.

Ok, just a guess,

Electrical current?

Chemicals?

Oxygen?

All of the above?

Yeah I get a 5 for smart #ss

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Anonymous Poster #1
#16
In reply to #15

Re: Help - Cause of Corrosion on Mixer

04/29/2013 1:53 PM

" What would possibly cause a metal object submerged in water to corrode? ... ...

... Oxygen?"

"Tsk-Tsk---for-shame-on-you"...

please, Please, PLEASE do not contribute to perpetuating the (various) MYTHS of corrosion-activity!

Only 4 things are necessary for REDOX Reactions (reduction/oxidation) to occur ... and NOTICE that not a ONE of them is 'oxygen'...:

Anode / Cathode / Metallic_Path / Electrolyte

"Oxidation" has NOTHING to do with "oxygen". [Period]

Oxidation=giving-up an electron. Reduction=gaining an electron.

Although 'oxygen' is frequently associated with the "by-products" of redox reactions, that is ONLY because it is "so available", in areas where cathodic reactions invariably take place! (O2, in addition to being a depolarizing agent, does get involved with *cathodic* reactions ... rarely-if-ever in the *anodic* reactions, where metal loss takes place).

Live / read / learn

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rashavarek
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#19
In reply to #16

Re: Help - Cause of Corrosion on Mixer

04/30/2013 9:39 AM

Oops, sorry left one out.

Fish!

and they wonder why comedians won't play engineering schools.......

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Users who posted comments:

Anonymous Poster (3); bwilko (1); I.L.M.E. (1); JBK (2); Just an Engineer (3); kramarat (1); lyn (1); PWSlack (3); rashavarek (2); SolarEagle (1); Tornado (1)

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