Chloride corrosion 3

[vjr0512]

In one of the Shell and Tube Heat Exchanger in the Polypropylene plant , the low temperature service calls for the LTCS material for the tubes . But the chloride content in the circulating cooling water is as high as 600 ppm. The temperature of cooling water outlet from the exchanger will be around 45 deg C.

The corrosion risk of the LTCS ( Low temperature Carbon steel) material for the above chloride contained Cooling water needs to be explored

If the corrosion risk is high then a suitable treatment system of the cooling water has to be thought off.

Thanks to clarify

 

[EdStainless]

how cold do you need to go?
You could use tubing of 2205 duplex stainless steel down to about -40C.

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P.E. Metallurgy, Plymouth Tube

 

2205 seems a good candidate.
Duplex HX tubing should always be specified 'seamless'.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

Now that comment is 'ironic'.
There is no reason to buy seamless stainless steel heat exchanger tubing, unless you want to explain why your seamless tubing leaks.
The key with duplex SS is the HT. The temperature must be correct and the cooling rate must be fast enough.
You can screw this up with either seamless or welded tubing.
And there are ways to test for it.
All duplex should require A923 testing for detrimental phases.
And requiring more that just basic NDT should be a part of this also.
Seamless tubing should always be Ultrasonic tested. The ID tears and laps from the piercing process can be significant.
You can require tighter ET on welded tubing.
And then there is welded and cold drawn tubing, which is usually overkill.
I have done failure analysis on dozens of cases of poorly made seamless tubing. That move alone assure you of nothing.


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P.E. Metallurgy, Plymouth Tube

 

[mcguire]

It's hard to imagine a product more prone to defects than seamless duplex tubing. Welded and fully annealed is far preferable.

Michael McGuire

 

EdS,
Because welded 2205 tubing is a data point in my failure analysis experience. The thin-walled ¾" welded tubing in question was made without filler metal and without subsequent heat treatment. The tubes came apart along perfectly straight longitudinal internal grooves. There was no problem with the process being out of control or alloy selection. My only recommendation was to upgrade the tube construction so as to achieve the full potential of 2205, everywhere (i.e., spend a little more money). One of my easier jobs.
MM,
Welded and fully annealed would also be my preference.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

There is no ASTM spec for unannealed duplex stainless steel tubing.
The only unannealed stainless is for ornamental applications.
Sounds like yours had some serious lack of penetration in the welds.

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P.E. Metallurgy, Plymouth Tube

 

The uncorroded tube away from the leaks was fine.
If there is no ASTM spec then the root cause was worse than I thought.
I will hold back from naming and shaming the manufacturer, but they sell a lot of tube...made in the cheapest way possible.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

"You may not always get what you pay for, but you never get more than you pay for"

They could have inline annealed it with such a short hold time that no homogenization occurred.

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P.E. Metallurgy, Plymouth Tube

 

That's possible, EdS.
Wouldn't be the first time high-speed electric tube welding has gone wrong.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

I have also seen laser welded tubes where they missed the seam.
They were close enough to get good fusion at the OD and ID surfaces, but there is a portion in the mid-wall that isn't welded.
I have seen this on material as thin as 0.028" wall and on heavier products also.

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P.E. Metallurgy, Plymouth Tube

 

[Dhurjati Sen]

Dear vjr0512,

Is it a once thru cooling water system? Or do you have a cooling tower?

Replacing LTCS is the last option. Upgrading material arbitrarily results in unexpected corrosion issues elsewhere that could easily get a place in the Corrosion Handbook !

Regards.

DHURJATI SEN

 

[Chumpes]

hello
Let's assume here that the original carbon steel (CS/LTCS) material is suitable for the cooling water below 45°C WITHOUT 600ppm chloride ions, because the cooling water is treated (O2 scavenger, pH buffer, others...).
will the presence of 600ppm chloride ions will render carbon steel no more suitable ? i don't think so... in my opinion chloride ions only have negligible influence on the corrosion and corrosion rate of carbon steel.

what do you think ?

Of course stainless steel may be still required if excessive corrosion of carbon steel, with or without chloride ions, is anticipated. Then DSS 2205 should preferably be selected.

regards

 

[Chumpes]

hello

if carbon steel is suitable (moderate to low corrosion rates) without chloride ions for this cooling water service, will carbon steel faill by corrosion when 600ppm chloride ions (not HCl) is added to the system ?

regards

 

[Dhurjati Sen]

Dear Chumpes,

Chlorine (previously) and now-a-days Bromine are dosed in CW systems since last two decades. That's why I wrote the earlier post.

Carbon steel will not fail when chlorine is added to the system. You will add the chlorine as gas but it reacts with various salts in the CW system and you monitor the effectiveness of Chlorine dosing by measuring the residual chlorine as chloride.

Cheers & regards.

DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[Chumpes]

@DHURJATI SEN: By chlorine you mean Cl2 gas dissolved in water, i.e. HClO (or more commonly bleach). It is an oxidative compound that is used as a biocide in CW. I think levels of residual active chlorine in CW shall be kept very low, i.e. (roughly) below 0.5-1ppm, in order to avoid excessive oxidation (corrosion) of steels. A bleach solution contains both chloride and chlorine.

I was talking about chlorides (chloride ions Cl-, which can maybe reach 600ppm) and not about active chlorine (bleach) which concentration should remain limited below ppm levels. The original poster did talk about chloride and not chlorine. I am not well aware of possible dosing technics, chloride/chlorine interactions at dosing...

600ppm of active chlorine is very very VERY high and to my knowledge it will corrode every materials from carbon to stainless steels including duplex (reference corrosion handbook outokumpu).

In the end i agree with you, keeping (if possible) the originally selected LTCS (that apparently performs well in absence of chloride ions Cl-) should preferably be targeted even with anticipated 600ppm chloride ions Cl-.

The original poster is apparently not comming back =)

 

[Dhurjati Sen]

Dear Chumpes,

Its really nice interacting with you.

CW treatment is not done on an adhoc basis. You first passivate the system using phosphate. So there is a layer of phosphate on every carbon steel component internally. Now you add chlorine / bromine to control iron bacteria or tubercles while you add biocides (again phosphoric acid based) to control sulphur reducing bacteria. Chlorine / bromine dosing is done on a continuous basis while biocides are shock dosed.

I am not a CW treatment specialist or vendor but have evaluated such programs for many years and the above is a generalized version of what is done.

I do agree with you that 600 ppm chlorine is too high that could only result from a mal-function in the chlorine dosing system. Normally, residual chlorides are maintained within 10 ppm.

And, as most of the time if not always, people who originally post the question are the ones who are least interested in the answers.

Regards.



DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[cap1a79]

I would add to quantitative calculation based on API 581..I think this can help to make solid logic why LTCS is suitable or not. Refer to API 581 2.B.11,

For example....

1. If the heat exchanger is operated at RSI =<6 and velocity =< 8fts (750 ppm of cl-, 45 c temperature)
--> Corrosion rate of LTCS will be 7 mpy (velocity is between 3 and 8 fts as assumed.)

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2. If the heat exchanger is operated at RSI >6 or velocity > 8fts (750 ppm of cl-, 45 c temperature)

--> Corrosion rate of LTCS will be 21 mpy (velocity is between 3 and 8 fts as assumed.)
--> Corrosion rate of LTCS will be 26.25 mpy (velocity is 9 fts as assumed.)
--> Corrosion rate of LTCS will be 31.5 mpy (velocity is 10 fts as assumed.)


Lee SiHyoung,
WorleyParsons Oman Engineering,