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Red Layer Formation in Austenitic Steel Reactor During NaOH Cleaning

02/05/2020 8:40 AM

ISSUE -

In typical Austenitic Steel reactor with agitator, there is a staining or colorization occuring in the region where NaOH (sodium hydroxide)+ water solution and air would be coming in contact with heated surface.

Top and Bottom portion of tank including agitator shaft, dip pipe are not affected.

The process of cleaning/neutralizing the reactor, post product batch is when the NaOH+water solution is used.

The NaOH+water (50%conc,) solution is 50-60% volume based filled in Austenitic steel reactor. The heating medium (steam) is supplied to the limpet area to boil the solution. With the continuous agitation during this process, droplet of these solution tends to settle on the hot unwetted surface of inner main body surface.

Once the droplet settles they quickly vaporized leading to high local concentration -NaOH scaling or localized deposition on surface results in corrosion like stain (red layer surface).

SUPPORTING DOCUMENT

The inner contact surface of reactor is certified SS316L Austenitic steel.

From metallurgical profile for Austenitic steel it affirms that it has good resistance to NaOH upto 50% concentration and temperature 93 degree Celsius.

Above 93 Degree Celsius the austenitic tends to exhibits unstable passivity and can suffer sever general corrosion.

QUESTION

The temperature has to be around 90 deg cel. as per requirement for cleaning the reactor with NaOH + water solution

The formation of stain like portion is in unwetted main body area only not on agitator profile nor inner top dish area.

The issue of red layer formation is with NaOH + water + steam Heating process only.

The issue of red layer formation is not formed under water + Steam heating process.

Request suggestion, on the implementation

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#1

Re: Red Layer Formation in Austenitic Steel Reactor During NaOH Cleaning

02/05/2020 3:42 PM

What is your goal here? Do you want to clean it? prevent it? What is the problem created by this deposit? Are you looking for some sort of coating or treatment to protect the metal? Is the metal being damaged or is it just unsightly in appearance?

..."Wet the surface with oxalic acid or 10% nitric acid, leave for 10 – 20 minutes, rinse with ammonia or sodium bicarbonate solution, rinse with clean cold water, wipe dry. This treatment may need to be repeated, depending on the severity of the rust staining."...

https://www.australwright.com.au/maintaining-cleaning-stainless-steel/

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#7
In reply to #1

Re: Red Layer Formation in Austenitic Steel Reactor During NaOH Cleaning

02/12/2020 3:16 AM

thank you Solar Eagle for your reply.

Goal is to neutralize the reactor with NaOH solution (50%). The sole purpose is to

Yes we want to clean it, more likely neutralize it.

prevent it ? what ? Can you please elaborate

The problem is reddish layer on the body calls for it to be contaminated surface. So the re-passivating the surface after draining the NaOH solution clears the surface.

We are not looking for coating or need any treatment to protect the metal, as the un-wetted surface is only concern where the reddish layer is shown its tricky question to answer at the point.

Is re-cleaning or Cleansing a good option, as the prime purpose of NaOH solution is to neutralize the reactor. we are in a way redoing the similar process. Please suggest.

Moreover - Why partial body surface (unwetted surface) is only affected remains unanswered.

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#13
In reply to #7

Re: Red Layer Formation in Austenitic Steel Reactor During NaOH Cleaning

02/13/2020 1:32 AM

It seems to me this is a buildup over time of deposit that has not been cleaned properly...try my suggestions #1 clean.... check rate of accumulation.... and determine cleaning maintenance interval....

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#2

Re: Red Layer Formation in Austenitic Steel Reactor During NaOH Cleaning

02/05/2020 3:44 PM

The red color is llikely iron oxide hydroxide.

Concentrations and temperatures are unlikely to be staying below the thresholds you assume. Boiling point of 50% concentration NaOH is above 140° C, so if the droplets are boiling dry then temps are definitely above 90° C.

...and boiling dry definitely increases conc. NaOH.

Your steam may be under more than atmospheric pressure. Alternately, the droplets might not be boiling away, but may be experiencing drying due to exposure to unsaturated atmosphere.

Consider purging the interior with steam to eliminate dryout of droplets to avoid spikes in concentration.

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#10
In reply to #2

Re: Red Layer Formation in Austenitic Steel Reactor During NaOH Cleaning

02/12/2020 4:42 AM

Thank you truth_is_not_a_compromise for your reply.

There is an update on our post with some clarity on the band area and conc.

we need to revisit on the deposition concern as it is on WETTED area actually NOT UNWETTED area.

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#3

Re: Red Layer Formation in Austenitic Steel Reactor During NaOH Cleaning

02/05/2020 4:10 PM

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#4

Re: Red Layer Formation in Austenitic Steel Reactor During NaOH Cleaning

02/05/2020 11:42 PM

Has the reactor been passivated prior to this?

Sodium hydroxide is commonly regarded as being compatible with iron base alloys; however, this is not completely accurate. It will react with iron that is in the 2+ or 3+ oxidation states to produce hydroxides. These can then precipitate as rust.

Since 316 stainless steel is still primarily composed of iron, it is possible that this it what is happening. Especially since you are WELL above the 93 C threshold if you are actually boiling it; 50% Caustic soda boils at around 140 C.

As far as why the other parts are not affected, they may be of a different alloy. When you are dealing with things like this, even 316 and 316L may exhibit different characteristics. Being an external element that may be a bit hotter or colder compared to the others could also have an impact.

Possible solutions:

  1. Change the reactor composition. Hastelloy will work well. Hate to have to pay for it though.
  2. Change the cleaning solution to something more compatible.
  3. Control the cleaning process better. If you do only require 90 C or above, set a reasonable target point just above that instead of boiling the solution.
  4. Clean the stains off with acid.
  5. Re passivate the tank. NASA has some very good information on hot Citric acid versus hot Nitric acid for this purpose. Building up the chromium oxide layer will help reduce the staining issue.
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#5

Re: Red Layer Formation in Austenitic Steel Reactor During NaOH Cleaning

02/06/2020 1:09 PM

Ferric oxide, Fe2O3 .H2O, is red and when it occurs in nature it is called hematite

http://www.corrosion-doctors.org/MatSelect/rouging.htm

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#12
In reply to #5

Re: Red Layer Formation in Austenitic Steel Reactor During NaOH Cleaning

02/13/2020 12:59 AM

thank you Whitephone for your reply.

Can you elaborate on how on Ferric oxide can be a concern in Austenitic steel. Something to do with NaOH + H2O reaction.

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#6

Re: Red Layer Formation in Austenitic Steel Reactor During NaOH Cleaning

02/07/2020 4:15 AM

<... From metallurgical profile for Austenitic steel it affirms that it has good resistance to NaOH upto 50% concentration and temperature 93 degree Celsius....Above 93 Degree Celsius the austenitic tends to exhibits unstable passivity and can suffer sever general corrosion...>

The phenomenon indicated seems to verify the information provided in the <...SUPPORTING DOCUMENT...> to hand.

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#9
In reply to #6

Re: Red Layer Formation in Austenitic Steel Reactor During NaOH Cleaning

02/12/2020 4:38 AM

Thank you for your reply. PWSlack.

We did not understood your comment. Please elaborate.

Also, please see the update we have done on our earlier post.

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#8

Re: Red Layer Formation in Austenitic Steel Reactor During NaOH Cleaning

02/12/2020 4:18 AM

There is Slight correction on the 2 accounts

  1. The conc. of 50% NaOH + 50% water is diluted to 10% NaOH - post this the Heating via Steam is initiated. This is when the heating reaction and issues are faced.
  2. The band of reddish layer is formed on WETTED surface (not unwetter surface) just around the surface of the solution.
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#11
In reply to #8

Re: Red Layer Formation in Austenitic Steel Reactor During NaOH Cleaning

02/13/2020 12:52 AM

Do anyone has any backup or process details - NaOH used in process reactor (Austenitic steel reactor) that would help to support our query.

Also any one has Austenitic Steel ASTM metallurgical properties with respect to NaOH process.

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#14
In reply to #8

Re: Red Layer Formation in Austenitic Steel Reactor During NaOH Cleaning

02/13/2020 10:29 AM

That makes more sense, as the boiling point of 10% NaOH is only 105 C not over 140!

Question relating to your reactor diagram- are the heating jackets actually as shown, especially in relation to the red band and the typical cleaning fluid level, and are they steam heated?

If so there's a good chance that you are getting localized overheating of the reactor wall by the fluid/air interface, which would explain why you only see it on the reactor wall and not on the prop shaft etc. Your solution would then be to either shut off the upper jackets or to increase the fill level to adequately cover them. Either way, 105 C is still well above the 93 C caution you mentioned earlier.

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#15
In reply to #14

Re: Red Layer Formation in Austenitic Steel Reactor During NaOH Cleaning

02/16/2020 12:45 AM

thank you JNB for your reply.

your observation is correct, the band is formed due to localization overheating. But don't have any parameter or method to prove our observation.

Secondly, the phenomenon occurs on band formation is exclusive to NaOH and body only at surface level of solution, which is unusual.

The temperature is certainly critical, but can we have a data linking temperature, NaOH solution and Steam the source of heating.

This can be helpful.

thank you.

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#16
In reply to #15

Re: Red Layer Formation in Austenitic Steel Reactor During NaOH Cleaning

10/11/2021 3:24 AM

That is nonsensical. The observation demonstrates the method.

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