question about coupling unlike metals

[vipereyes]

Hi all,

We will be installing a bag filter on our heated alkaline degreaser tank to extend the life of the bath. The bath composition is a basic Sodium Carbonate, Sodium Hydroxide and Sodium Metasilicate solution. The tank and heating tubes are constructed of 316 Stainless Steel.

The quote we received calls for a 316 stainless pump and a carbon steel bag filter housing. In addition, we will be installing a number of eductors in the bottom of tank on the filter return line to keep the solids in suspension. They quoted cast iron educators. We plan to use 316 stainless for the couplings, flanges and plumbing lines.

I have been assured by the company who quoted the system that we do not need stainless steel for the bag filter housing or eductors based on the tank chemistry, but I am concerned about coupling cast and carbon steel to 316 stainless. They were not.

Do you think I should be concerned?

Thank you in advance for your help.

 

[kenvlach]

The silicate should protect the bare iron & steel. It's commonly used in multi-metal automotive cooling systems.

Can you possibly get the eductors in Ni-resist cast iron?

 

[happychemist]

Is it an open-loop process, or is it a pure closed process ? In the second case, You may have problem with hot caustic with Fe traces due to your bag filter;
Fe traces may cause corrosion "pitting like" on austenitic ss, in particular on cold worked (eg. swifting elbow), parts without annealing or near weldings. Dead volume, or bad surface quality are worsening factors.
Case observed in a KOH20% 80°C plant, carbon steel apparatus, 316L and 321 ss pipes, and Ni-coating for some parts . Severe pitting on elbow and flanges observed with 316 and 321 ss after less than two years.

 

[kenvlach]

I wouldn't worry about the SS; the alkalinity of the solution will be way less than mentioned by happychemist. vipereyes gave a very generic formula for soak cleaners & electrocleaner solutions for ferrous metals:

http://www.degreaser.net/additionalcleaning.htm

Note: most are now phosphate-free since EPA restrictions on alage-growing phosphates. It probably also has some surfactant, a chelator such as sodium gluconate to prevent scaling, and a rust inhibitor such as sodium nitrite to prevent flash rusting upon rinsing.

Oakite 90 Electrocleaner (for which a mild steel tank is preferable to SS due to electrical conductivity; only the steam HXer is SS), contains the 3 primary ingredients above and is typically operated at free NaOH = 35 g/L (slightly less than 3.5 wt%)