50% sulfuric acid and stainless steel 4

[Subystud]

Hello -

I use recovered 50% sulfuric acid. This liquid has a slight bluish hue or is clear.

When this comes in contact with stainless, there is a black discoloration.

This is black material leached nickel from the stainless?


Any help is appreciated.

 

[moltenmetal]

50% sulphuric acid will eat most stainless steels rapidly and without remorse.

What is the black discolouration? Corrosion products. Which ones? Probably more iron than nickel in it.

 

[MiketheEngineer]

About the only thing 50% sulphuric acid can't/won't attack is glass!!

 

[kkala49]

Perry's Chemical Engineers' Handbook (4th ed, Materials of Construction) reports good corrosion resistance to 50% sulphuric acid of polyethylene and other plastics (butyl rubber, nitrile rubber, unplasticized PVC, high impact styrene copolymers); at least up to recorded temperatures, 130 oF or higher. However (industrial) practice may be different, can you advice whether some of mentioned plastics are actually used for 50% H2SO4 at ambient temperatures? Glass is fragile, limiting its application. On the other hand polyethylene pipes (up to 16 barg design pressure) or polyethylene coatings are common, probably used satisfactorily in 50% H2SO4 service (but not known to me).

 

[moltenmetal]

You do not need to use glass.

A variety of plastic and elastomeric materials are used industrially for these diluted acid services. They are used alone, with FRP for exterior reinforcement, and as linings of carbon steel pipe, tanks, valves etc.. The required combination of pressure, temperature and mechanical robustness determine what system is best to use.

 

[25362]

The following link shows a graph (Fig.2) about the corrosion of SS 316 by H[sub]2[/sub]SO[sub]4[/sub] solutions:

http://www.al6xn.com/SSSguide.pdf

​

 

[hacksaw]

recovered acid from what source?

 

[lukman143]

If sulphuric acid attacks stainless steel, why do we use that chemical for pH adjustment in cooling water initial treatment? Can anybody know why?

http://chemicalengineeringmagazine.com,

Free Chemical Engineering Magazine Subscription

 

[moltenmetal]

Sulphuric acid is a relatively cheap and widely available acid for pH adjustment. The resulting anion sulphate (SO4^-2) is usually much less worrisome in solution to downstream equipment than the alternative (Cl- when HCl is used).

The acid resistance of all metals and alloys depends on the acid concentration, temperature, the presence of co-contaminants, and many other factors. At room temperature, commercial concentrated sulphuric acid (93%) is compatible with ordinary austenitic stainless steels. The same is true at dilute concentrations in otherwise clean water (concentrations < 1% acid). However, between 1% and 93% even at ambient temperature is a no-mans land for many metals and alloys, with each having its own limits. Only tantalum is fully resistant across that entire range.

In a water treatment application, the injection quill and the section of pipe where dilution will happen, need to be designed properly, with materials selected to withstand both the dilute acid AND the heat of dilution which can be significant. This is a great application for plastics, many of which are immune to corrosion by acid within this range, provided that the heat of dilution is taken into account.

 

[cloa]

Duiron 14.5% Silcon Cast Iron will handle any almost any concentration of sulfuric acid provided it doesn't contain fluoride ions. Only slightly better mechanical properties than glass.