Chrome and Stainless Steel Resistance

[Constructe]

Is Chrome or Stainless Steel resistant to corosion by salt and/or acid?

[Borek]

Depends on many factors. First of all - there are many types of stainless steel, some more, some less resistant.

[Astronuc]

Is Chrome or Stainless Steel resistant to corosion by salt and/or acid? Not necessarily. As Borek indicated there are environmental factors, such as pH, dissolved oxygen, cation species and concentration, temperature, and local fluid velocity (convection) involved.

SS304 (18-8) will 18-20% Cr, 8-12% Ni, will corrode in saltwater (seawater). SS316 with 2-3% Mo gives better resistance to pitting, and 18-8 SS with 6% Mo gives even better resistance. Specialty steels like Allegheny's AL-6X (AL-6XN) and Sandvik's 254 SMOTM were specially designed for saltwater environments.

[Constructe]

Oh thanks for the feedback. Stainless steel sure is much cheaper than titanium to protect against salt water and mild acid. I guess platinum is also relatively resistant although not resistant to theivery... lol.

It is because of the surface oxidation coating protects further oxidation and wear right?

[mgb_phys]

Remember that the more corrosion resistant steels are often harder to work with (machine or weld), 316 stainless is probably the most common in marine applications.

[Astronuc]

Oh thanks for the feedback. Stainless steel sure is much cheaper than titanium to protect against salt water and mild acid. I guess platinum is also relatively resistant although not resistant to theivery... lol.

It is because of the surface oxidation coating protects further oxidation and wear right? All metals protect themselves by building an impermeable oxide at the metal/environment interface. In the case of SS, it's the layer of Cr2O3. Fe and Ni do not form much of a protective oxide. Ti and Zr do.

Pt could be plated on a surface, but it is quite expensive. Noble metals are used in special cases to change the electrochemical potential (ECP) of metals, e.g. steels.

I'm not sure that Ti is necessarily more expensive than SS. Several companies provide Ti-alloy tubing in addition to SS. I think it depends on the application and size, as well as environment. The 6% Mo stainless steels are much more expensive than 316, which is more expensive than 304.

Austenitics are problematic with respect to cutting.

[Constructe]

Thanks for your chemical clarification of oxidation protective barriers for stainless steel. So am I correct in believing that if I expose stainess steel to other metals in a harsh environment it may chemically react with it and strip away the barrier and cause erosion until a new protective layer forms (provided the stainless steel is not just a thin coating)? I suppose if that is the case if I build something with it I need to looki for stainless steel ingots or weild joints together.

[Astronuc]

Thanks for your chemical clarification of oxidation protective barriers for stainless steel. So am I correct in believing that if I expose stainess steel to other metals in a harsh environment it may chemically react with it and strip away the barrier and cause erosion until a new protective layer forms (provided the stainless steel is not just a thin coating)? I suppose if that is the case if I build something with it I need to looki for stainless steel ingots or weild joints together. Chlorides attack the oxide layer on many metals, particularly SS, and particularly at the grain boundaries and inclusions. One remedy is to reduce impurities like S, or P, and increase Mo, as previously mentioned.

Abrasion of the oxide is another factor that must be considered in the environment.

Surface roughness is another factor - the smoother, the better.

These articles might be useful:

http://www.enerquip.com/Chloride%20Induced%20Failure.pdf

http://www.mcnallyinstitute.com/04-html/4-1.html

http://www.plant-maintenance.com/articles/Tea_Staining.pdf

http://www.ferret.com.au/c/Australian-Stainless-Steel-Development-Association/ASSDA-says-the-choice-of-stainless-steel-depends-on-chlorine-and-chloride-content-in-water-n675543

http://www.tpub.com/content/doe/h1015v1/css/h1015v1_134.htm

http://www.azom.com/details.asp?articleID=1177

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

http://www.bssa.org.uk/cms/File/SSAS4.92-Stainless%20Steels%20in%20Supply%20%20Waste%20Waters.pdf


Welding is a concern because it has a different microstructure than the base metal, the local composition could very from base metal (some alloy segregation), and there could be residual stress. Autogenous or semi-autogenous welds are generally preferred.

The base metal composition is determined at the ingot stage, so one must pay attention to the ingot chemistry. Now there is the possibility to clad steel during manufacture, e.g. co-rolling slab on billet, or sheet of slab (say cladding 304 in SS316 or AL-6X), but one must careful control dimensions throughout the process, and joining the sheet must be done with due consideration. Explosive bonding is done some special applications, particularly where the two materials are rather disparate in properties, i.e. they don't alloy well. Then there are now flame or plasma spary process that apply a coating on the nearly finished product.

Consideration must be given to the dynamics of the environment. A static structure (bridge or platform) may have little wear, but a dynamic structure (shaft, bearings, impeller or propeller) will have considerable potential for wear.

Correction - 254 SMO is apparently a registered TM of Avesta Polarit AB
http://www.avestapolarit.com/upload/documents/technical/datasheets/AVPHighAlloyed.pdf

[Borek]

So am I correct in believing that if I expose stainess steel to other metals in a harsh environment it may chemically react with it and strip away the barrier and cause erosion until a new protective layer forms (provided the stainless steel is not just a thin coating)?

Sounds to me like you are talking about electrolytic corrosion. That's a little bit different story. In general you should avoid using different metals in construction, or at least find a means to isolate them so that there is no electric contact.

[Constructe]

Thanks. I noticed you are both frequent posters and appreciate the info. How do you find the time?

[Astronuc]

Thanks. I noticed you are both frequent posters and appreciate the info. How do you find the time? We think PF is important - so we find time.