Is Chrome or Stainless Steel resistant to corosion by salt and/or acid?
Depends on many factors. First of all - there are many types of stainless steel, some more, some less resistant.
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.
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?
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.
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.
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.bssa.org.uk/cms/File/SSAS4.92-Stainless Steels in Supply Waste Waters.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
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.
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.
I work in the stainless fabrication industry, manufacturing for the food/beverage industry.
How salt water affects 304 and 316 grades, the salt sits in scratches, unpassivated welds and pits in material and creates whats called tea stain (tea stain pits the passivated surface and creates at star shaped corrosion underneath the passivated surface, so sanding/polishing is a poor option for removal), To reduce this effect mirror finish polish is required. This means the elements have nothing to attach itself too. Mirror finish 304 can work just aswell as 316 in sea water.
Only instance that I have found where tea stain occurs on stainless steel away from salt water is in the pressence of ammonia, ammonia will tea stain the passivated surface.
I work for JL Lennard Engineering and in conjuction with the University of Queesnland, Australia, we did testing on chemicals for the removal of the tea stain and repassivation of the material surface.
University Researcher: Mr Jeff Gates Bsc.Phd.MIEA (University of Queensland Australia)
For more information on the product corosion removal (warning: this chemical will eat through galvanised and mild steel, turn concrete porus, even eat through nylon, so protection of items in the area require complete plastic coverage including PPE for yourself, can even lift membrane surface coating on concrete, make sure everything is covered in plastic and appling neutralising solution around the area is a must.
MSDS- Chemwatch 4543-88 Stainless Steel Passivating Gel
Callington Technical data sheet code 1130 passivating gel for stainless steel
Neutralising Solution we use
MSDS- 0210 BOC Weld-Guard Neutralising Solution
Callington Technical data sheet code 1150 S-weld neutraliser
The product can be purchased from Industrail Cleansers here in australia
Product Code : P00100A
Just stumbled across this thread. If there is electrolytic corrosion as Borek suggested, then you have a whole new field to delve into, including introducing charged (sacrificial) probes into the solution to protect the stainless steel. Even really exotic alloys can be eaten up when there are small differences in electrical potential between objects in a salty solution.
oh I forgot to add the simplest way to prevent the corrosion is Electroless Nickel plating, only afew microns will be required, this will eliminate most instances where tea stain can occur
Nice comment Turbo-1, to my knowledge no-one has done much research in the use of electrolisis and the use of sacrifical elements, cost of research and development is more than replacing the product.
I do know of 1 brewery that uses old ammonia cooled tanks, and the leaking ammonia vapour will create the stain in a matter of days and the only real long term solution is electrolisis.
I went to this brewery and turned their 2 brown stained tank into a shiny stainless
I was a process chemist in a brand-new pulp mill, and after suffering corrosion damage in the drum of the first-stage bleaching equipment (using stainless steel) the company opted to replace the drum with a titanium alloy (over 1/4 M $ for the drum alone) only to see it destroyed even sooner than the SS drum was. We had to invest in a cathodic protection system with sacrificial probes in the solute to get decent life out of the drum, and invest in non-metallic filter media to cover the drum. Eventually, it all got sorted out so the losses were tolerable, but when you're dealing with chlorine (bleaching agent) being introduced to Kraft pulp (high in NaOH and other inorganics) things get dicey pretty quick.
Separate names with a comma.