Strange corrosion of AISI 304 near the sea

[xiano]

I work for an Italian company that produces car-wash machines.
Some parts of a type of machines are in AISI 304.
In some locations, near the sea (ca. 10 km), we had some strange corrosion: after few months the SS is "reddish", but you cannot see any specific point of rust.

Can anyone tell me whether it could be due to the type of SS (maybe not the best quality??) or to the type of SS?

What can we use to polish and protect it?

Thank you

 

[Metalguy]

Hmmm, I'm in Italy. I suppose for a few years worth of free car washes I could go take a look. <g>

Anyway, sea water has a little Cl in it-about 2%. 304 SS is likely to pit in the presence of Cl (the ions), even at room temp. The red can also be cause by MIC of if the SS surface has any imbedded iron/steel from a manufacturing step.

Dilute nitric acid will remove the iron/steel, but obviously must be handled with care.

 

[Metalguy]

BTW, the next time you get to Pisa take a good look at the roof of the bapistry next to the leaning tower. Notice how one side of the roof is reddish terra cotta, while the other side is grey/black. That side is lead sheets, because the water used to be a lot closer to the tower than it is now, and whoever made the roof didn't trust terra cotta on the sea side.

 

[quadswift]

There could be some strange combination of halogens / chlorides ( in the sea-water, towns water, your detergents) and / or irons or other elements in the towns water in action.
I would think that you would have to be near the sea-front for the location to have a significant effect.

If the cost / effort is not too high, you could just introduce a periodic inspection and replace them or try increasing the flushing rate with clean water. (But see my last comment)

It is easy to think of 'stainless steel' as being very corrosion resistant, but many engineers and metallurgists have found, to their cost, that given the right (i.e. wrong) conditions and presence / abscence of certain chemicals / elements, it is anything but.

Abrasion , galvanic action / attack from dissimilar metals may start the process. Once the 'passive' surface is penetrated or removed and for some reason cannot be replaced by the action of oxygen, then corrosion, from a number of sources, can occur.

Iron typically leaves a reddish-brown residue.

Rate of attack is affected by concentration, temperature, velocity of flow and stress in the metal parts subject to attack.
Typically, elevated temperatures can increase the rate of attack. Problems can occur in, or adjacent to, welded areas.

Is there a difference? i.e is the problem on the surface most exposed to atmospheric / splash action or the side most exposed to the wash process?

Do you use micro-organism based cleaning fluids / processes?

Do you do something different in the plants with the problems.
If the material is OK in machines in other areas, and the materials / mechanical process are the same, then you need to start examining specific local conditions. Start with local water analysis.

 

[quadswift]

ps. Maybe they have bigger temperature ranges and use more de-icing salt on the roads in those areas?

Do a google search and look at IMOA (International Molybdenum Association) web site where there is document 'Which stainless steel should I specify for exterior applications'. It has some interesting content.
There is a little questionnaire that may give you some further guidance.

http://www.imoa.info/technical

downloads/which%20stainless%20steel
Regards,
Quadswift

 

[mcguire]

This same thing happens in the US with gasoline pumps which are not exposed to rain's claening action. When these pumps are made of polished ( #4 finish) 304 they develop red rusty corrosion from profuse tiny corrosion pits. If they are made of 304 with bright anneal or a rolled on finish, they do fine. Polishing reduces the critical pitting temperature by around 20C.
304 is quite marginal in salt air unless you have the ideal surface finish.

 

[Cello]

Parts made from 304 and supplied in the cold work condition could behave against salt air attack worser then those supplied in the solution annealed condition. Try then to figure out the supply condition and see if this match stands.

Ciao

 

[stevenal]

As a coastal resident, I can tell you that stainless steel of any grade is misnamed. They all discolor around here. The stain, however, is superficial. Polishing away the red will only temporarily remove the protective oxide coat.

 

[mcguire]

If you want to remain rust free in coastal areas, you should have 316. 304 is marginal. The surface finish should be bright anneal, 2b, or 2d. #3 or #4 polish is not acceptable. Welds will rust unless passivated thoroughly.
Salt should not be allowed to collect and concentrate on the surface. Washing or rinsing by rain is important.