These small spot stain are most likely caused by insects. There is no way to prevent this but you can coat the surface with a silicon polish this will prevent the metal surface becoming permanently stained.

Another solution might be to coat the surface of the enclosure with a protective lacquer prior to their being installed in the field.

I have a customer having the same problem and we discovered it's corrosion (spot) on SS 316 and SS 304. It is mainly cause by (more) acidic rains and presence of salts (like calcium, de-icing salts, atmospheric pollution increasing etc) which are found on hyghway or railroad tracks.

We solve part of the problem by spraying 2 coats polyurethane clear coat (DFT=1-3 mils). We found out that the urethane clear coats is degreading faster as the human activities increase. We switch to a siloxane clear coat (DFT= 5-7 mils) and seems we solve the problem for the next 20 years+. It's been in the field for 8 years now and shows outstanding gloss and color retention.

Thanks a lot for the fast responce;

What is the application methode of the siloxane coating?

depend of your equipment: you can use plural pump, conventional air spraygun or a 30:1 airless system. The size of your part will dictacte what kind of equipment. I like using the 30:1 with 5 to 10 % thinning. A good siloxane is at least 75% solids (V/V)

Airless Spray Application

Pump Ratio: 30:1 (min.)

Volume Output: 2.5 gpm min........... (11.5 l/min min.)

Material Hose: ½" I.D. min...............(12.5mm min.)

Tip Size: 0.017-0.021" ....................(0.43-0.53mm)

Output Pressure: 1500-2000 psi .........105-140kg/cm²

Usually 2 coats of Carboxane 2000 to get DFT = 5 to 7 mils (125-175 microns)

Is all the screws stainless 316? Could be valance differences in the body material with the connecting (screws and nuts) or supporting material (poles and posts) allowing micro currents to occur. When wind blows over a sign a slight current is produced. Add differences in these other materials, add acid rain, add brushed finish. Or it could be something as simple as drivers spitting out their windows. The bacteria produces powerful acids. Solutions is a clear coat. Brushed materials are more prone to oxidation than a high polished.

Stainless steel is practically the most compatible materials of construction for the resistance to normal atmospheric corrosion but gets affected in acidic, saline envornment and leads to localised effect. It seems, the highway is either passing through or near chemical factory zone, coastal area where either acidic fumes, acidic condensate along with fine particulates deposition on the surface(more on the horizontal surface - how about yours?) creates spot on the surface. A saying goes with stainless steel exposure to the envronment that Good Surface - Clean Surface keeps corrosion away , hence a cleaning brush at the stop over on the way with good cleansing agent will keep away this problem and if you have good profit margin in your business go for a siloxane clear coat and be happy. The manifastation of affected surface photographs may add more information and I can be of more help to you !!

Thanks a lot all for the feedback

I am already in contact with Carboline that suggested a polyurethane (Carbothane 133) coating.

I am a little sceptical against this option espesialy after your experience Madmax1997 .

In addition our enclosures have some elasticity inherented from their sheet metal nature. I feel that polyurethane is possible to crack on such conditions...

The carbothane 133 HB will work; the finish is satin and will resist to acidic environment. The Carbothane 134HG Clear coat in my opinion is superior, come in high gloss and semi-gloss. Because of the gloss difference the 133 HB will chalk more rapidely than the 134HG: so you got less life expectancy... Which sometimes is not too bad: you can take them off and get them repainted or upgraded... repeating business.

I have some interresting data (photos and tests) on this,comparing different coatings, 134HG and Carboxane 2000. Please provide e-mail I can send attachment, I will be please to sent these data.

Carboxane 2000 will give you more years of service: there is a cost attached to this. IS your margin reasonnable or if you provide something that competion does not have customers are willing to pay more for the quality?

I do have customers willing to pay more for service and excellent quality product, some are just not ready yet and sometimes got surprises.

Hi Madmax1997. I work at a hydroelectric power plant where the wicket gates ( water distribuitor to the turbine runner) are made of SS 316. When we disassenbled these Wicket gates we found golden and metallic blue straigth stains running in the direction of the water flow. Most of the time there are small corroded blisters or spots upstreams from the stains. The head of the water is 75 feet and the diameter of the penstock is 11 feet. The area of the wicket gate assembly is +/- 33"x84"*pi where 33" is the height of the wickets and 84" is the turbine O.D. We do not know what could produce this coloring pattern but to my looks like they could be produced by porosity inclusions formed on the SS surface during the forging process. We are at the Gatun Hydro Station at Panama Canal Area, Central America. Any idea?. Thanks for your comments.

P.D. on your picture: is that Island on the Caribean? Looks similar to our Kuna Yala Islands at Panama. Regards, Luis. llizarraga@pancanal.com

Yep it's Panama... Kuna Islands...land of the free man...where the Sylas (Goedup village, Mormaquedup...) gave permission to stay in 2005... wonderful way to spend vacation!

I will talk to my friend, he is working at formely called GE Hydro for the turbine runner. I have seen the way they forge or refurbish these huge runners but do not know how to avoid these spots. Excesive heat during forging may cause a weakness in the SS...then water attack these weaks spot and start corroding... because it is mostly in full immersion it does not have the time to corrode more then that.. I will ask him and keep you inform.

In penstok we apply a paint like polyurethane which helps the flow of the water and having a great abrasion resistance. Hydro Quebec (electric company in Canada) is using sometimes an epoxy with special additive (teflon, some got carbide in) this helps the SS stopping corrosion and last longer between maintenance schedule

Maybe it could help in your case. I will send you some photo to see if its look like what you got...

Kuna Yala ....I think they are one of the most beautifull island around this blue marble... and they are really free men... they have managed to keep their ancient culture and Saylas goverment over the time and they are even respected by Panama's goverment. I will send you my last pictures from Yantupo...

Thanks for your advise. I will send you some pictures and details over the net in the next couple days.

We have done all the repairs that you mention and finally over teh years we replaced them with new SS wicket gates. They require a lot less maintenace over the years and we are happy with this choice.

The turbine runner are going to be painted and we are going to use your advise. We have used plasite before and works fine... maybe this new formula will work better.

The penstocks are scheduled for painting in the near future and we are going to check your recomendation.

Thanks a lot for your interest and hope we can exchange more information in the coming years, and also thanks for showing our little piece of land to the CR4 comunity.

Regards. Luis llizarraga@pancanal.com

hi all, i've found this thread pretty informative, as we are having a similar problem. but instead of externally used devices, we are having problems with the brushed stainless steel finish of our elevators. upon installation of the elevators, they have been covered up with a protective mat but upon its removal, we noticed spots that have been impossible to remove.

what could've been the cause of this? any way to restore the surface apart from replacing the entire panel?

thanks for your advise and expertise.

When you say brushed SS do you mean sandblast or lightly abraded SS finish? How long did it take you to realize you got spots on SS? Grade of the SS?

Your elevator is use for ? grains, humans, animals etc....

Are your spots located all at the same height or different places? do they seem to be more local or randomly dispersed?

What kind of protective mat did you use? How long the SS has been covered w this mat?

Send more details or photo. Too many things could produce spots on SS

it's actually hairline finish, probably type 304. we only realised the extent of the spots after we removed the protective covering, which is one of those canvas type with somesort of foam infill. they are more local, and start at about 4' height and above. the mat had been in place for 1.5 years.

it's a passenger lift in a residential building. that being said, apart from humans it's also been used to transport furniture as people move in and out of the building.

the spots are difficult to photograph. they hardly show up, and can only be seen clearly when inspected from an angle.