Which material to use for bolts, washers and nuts?
Which material to use for bolts, washers and nuts?
(OP)
Greetings,
Please see the pictures below. The white pipe is part of a 15x20 foot tank made of carbon steel(fully painted). The bolts, washers and nuts are made of galvanized steel and the valve is made of brass. This corrosion appeared after the assembly was left out in the rain for just a few days. Galvanized bolts are used at other points, but do not show this problem.
Which material should be used for the bolts, washers and nuts if this is to withstand the elements for several years?
Thanks,
David


Please see the pictures below. The white pipe is part of a 15x20 foot tank made of carbon steel(fully painted). The bolts, washers and nuts are made of galvanized steel and the valve is made of brass. This corrosion appeared after the assembly was left out in the rain for just a few days. Galvanized bolts are used at other points, but do not show this problem.
Which material should be used for the bolts, washers and nuts if this is to withstand the elements for several years?
Thanks,
David







RE: Which material to use for bolts, washers and nuts?
RE: Which material to use for bolts, washers and nuts?
RE: Which material to use for bolts, washers and nuts?
I understand that I should use non-conductive washers. Is this correct? Anything else?
RE: Which material to use for bolts, washers and nuts?
RE: Which material to use for bolts, washers and nuts?
RE: Which material to use for bolts, washers and nuts?
RE: Which material to use for bolts, washers and nuts?
CoryPad brings up a good point too. Your need to do this may be more for what is inside the pipe to protect the less noble steel from the more noble brass. Your real corrosion problem may not be the bolts.
And, if you are only a mile from the sea, you are by definition a Marine environment.
rmw
RE: Which material to use for bolts, washers and nuts?
The others have clearly outlined what is required to make zinc-coated steel work with a brass flange. Another option would be to use brass fasteners. Here is a link with some information on standard brass grades (ASTM F468):
http://www.cookfasteners.ca/faqs_1.php
RE: Which material to use for bolts, washers and nuts?
TVP, thanks, I would prefer brass fasteners to trying to electrically insulate galvanized fasteners. I will look into this.
So does this mean I should avoid using galvanized parts attached to the carbon steel main tank, including galvanized cable conduit and galvanized bolts?
RE: Which material to use for bolts, washers and nuts?
If the carbon steel is painted well, it will minimize the effect. Your biggest problem with the initial photos you posted is the bare brass. While many grades of brass are relatively corrosion resistant, the zinc galvanizing will still sacrifice itself to protect the bare brass.
RE: Which material to use for bolts, washers and nuts?
RE: Which material to use for bolts, washers and nuts?
In your materials selection, some of the major galvanic corrosion factors to consider are
1) Potential (voltage) difference
2) Time-of-wetness (rain, dew)
3) Chlorides (chlorides are awesome at driving corrosion)
4) Other conductive salts
5) Cathode/Anode area ratio.
6) Projected lifespan of the item
0.8V is a LOT - it's way too much for most outdoor environments. You have to start thinking about mitigation to break the electrochemical cell - isolation kits, painting, et cetera.
In your case, you have high time-of-wetness, high chlorides, high other salts, a long projected lifespan, and with the small galvanized parts, you have the wrong Cathode/Anode ratio and a large potential difference.
The first good report on this effect (including isolation) is the British Admiralty report on the iron-nailed copper sheathing on HMS Alarm - which only had an 0.5V difference. Similar Cathode/Anode area ratio (small iron nails) but higher chlorides, salts and time-of-wetness (saltwater immersion.) The report on the corrosion was issued just two years after Alarm launched.
The "isolation kit"? - the copper sheathing was wrapped in brown paper to prevent damage prior to installation. Some of the nail heads had brown paper trapped underneath: workers had simply nailed through the paper, and removed the bulk of it later. This made a noticeable difference in the corrosion rate.
RE: Which material to use for bolts, washers and nuts?
RE: Which material to use for bolts, washers and nuts?
The investigation found some stainless/galvanized mismatch and some electrogalvanization instead of hot dipped galvanized, both of which were corrected.
But the fundamental problem(which was the cause of the corrosion in the pictures in the first post) is the extreme acidic nature of the monsoon rains in Mumbai. I wound up changing hundreds of nuts, bolts and screws before applying zinc appropriate primer and painting these parts.
The chemistry professor was saying that Mumbai rain results in a diluted version of Aqua Regia, the only substance capable of corroding gold. Smog leads to sulfuric acid in the rain, abundant lightning results in nitric acid in the rain, and salt from the sea combines with either of these to produce hydrochloric acid.
Here is another picture:
RE: Which material to use for bolts, washers and nuts?
RE: Which material to use for bolts, washers and nuts?
They offer superior corrosion resistance as compared to galvanized fasteners and are cheaper than a Monel type solution.
http://www.metcoat.com/ptfe-coatings.htm?_vsrefdom...
http://www.eng-tips.com/viewthread.cfm?qid=325662