So, do you think the copper will rust?
Copper can corrode. Copper.org is a good reference for things that can cause copper gas lines to degrade. From their site:
The majority of the literature available on the causes of soil-side corrosion of copper tube is based, for the most part, on the corrosiveness of the soil and backfill materials used. Based on studies undertaken by Denison
1 , and others, like
James R. Myers and
Arthur Cohen the following are conditions that may render soils corrosive to copper
2 :
- Elevated sulfate or chloride contents together with poor drainage, retained moisture, and an annual rainfall exceeding 30 in.;
- Very low resistivity of the soils (below 100 to 500 ohm-cm);
- Large quantities of organic matter, particularly organic acids;
- Moist cinder fills, either because of the sulfides present in the cinders or because of galvanic action between the copper tube and the cinders;
- Anaerobic SRB (sulfate reducing bacteria), which produce sulfides and are aggressive to copper;
- Inorganic acids; and
- Appreciable amounts of ammonia or ammonia compounds, which may be introduced by lawn fertilizers.
Other factors that could promote underground corrosion of copper include:
Oxygen differential concentration cells. Preferential corrosion is sometimes found on the underside of copper tubes because they are in contact with undisturbed soil where the oxygen content is reduced, in contrast to the upper portion of the tube which may be exposed to aerated backfill where oxygen content is high.
Variable aeration characteristics. These depend on particle size and distribution, the degree of compaction, and the drainage characteristics of the soil or backfill material.
Deicing practices. If the chloride content of the soil is elevated because of deicing (thawing salts for roadways and side walks) the metal becomes more anodic than the areas where the chloride content of the backfill material is lower.
Stray currents. Direct current (DC) from impressed cathodic current corrosion protections systems or from the grounding of alternating current (AC) systems to the underground copper service can be detrimental.
Cinders. Although connections of copper tube to ferrous water mains can be very favorable for the copper tube, the copper could still be attacked if it is embedded in backfill materials containing cinders.
...Laying the tube on the bottom of a trench, directly on the undisturbed or virgin soil can allow an oxygen differential cell to develop along the bottom of the tube where it is in contact with the less oxygenated (less aerated) soil, possibly leading to accelerated corrosion in this area....When sleeving is required it is very important that the sleeve be sealed water-tight against the infiltration of ground water. It has been determined that many unsealed, sleeved installations have exacerbated the soil-side corrosion of the copper tube by allowing the thawing salts, fertilizers, or other contaminants in the ground water to collect and concentrate between the copper tube surface and the sleeve...
Sealing of sleeves can be accomplished several ways, although many practices which are thought to be satisfactory are actually very detrimental to the copper tube. One such practice that should be avoided is the sealing of the space between the tube and sleeve with silicon caulk. Many silicon caulking materials outgas ammonia and/or methanol during their curing process. The outgassing of ammonia, within the sleeve, can lead to stress corrosion cracking of the copper tube, and eventual failure. Another often employed practice that should be avoided is the sealing of the sleeve with hydraulic cement. Though no adverse reaction with the copper tube will occur, the fact that the tube is held very rigid at the location of the hydraulic cement may contribute to stress fracturing of the tube at that point. Stress fracturing is caused by overworking the tube at that point through motion from thermal expansion/contraction, movement of the backfill material by freezing and thawing, or settling of the soil or structure around the sleeve and tube by improper compaction of the backfill....
Refernce:
http://www.copper.org/applications/plumbing/techcorner/prevent_corrosion_cu_tube_buried.html
So yes, in a sense, copper can "rust" if by rust you mean deteriorate and eventually fail.
I am going to a lot of expense to replace a previous 100ft Tracpipe installation that has whistled since the day it was installed. I want this to be a 50 year install. It costs thousands of dollars in patio cutting and flagstone repair to do this and I want the most headache-free material available for the new gas line. I specified PE and the plumber initially agreed, only to tell me at the last minute he was using copper on the fire pit itself. Even so, I think he messed up on the copper size. We specified 300-400k btu at the fire pit. He ran 3/4 PE at 2 psi to the back of the house and will be configuring a manifold with a couple of regulators - one for the fire pit on the patio and one for the grill and smaller fire pit on the deck above. Problem is that the patio fire pit is 30-40 ft from the manifold due to where the manifold had to be installed to hide it, and 3/4 copper to the fire pit at 30-40ft low pressure only delivers 118-138k btu. The trench is still open so we can fix this (who pays?).
That leads me astray for another question - has anyone ever placed a regulator at the bottom of the fire pit (maybe 12" bewteen the burner pan and the bottom of the pit)? Regulators can be installed next the grill inside the cabinet. I can't fine any language the expressly forbids one in the fire pit, but I do see language about venting and temperature limits and being three feet from a flame source so I'm assuming it's a non-starter.
Regards
