All of the coppers share the same broad corrosion behaviour, which is excellent in the environments copper is actually bought for. Copper resists fresh and potable water, atmospheric exposure — where it develops the familiar adherent patina and then corrodes at a negligible rate, which is why copper roofing lasts a century — non-oxidising acids free of dissolved oxygen, neutral and alkaline salt solutions, and seawater at moderate flow velocity. It is biostatic, which is the basis of its use in potable plumbing and in antimicrobial touch surfaces.
Its limits are equally consistent. Copper is attacked by oxidising acids — nitric acid rapidly, and sulfuric or hydrochloric acid as soon as oxygen or an oxidising contaminant such as ferric iron is present. It is attacked by oxidising salts, by sulfides and by hydrogen sulfide, which is the limit on copper in sour service and in polluted harbour water. It suffers erosion-corrosion and impingement attack in fast or aerated water — conventionally about 1.2 m/s is the design ceiling for copper tube in seawater, well below what a copper-nickel will take — and it will pit under deposits. Elevated-temperature oxidation in air becomes significant above roughly 200°C.
Hydrogen embrittlement is the defining weakness of tough-pitch copper and the reason the rest of this family exists. Tough-pitch copper contains its residual oxygen as a fine dispersion of cuprous oxide, Cu₂O. Heat it above roughly 400°C in any reducing atmosphere — the hydrogen-bearing flame of an oxyacetylene torch, a hydrogen or cracked-ammonia brazing furnace, a reducing process gas — and hydrogen diffuses into the metal, reduces those oxide particles in situ and produces steam at the grain boundaries. The resulting internal pressure opens intergranular cracks and porosity, and the copper loses ductility catastrophically and permanently. Nothing in the surface appearance warns of it; ASTM B577 exists specifically to test for the susceptibility. The practical rules are that tough-pitch copper must not be oxyacetylene welded, must not be brazed in a reducing furnace atmosphere, and should not see sustained high-temperature service in hydrogen-bearing gas. Where any of those is required, specify an oxygen-free copper (C10100 or C10200) or a deoxidised copper (C12000 or C12200) instead.
The welding and brazing ratings follow directly from that. CDA rates C11000 as Excellent for soldering but only
Good for brazing,
Fair for gas-shielded arc welding and
Not Recommended for oxyacetylene welding, coated metal arc welding, spot welding and seam welding — a markedly poorer set than the deoxidised grades, which rate Excellent for both brazing and gas-shielded arc welding. If the joint must be welded or furnace-brazed, specify
C12200 or
C10200 at the design stage rather than trying to weld ETP.
Ammonia and amines cause stress-corrosion cracking in copper, and this is the one corrosion mechanism that will fail a copper fastener without warning. A copper component carrying residual or applied tensile stress — and a tightened bolt is exactly that — will crack in moist ammonia, in ammonium compounds, and in the amine corrosion inhibitors used in boiler and steam systems. Nitrites and mercury compounds do the same. The practical consequences are that copper fasteners should not be used in refrigeration plant handling ammonia, in fertiliser service, in amine-treated condensate systems, or anywhere animal waste or decaying organic matter concentrates ammonia; and that where copper has been cold worked and must sit in a borderline environment, a stress-relief anneal to remove residual stress is worthwhile even though it costs some strength.
Copper is galvanically noble, so it damages what it is bolted into. Copper sits well above carbon steel, cast iron, aluminium, zinc and galvanised coatings on the galvanic series. A copper fastener in an aluminium or steel structure will survive, but it will drive rapid corrosion of the surrounding metal, and the small-cathode-is-safe rule works against you here: a small copper bolt in a large aluminium plate is the less damaging geometry, but a copper bolt clamping a thin aluminium or galvanised sheet will eat through it. Copper must be isolated from aluminium and from galvanised steel with insulating washers and sleeves or a barrier coating, which is why aluminium-to-copper electrical connections use bimetallic lugs rather than a direct bolted lap. Copper is, conversely, compatible with the other coppers and bronzes, with the copper-nickels and with Monel.
With copper, temper — not heat treatment — sets the strength. None of the coppers on this hub is hardenable by heat treatment; there is no quench-and-temper and no precipitation cycle to call on. Strength comes almost entirely from cold work, and the temper designation is therefore the single most important thing on the order after the UNS number. The sequence runs from the annealed tempers (O60 soft anneal, or the OS grain-size tempers such as OS025 and OS050, which specify the annealed grain size rather than a strength) up through H00 one-eighth hard, H01 quarter hard, H02 half hard, H04 hard, H06 extra hard, H08 spring and H10 extra spring. Across that range the tensile strength of a plain copper roughly doubles — from about 220 MPa annealed to about 390 MPa at H10 — while elongation collapses from 45–50% to around 4%. A hard temper bought for strength is therefore also a temper that will not take further forming, and cold heading a fastener head is itself cold work: heads are normally formed in an annealed or light temper and the shank drawn to the specified temper, or the part is formed and then the whole fastener left in the as-formed condition. Tell Exotic Alloy the temper you need, or tell us the property you need and we will specify it.
Copper fasteners are not chosen for load. This is the most common specification error in the family, so it is worth stating plainly: even at hard temper a copper bolt develops roughly 330–380 MPa tensile, against 830 MPa for an ordinary ASTM A325 structural bolt and 1040 MPa for a property class 10.9. Copper is specified when the fastener has to carry current, conduct heat, resist a corrosive environment, stay non-magnetic or avoid sparking — and the structural duty is light or carried by something else. Where a copper-coloured fastener is wanted at real strength the answer is a copper
alloy — silicon bronze, aluminium bronze or beryllium copper — not one of the high-conductivity coppers here.
Stress relaxation is the companion problem. Copper creeps at modest temperature and under sustained load, so a bolted copper joint loses preload over time; the softer the temper and the warmer the joint, the faster it goes. Electrical connections in copper are therefore designed either to be re-torqued on a schedule, or to use a spring element (a Belleville or conical washer) that takes up the relaxation, or to use C19400 or a copper alloy whose stress-relaxation resistance is far better than that of a pure copper. A copper busbar bolt torqued once and forgotten is a loose, hot joint later.
How this family divides. The seven coppers Exotic Alloy stocks are not interchangeable grades at different price points; each exists to solve one specific problem, and the trade-off is almost always conductivity against something else.
C10100 and
C10200 are the oxygen-free coppers — buy them for immunity to hydrogen embrittlement and for the highest conductivity, C10100 when electronic-grade purity and vacuum-tightness genuinely matter and C10200 for everything else.
C11000 ETP is the workhorse: the cheapest and most available copper, the standard for busbar and electrical hardware, and the only one in the group covered for bolts and nuts by ASTM F468 and F467 — but it is the grade that embrittles in hydrogen.
C12000 DLP and
C12200 DHP are the phosphorus-deoxidised coppers, bought for weldability and brazeability; C12000 keeps 98% IACS with a low phosphorus residual, while C12200 trades conductivity down to 85% IACS for the best weldability in the family and is the standard plumbing and HVAC copper.
C14500 tellurium copper is the free-machining grade — the only sensible choice for a turned or heavily machined copper part.
C19400 is the odd one out and the strongest: a copper-iron alloy that reaches 550 MPa and resists stress relaxation, at 65% IACS.
