Electrolyte Filtration
Stainless Steel in Copper Electrolyte: Why 316L Survives Hot Acid, and What Takes the Margin Away
Why 316L stainless steel holds up in hot copper electrolyte when sulphuric acid charts say it should not, and how chloride, temperature, welds and copper-free acid erode the margin.
The short answer
A copper electrorefining electrolyte is hot, roughly 13 to 17% sulphuric acid by weight, and by the published sulphuric acid tables it should be too aggressive for 316L stainless steel. Alleima's corrosion table for pure sulphuric acid rates its 316L grade as corroding faster than 1 mm a year at 20% acid and 60 °C, a rating it describes as "not usable". Yet 316L is a standard material for the permanent cathode blades that sit in that same electrolyte.
The reconciliation is the copper. Dissolved cupric sulphate is an oxidising salt, and oxidising salts, in significant amounts, help hold stainless steel in its passive state beyond the range it manages in plain acid. Alleima's table for sulphuric acid with copper sulphate rates the same 316L grade "corrosion proof" in laboratory tests, under 0.1 mm a year of general corrosion, at 16% acid with 13% copper sulphate and 90 °C.
So for 316L in copper electrolyte the material question becomes a question about the margin, and four things take that margin away: higher temperature, acid that has lost its copper, chloride, and weld zones that have become sensitised. This article works through each, with the published data behind it, and ends with what to put in an enquiry so the material is chosen for the stream the filter will actually see.
What the electrolyte is, chemically
Glencore Technology's published guidance on electrorefining gives the working envelope. Most refineries run their electrolyte at 60 to 65 °C, with sulphuric acid usually 160 to 200 g/L, copper typically 40 to 50 g/L, and chloride added deliberately at typically 20 to 50 mg/L to work with thiourea as a levelling agent. A described operating plant reported by 911 Metallurgist matches that envelope on temperature and acid: electrolyte at 60 °C, 200 g/L free sulphuric acid and a specific gravity of 1.2. The filter cloth article uses the same figures for the cloth side of the duty.
Corrosion tables are written in weight percent, so the conversion matters. Taking a specific gravity of 1.2, 160 to 200 g/L of acid is about 13 to 17% by weight, and 40 to 50 g/L of copper, expressed as copper sulphate, is about 8 to 11% copper sulphate by weight (anhydrous basis). That is our arithmetic, and it is approximate, because specific gravity moves with composition and temperature. It is close enough to find the right columns in the tables.
What a plain sulphuric acid chart says
Alleima, a stainless steel and special alloy producer, publishes corrosion tables from laboratory tests in pure chemicals. Its 316-type grade is 3R60, which its datasheet certifies to both ASTM TP316L and TP316 (UNS S31603 and S31600), with about 2.6% molybdenum, so its ratings below describe a material that meets both grades. The tables use three ratings: 0, under 0.1 mm a year, "corrosion proof"; 1, 0.1 to 1.0 mm a year, "not corrosion proof, but useful in certain cases"; and 2, over 1.0 mm a year, "serious corrosion. The material is not usable."
In plain sulphuric acid, the 316L grade rates:
10% acid at 60 °C: 1. Marginal.
10% acid at 80 °C: 2. Not usable.
20% acid at 50 °C: 1.
20% acid at 60 °C: 2. Not usable.
A copper electrolyte sits between the 10% and 20% columns at 60 to 65 °C. Read on its own, this table would rule out 316L for a copper tankhouse.
What changes when the acid carries copper
The same publisher has a table for sulphuric acid with copper sulphate, and the picture reverses. For the 316L grade:
16% acid with 13% copper sulphate at 90 °C: 0. Corrosion proof, at a temperature 25 to 30 degrees above the tankhouse, and with somewhat more copper than a typical electrolyte (13% against about 8 to 11%).
10% acid with 10% copper sulphate, boiling: 0.
16% acid with 12% copper sulphate at 120 °C: 1. The margin starts to close at very high temperature.
A 911 Metallurgist article on corrosion control in copper SX-EW plants states that oxidising salts such as cupric and ferric sulphate "exert a corrosion-inhibiting effect" on stainless steels in sulphuric acid and "help maintain the passive film past the normal range of resistance". It reports work by Kiefer and Renshaw, who added a series of metal sulphates to 5% and 30% sulphuric acid and tested types 304 and 316 at 100 to 200 °F, finding that "higher valence sulphates are extremely effective even at low concentrations", that the effect weakens as temperature rises, and that "none provides complete inhibition". An independent source points the same way from a different industry. A US patent on urea and sulphuric acid fertiliser solutions states as known background that "a wide variety of compounds, including cupric sulfate, reduce sulfuric acid corrosivity toward stainless steels", adds cupric ion, generally at least about 50 ppm, to its own solutions for that purpose, and notes that more is needed at higher temperatures and flow rates.
The field evidence is the cathode. Glencore offers its permanent stainless steel cathode in 316L, LDX 2101 duplex and a newer alloy aimed at "high-chloride and high-current-density applications". The ISA PROCESS patent describes the cathode plate as fabricated from 316L, with under 0.03% carbon and 2 to 3% molybdenum, the molybdenum giving "higher resistance to pitting and crevice corrosion in acidic environments". The cathode corroborates this, though it is not proof: the blade sits in the same electrolyte, but it is cathodically polarised and covered in copper for much of the cycle, which a filter is not.
The practical reading is the one that matters for a filter: 316L in a copper electrolyte depends on the copper. Change what the steel is exposed to, and you are reading a different table.
What takes the margin away
Temperature
Kiefer and Renshaw found the inhibiting effect of the sulphates "becomes less pronounced" as temperature rises, and Alleima's rating for 316L moves from 0 at 90 °C to 1 at 120 °C in comparable copper-bearing acid. By our reading of Alleima's laboratory data, a tankhouse at 60 to 65 °C sits inside the range where 316L rates 0. The cases to look at are the hot ones: heat exchanger outlets, any electrolyte heated above the normal set point, and hot cleaning solutions.
Acid that has lost its copper
This is the case most specifications miss. In Alleima's copper-bearing table, at 8% acid with only 0.05% copper sulphate at 80 °C, about a two-hundredth of a working electrolyte's copper, the 316L grade still rates 0, but 304L-type stainless rates 2, and so does commercially pure titanium. At low copper, 316L holds where 304L and titanium do not.
Remove the copper entirely and the steel is back on the plain acid table, where 10 to 20% acid at 60 °C rates 1 to 2. That is our reading of the two tables together, and it applies to any stream the filter sees that carries acid without copper: an acid make-up line, a decopperised bleed, or an acid wash. We do not suggest that every plant runs such a stream through its filter. We do suggest the question is asked before the material is fixed, because the filter's material should be chosen for the worst stream it will see, not the normal one.
Chloride
Chloride is something a refinery tankhouse doses deliberately, and Glencore flags it directly: "Excess chloride attacks stainless steel cathode blanks and must be kept within specification." A patent on preventing cathode corrosion in copper electrowinning, which tested a 316L cathode in electrolyte carrying 80 to 200 ppm chloride, tentatively attributes the attack, which occurred in the splash zone just above the liquid, to free chlorine evolved from the chloride.
A pressure filter has no anode, so that specific route is a cell mechanism. What carries over is the general behaviour of chloride in sulphate solutions. A 2022 study by VTT, Outokumpu, Metso and others tested 316L and two other grades in 1% and 10% sulphuric acid with 500 to 10,000 mg/L sodium chloride, from 22 to 130 °C. According to its abstract, sulphate acts as a corrosion inhibitor, pitting was the main attack in 1% acid with chloride while uniform corrosion dominated in 10% acid, and no pitting occurred where the chloride to sulphate activity ratio was below 10. Converted from sodium chloride to chloride, those levels run from about 6 to 300 times a refinery's 20 to 50 mg/L (our arithmetic), and the solutions held no copper, so the study does not clear a copper electrolyte. Our reading: at 20 to 50 mg/L chloride in a 13 to 17% acid electrolyte, the bulk ratio sits far below the study's pitting threshold, so attention goes to places where local chemistry departs from the bulk.
The 911 Metallurgist article names those places and the mechanism, and it ties straight back to the copper. At flanged joints and incompletely welded attachments, "corrosive media penetrate between the surfaces to form a stagnant area or pocket, resulting in a depletion of inhibiting copper salts", which "typically results in failures under gaskets at flanged joints, and under sludge or slimes settling in tanks". It also warns that "accumulation of solids on metal surfaces is conducive to pitting". A crevice is a small pocket of acid that has lost its copper. Our reading for a filter: gasketed closures, the joints between leaves and manifolds, and any solids that settle on bare metal outside the cloth or in dead zones are where to look first. Full cake discharge, clean crevice geometry and gasket faces that do not trap liquor are materials decisions as much as mechanical ones.
Welds
The solution a copper refinery circulates belongs to the same family of liquids that metallurgists use to find bad welds. ASTM A262 Practice E, the "Copper-Copper Sulfate-Sulfuric Acid Test for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels", detects attack associated with chromium carbides precipitated at grain boundaries. The 16% version boils the specimen in a copper sulphate solution with 16% sulphuric acid and then bends it to look for cracks. 911 Metallurgist states it more bluntly: hot copper sulphate and sulphuric acid solutions "strongly promote intergranular corrosion".
A filter vessel at 60 to 65 °C is not a boiling test, and it is not in contact with metallic copper as the Practice E specimen is. The point is the family of solution, not equivalence: if a weld's heat-affected zone has been sensitised, a hot copper electrolyte is the kind of liquid that can find it. The same 911 Metallurgist article gives three ways to avoid it: "heat treating equipment after weld fabrication, by using low-carbon stainless steels (such as types 304L and 316L), or by employing stabilized stainless steels." On the low-carbon route it is plain: "In low-carbon grades containing a maximum 0.03% carbon, there simply is not enough carbon available to produce a damaging amount of chromium carbide." Which route a fabricator takes, and the welding procedure, heat input and post-weld cleaning that go with it, belong in the specification next to the grade.
Titanium and nickel alloys are not automatic upgrades
A materials ladder that runs from 304 to 316L to titanium to nickel alloy is a poor guide in this service. Titanium shows why. In Alleima's tables, commercially pure titanium rates 2 in plain 10% sulphuric acid at 60 °C and 2 in low-copper acid at 80 °C, yet 0 at 16% acid with 13% copper sulphate at 90 °C. A 2014 electrochemical study in Corrosion, in work aimed at pressure-leach autoclaves, found that cupric, ferric and chloride ions all "facilitated the anodic passivity of titanium in sulfuric acid" up to 85 °C. Titanium, like stainless, depends on the oxidising species in the stream. It is the right choice for some duties and the wrong one for a copper-free acid at temperature.
The same logic runs the other way for high chloride. Where chloride climbs well above the normal tankhouse range, or where a leach or SX circuit brings it in, the case for a higher alloy is made by the chloride and temperature, and the choice is made per duty. The SX-EW duty map covers where those streams sit in a leach circuit.
What to put in the enquiry
The material can only be chosen for the stream described. These are the lines that change the answer:
Free sulphuric acid and copper concentration, normal and range.
Chloride, normal and the highest excursion the plant has seen.
Temperature, normal and maximum, including any heated stream. (Temperature also drives filter sizing through viscosity, covered in the viscosity article.)
Every stream the filter will see apart from electrolyte: acid make-up, bleed, wash or cleaning solutions, and whether any of them carry acid without copper.
Iron, and whether it is present as ferric (another oxidising species), plus any other impurity the plant tracks.
Solids handling: whether cake is fully discharged each cycle or solids can sit on internals between cycles.
Welding and fabrication requirements the plant's own standards impose.
What we could not establish
A corrosion rate in an actual copper electrolyte. The Alleima ratings are laboratory data in pure chemicals, in aerated solution, on annealed material with clean surfaces; they rate general (uniform) corrosion only, not pitting, crevice or intergranular attack; and Alleima says plainly that laboratory tests "are not strictly comparable with actual service conditions". No table here is a design allowance.
The full text of the 2022 chloride study. We worked from its abstract.
Kiefer and Renshaw's original paper. We read their findings as reported by 911 Metallurgist, and the direction is consistent with the Alleima tables and the fertiliser patent.
A published chloride limit for 316L in a copper ER electrolyte. Glencore says excess chloride must be kept "within specification" without giving the number.
Where Sharpenn fits
Sharpenn builds pressure filters for electrolyte clarification, running in leaf (body-feed) or precoat mode. SS316 is the baseline wetted material for our leaves and internal manifolds. Our welded parts are usually made in 316L, the low-carbon grade, with the grade depending on the material selected for the duty, and we have built in SS316L, Hastelloy and titanium where a duty calls for them. Which one is right follows the electrolyte's chemistry, temperature, chloride and the other streams the filter will see, which is why we ask for them. Our record: ~1,000 units installed worldwide across 460+ installations (all applications); 32 units in copper refining; capacities 5 to 350 m³/hr; repeat orders after 20 to 25 years. For the wider picture of the duty, see the guide to electrolyte filtration in copper refineries.
If you are specifying an electrolyte filter, send us the chemistry above, including the streams other than electrolyte, and we will tell you what we would build it in and why.
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