Electrowinning Cathode Plates Compared: Stainless Steel, Titanium and Lead Alloy

The cathode is where every gram of recovered metal finally lands, which makes it the most consequential consumable in an electrowinning cell. Choose the wrong plate and you pay twice: once in current efficiency, and again in cathode purity — iron from a dissolving plate is invisible in the tankhouse until a buyer rejects a shipment. This note compares the three plate materials we build and ship most often and, more usefully, explains which failure each one is prone to.

The comparison is often framed as a price question. It is really a question about the electrolyte and the campaign. A stainless blank that costs a fraction of a titanium one is the correct answer for a three-month copper pilot and the wrong answer for a nickel circuit, not because of the metal price but because of chloride, deposit adherence and how long the plate has to survive. Lead belongs in the same conversation only as an anode.

The three plate materials side by side

What the plate actually has to survive

In the cell the cathode is the working electrode: copper, zinc or nickel ions are reduced and deposit on its surface. Everything about the plate — surface condition, flatness, electrochemical stability — controls how that deposit grows, and the deposit is not passive: it grips the plate, carries internal stress, and is pulled off by hand or machine at the end of every cycle. The plate must survive that hundreds of times without bowing, cracking at the hanger bar or shedding particles.

The failure that ends plate life: warping and edge growth

Most stainless blanks are not retired because they dissolved. They are retired because they bowed. A plate that warps closes the gap at one point and opens it elsewhere, and current follows the gap: the close point runs hot, the deposit grows faster, nodules form, and eventually a nodule touches the anode. That short circuit costs current efficiency immediately and can burn a hole through the plate. The usual starting point is above roughly 250 A/m², where deposit stress and plate heat exceed what a thin blank can hold flat.

Edge growth is the second mechanism. Deposit creeping past the masked edge bridges the gap and creates a short at the least accessible place in the cell. On titanium mother plates this is handled with edge strips that overhang the deposition zone by at least 10 mm on each side; on stainless, with wax or a bonded mask, renewed every cycle. Where the mask is damaged, the deposit wraps the edge and the plate is effectively lost.

Surface condition feeds both. A plate that has been mechanically damaged, contaminated with iron by a grinder or a steel wire brush, or left with a rough weld will plate unevenly and strip badly. The surface treatment note covers pickling, passivation and finish options and what each one does to release behaviour.

Chloride and nickel: where a stainless blank stops

For laboratory and pilot copper and zinc cells, 316L is what we recommend in most cases. The molybdenum-bearing grade resists acidic sulphate electrolyte far better than 304, strips cleanly after a light acid pickle, and costs the least of the three. Its limit is chloride: the working guideline is around 30 ppm, above which pitting starts at the waterline, under the mask and at any scratch. If your electrolyte carries a deliberate chloride addition for deposit control, count it in the total before choosing the blank.

Titanium earns its premium in three situations. High-chloride or mixed sulphate-chloride electrolytes, where stainless pits within days. Nickel electrowinning, where titanium mother plates with edge strips are the industry standard. And long demonstrations, where replacing blanks mid-campaign costs more than the titanium did on day one. Its oxide film is an excellent barrier to dissolution, so the plate contributes essentially zero contamination; the trade-offs are cost and a lower modulus, which makes flatness and hanging design more critical. Our stainless steel and titanium cathode plate pages list standard sizes.

Lead belongs on the anode

Lead alloys — Pb-Sn-Ca or Pb-Ag — belong on the anode side of the cell, where they form the PbO₂ layer that releases oxygen and slowly consumes itself. Put lead on the cathode and you contaminate the deposit directly: lead reports into the metal you are trying to sell. Cathode specifications for high-grade copper cap lead at a few parts per million, a level a lead cathode will exceed in a single campaign. If a flowsheet calls for “lead cathodes”, what it almost certainly needs is a lead anode paired with a stainless or titanium cathode.

Lead anodes have their own costs. They corrode into the electrolyte, generate sludge that has to be removed and disposed of, and contribute lead-bearing mist to the tankhouse atmosphere. An MMO-coated titanium anode avoids all three in laboratory work: no lead in the tankhouse, no sludge management, and a dimensionally stable surface. The coating is sensitive to fluoride and to current reversal, but for a pilot cell it removes a whole category of contamination risk.

How current density and circuit design move the boundary

Current density decides how hard the plate works. Copper EW runs at roughly 180–250 A/m²; above that the flatness requirement tightens quickly and the plate becomes the limiting item rather than the rectifier. Sizing follows Faraday’s law: at 200 A/m² and 100% current efficiency, one square metre of cathode recovers about 0.24 kg of copper per hour, or around 5.7 kg per day. Our current density and production calculator does the arithmetic for other metals and efficiencies.

Circuit design moves the boundary as much as the material. Keep 20–30 mm between anode and cathode faces; tighter gaps raise cell voltage sensitivity to sludge and misalignment and turn a small warp into a short. Busbar contact resistance at the hanger bar matters more than most operators expect, because one warm contact redistributes current across the cell and overloads the nearest plates. Acid mist is the other variable: oxygen bubbles burst at the surface and carry acid into the freeboard, where it attacks hanger bars, busbars and the top edge of every plate.

Cost structure across the campaign

Relative plate cost is roughly 1 : 3–5 for stainless : titanium, with lead anodes at 2–3 times a stainless blank in the same size. Per plate, stainless wins almost every comparison. Per campaign, it depends on what the plate costs in service: stripping labour per cycle, pickling chemicals, blanks consumed, and the value of a rejected cathode batch. A titanium plate lasting thousands of cycles against a stainless one lasting hundreds changes the arithmetic beyond about a year. Add the cost of a shutdown to change a set of blanks and the comparison usually resolves itself; see what actually drives equipment cost.

Selection sequence

1. Name the metal and the electrolyte: copper or zinc sulphate, nickel sulphate, or a chloride-bearing leach liquor.
2. Get the chloride and fluoride numbers, including any deliberate chloride addition for deposit control.
3. Fix the current density and the anode-cathode gap, then check what flatness that demands.
4. Estimate campaign length and the number of strip cycles the plate must survive.
5. Decide the stripping method — manual, mechanical or automated — because it sets the plate’s stiffness and surface finish.
6. Choose the anode and work out where its corrosion products go.
7. Size the plate area from the production target, then confirm with a pilot cell that counts actual strip cycles.

FAQ

Can I use a stainless cathode in a chloride leach liquor?

Not for long. The working guideline is around 30 ppm chloride, and a chloride leach liquor is usually one to three orders of magnitude above that. Pitting starts at the waterline, under the edge mask and at scratches, and once a pit penetrates the plate the electrolyte sees iron. If the liquor is chloride-bearing, titanium is the honest answer; if the chloride is only an occasional upset, keep a stainless set as spares and monitor the waterline.

Why not use lead cathodes if they are cheaper?

Because lead on the cathode side contaminates the product. A lead surface in cathodic contact is not stable, and lead reports into the deposit; cathode specifications for high-grade copper cap lead at a few parts per million. Lead alloys belong on the anode, where the PbO₂ layer is stable and releases oxygen. A “lead cathode” flowsheet almost always means a lead anode with a stainless or titanium cathode — worth checking before any plates are ordered.

How long does a 316L blank actually last?

Expect hundreds of cycles rather than thousands, with the exact number set by current density, chloride, stripping method and how carefully the edges are masked. For a pilot campaign of a few months the plate outlives the project, which is why stainless is the right first choice. For continuous operation the arithmetic changes, and the cost of replacing a set of blanks mid-campaign usually justifies titanium on the second order rather than the first.

Does a titanium plate need edge strips?

Yes, on mother-plate duty. Deposit that wraps the edge bridges the anode-cathode gap and shorts the cell, and titanium is too expensive to risk that way. Strips should overhang the deposition zone by at least 10 mm on each side and be inspected every cycle, because a lifted strip lets deposit creep underneath. On a laboratory plate the same rule applies at smaller scale — it is the geometry that matters, not the size.

What plate area do I need for a given production rate?

Work from Faraday’s law. At 200 A/m² and 100% current efficiency, one square metre of cathode recovers about 0.24 kg of copper per hour, roughly 5.7 kg per day; nickel and zinc differ because of valency and atomic weight. Laboratory cells typically use 0.01–0.03 m² plates, so expect tens of grams to a few hundred grams per day. Allow for efficiency below 100% and note that only one face of a plate is usually counted.

Related reading

• Copper electrowinning pilot, 10–50 kg/day: reference configuration
• Low current efficiency in electrowinning: the five losses you can measure
• Cathode plates: stainless and titanium
• Buying small quantities of titanium cathode plates