Paul Industries carries out corrosion assessment, materials consultation and passivation for South Dakota ethanol plants. These plants have a corrosion problem they largely created for themselves, and they created it by doing something sensible. Recycling thin stillage saves water and energy, and it also concentrates chloride in a closed loop until the process water is considerably more aggressive than anything that entered the plant. Nobody specified that environment. It emerged.
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How a water-saving measure becomes a corrosion problem
Chloride enters an ethanol plant from several directions: the incoming water, the corn itself, process chemicals, and pH adjustment where hydrochloric acid is used. Some of it leaves with the ethanol, very little; some leaves with the dried co-product. The rest stays in the water, and the water is recycled.
So the loop reaches a steady-state chloride concentration determined by how much comes in and how much leaves, and the higher the backset rate, the higher that steady state. A plant drawing water at a modest chloride level can be running process streams at a multiple of it. As set out on our South Dakota water page, raising backset to save water and energy raises this concentration as a direct consequence, and the two decisions are usually taken independently by people looking at different numbers.
That produces the characteristic failure of this industry: equipment specified in a grade that was entirely appropriate for the water analysis on file, corroding in service, with nobody able to point to what changed. What changed is that the plant got better at recycling.
Two mechanisms, and the temperature decides which
Chloride attacks stainless in two distinct ways and an ethanol plant provides conditions for both, in different places.
Pitting and crevice corrosion occur across the temperature range and concentrate wherever solution stagnates or where geometry prevents exchange with the bulk: under gaskets, in dead legs, beneath deposits, at lap joints and around fixings. This is what damages stillage handling, tanks, and anything with a crevice in it.
Chloride stress corrosion cracking needs tensile stress, chloride and temperature together, with roughly 60 degrees Celsius conventionally treated as where the risk becomes a serious design consideration for austenitic grades. An ethanol plant has plenty of places above that: the cook system, beer column bottoms, the evaporator train, and hot stillage lines. Where cracking occurs it produces through-wall cracks in metal at full thickness, which thickness monitoring will never find, exactly as described on our Oklahoma corrosion page.
The practical consequence is that one plant needs two different inspection approaches. Thickness measurement on the cold, crevice-prone equipment. Crack detection at welds and heat-affected zones on the hot equipment. A program applying one method everywhere will miss half of what is happening.
| Equipment | Likely mechanism | Inspection method |
|---|---|---|
| Evaporator train | Cracking, plus concentration under deposits | Crack detection at welds; deposit removal first |
| Beer column and bottoms | Cracking | Crack detection, heat-affected zones |
| Hot stillage lines | Both | Both; and check insulation condition |
| Fermenters and cold tanks | Pitting and crevice | Thickness, gasket faces, fixings |
| Centrifuges and handling | Pitting plus erosion | Thickness at wear points |
| Under insulation on hot lines | Cracking, from concentrated external chloride | Remove a sample of insulation and look |
The last row is worth a specific note. External chloride reaching a hot insulated line, from a leak, from washdown or from the insulation itself, concentrates by evaporation at the metal surface, and the line is hot enough for cracking. That is the same under-insulation mechanism described on our Oklahoma page, and on an ethanol plant it is easy to overlook because attention is on the process side.
What to do about it
Measure the loop, not the inlet. This is the first and cheapest action. Chloride concentration measured in the actual recycled process streams, at the points where equipment is failing, gives the environment the material is really in. Plants specifying replacements from the incoming water report are specifying for a condition that exists nowhere in the plant.
Treat backset rate as a materials decision as well as a water one. If a plant is pushing recycle to cut water, it is changing the corrosion environment, and that should be a conscious trade with the maintenance budget in the room rather than an unintended consequence.
Select material against the measured environment and the temperature. Where 304 has failed, 316 is a step and not always a sufficient one. For genuinely aggressive hot chloride service, duplex grades resist both pitting and stress corrosion cracking considerably better than austenitics, as set out on our Washington alloy page, and they require welding procedures matched to the material rather than borrowed from 316 practice.
Passivate, and understand its limit. Cleaning per ASTM A380 and passivation per ASTM A967 removes free iron and weld heat tint and restores the film. In this environment that matters, because heat tint and embedded iron are exploited quickly by chloride. It does not raise the alloy’s inherent resistance, and it does not address cracking at all, which needs stress relief or a material change. We would rather say that than sell a treatment for a mechanism it cannot touch.
Passivate after every repair, without exception. Every weld and grind in this plant reintroduces exactly the defects the environment punishes, and post-repair treatment belongs in the maintenance procedure rather than the construction one.
| Heating load | Per treatment | Per 20 treatments |
|---|---|---|
| 30 kW | $19.87 | $397 |
| 60 kW | $39.74 | $795 |
| 120 kW | $79.49 | $1,590 |
At 8.28 cents per kilowatt-hour, essentially the national average of 8.13 (EIA, 2024), treatment energy is immaterial. The governing cost is plant availability, since most of this work happens inside equipment during a shutdown, competing for the same window as everything else.
We carry out corrosion surveys using the right method for each area rather than one method everywhere, loop chemistry assessment, materials consultation for replacement decisions, in-place and shop passivation, heat tint removal, and fabrication in duplex and higher alloys with procedures matched to the material.
Standards referenced: ASTM A380 · EIA electricity price data · ASTM A967 · ASME BPE
Frequently asked questions
Do you handle corrosion work at South Dakota ethanol plants?
Yes, across Sioux Falls, Aberdeen, Watertown, Mitchell, Huron and statewide: corrosion surveys matched to each mechanism, loop chemistry assessment, materials consultation, in-place and shop passivation per ASTM A967 after cleaning per ASTM A380, heat tint removal, and fabrication in duplex and higher alloys.
Why do ethanol plants have a chloride problem?
Because recycling thin stillage concentrates it. Chloride enters from water, corn, process chemicals and any hydrochloric acid used for pH adjustment, very little leaves with the ethanol, and the rest stays in a recycled loop until it reaches a steady state well above the incoming level.
Why does this catch plants by surprise?
Because equipment was specified from the incoming water analysis, which describes a condition that exists nowhere in the process. Nothing appears to have changed except that the plant got better at recycling, and the corrosion environment moved without anyone deciding it should.
Is it one corrosion mechanism or two?
Two. Pitting and crevice attack occur across the temperature range wherever solution stagnates or geometry prevents exchange. Stress corrosion cracking needs stress, chloride and temperature together, and the cook system, beer column bottoms, evaporators and hot stillage lines all provide it.
Does that change how we inspect?
Substantially. Thickness measurement on cold crevice-prone equipment, crack detection at welds and heat-affected zones on hot equipment. A program applying one method everywhere misses half of what is happening, and thickness monitoring will never find cracking.
What is the first thing we should do?
Measure chloride in the actual recycled process streams at the points where equipment is failing. It is inexpensive and it gives the environment the material is really in, which is the basis every subsequent decision depends on.
Should we reduce our backset rate?
Possibly, and it should be a conscious trade rather than an accident. Raising recycle to cut water and energy raises loop chloride as a direct consequence, so that decision belongs in a conversation with the maintenance budget in the room.
Is 316 stainless sufficient?
Sometimes, and it is a step rather than a guarantee. For genuinely aggressive hot chloride service, duplex grades resist both pitting and stress corrosion cracking considerably better than austenitics, and they need welding procedures matched to the material rather than borrowed from 316 practice.
Can passivation solve this?
It helps and it has limits. Removing free iron and weld heat tint matters here because chloride exploits both quickly. It does not raise the alloy’s inherent resistance and it does nothing for cracking, which needs stress relief or a material change.
How do I get a quote for South Dakota corrosion work?
Use the form on this page or call 201-450-8280. Useful inputs are measured loop chloride and operating temperatures by area, current backset rate, which equipment is failing and how, existing materials, and what inspection has been done and by which method.
Which duplex grades are used for ethanol evaporators?
Lean and standard duplex grades for most stillage and evaporator service, with super duplex where chloride and temperature are highest, because their pitting resistance and strength exceed austenitic grades at a cost between 316 and nickel alloys. Duplex has become the usual evaporator tube material at high-backset plants.
Where is chloride highest in the plant?
In thin stillage, backset and the evaporator feed and concentrate, where recycling concentrates it, and in the evaporator syrup. Equipment in those streams sees the most aggressive water in the plant.
How is loop chloride measured and tracked?
By sampling backset and thin stillage for chloride on a schedule and trending against backset rate and incoming water quality. The trend shows whether a process change has raised the concentration.
Does microbial corrosion occur in ethanol plants?
Yes, in stillage and cooling water systems where bacteria form deposits under which local chemistry becomes aggressive, producing pits that look like chloride attack but follow the biofilm. It is identified by deposit analysis, and it responds to biocide and cleaning rather than to alloy alone.
What corrosion occurs in distillation?
Organic acids and carbon dioxide in the beer column and rectifier condense at the top and attack carbon steel and low-grade stainless, while the hot bottoms carry the loop's chloride. Distillation equipment sees both acid and chloride, and its materials are chosen for each section.
How do gaskets and elastomers degrade in the stillage loop?
Ethanol, hot water and organic acids swell and harden common elastomers, and chloride attacks the metal beneath a failed gasket, so gaskets are specified for the chemistry and inspected as part of the corrosion programme. A leaking gasket is often where a pit is found.
What corrodes in centrifuges and dryers?
Centrifuge bowls and internals see hot, chloride-bearing stillage at high velocity, combining erosion and corrosion, and dryer internals see hot, acidic, wet material and condensation. Both are inspected for pitting and thinning on a short interval.
How are corrosion coupons and probes used in the loop?
By exposing coupons of candidate alloys at points in the stillage and evaporator system and weighing and inspecting them on a schedule, and by installing electrical resistance or linear polarisation probes for continuous rates. The data supports alloy decisions with the plant's own conditions.
How does incoming water quality affect the loop?
Chloride in the well or municipal supply enters the loop and accumulates, so a plant on high-chloride supply reaches aggressive concentrations at a lower backset rate. Supply chloride is part of the balance.
How does cooling tower chloride relate to the process loop?
Cooling towers concentrate the make-up water's chloride by their cycles of concentration, and where process water or condensate serves as make-up, the loop's chloride enters the tower too. Tower chemistry and the process loop are managed together, because they share the plant's chloride.
How is corrosion under insulation handled here?
Hot insulated stillage and evaporator piping is at risk of external chloride cracking as well as internal attack, and inspection at insulation breaks and sealed jacketing apply. Both sides of the pipe are at risk.
What about carbon steel equipment in the plant?
Carbon steel in stillage and syrup service corrodes generally and is protected by coatings or replaced with stainless where the duty demands. The mechanism differs from stainless and so does the fix.
How is spent passivation chemistry handled?
Through the plant's effluent treatment after neutralisation. Ethanol plants have treatment for their process streams and the small passivation volume fits within it.
How is a materials upgrade prioritised?
By failure history and by the chloride and temperature at each location, so that the hottest, most concentrated sections are upgraded first and cooler sections are left. Upgrading everything is unnecessary.
What is the commonest corrosion finding at South Dakota ethanol plants?
Stainless evaporator and hot stillage equipment pitting and cracking under chloride that accumulated as the backset rate rose, in equipment specified before the loop chemistry changed. The water analysis explains it.
Does citric passivation help a plant with recycled process water?
It removes free iron and restores the passive layer, which raises the chloride concentration needed to start a pit. It does not lower the chloride in the water. On an ethanol plant that recycles to save water, citric passivation buys margin while the water chemistry keeps eroding it.
Citric or nitric where chloride has already caused damage?
Chemistry choice is the smaller question. Existing pits have to be assessed first, because passivating over an active pit treats the surface around a defect that is still growing. Where the surface is sound, citric is the practical in-place treatment. Where it is not, the answer is repair or material change before any passivation.
Unexplained corrosion at a South Dakota ethanol plant?
Send measured loop chloride rather than the incoming water report, with the temperature where the failure is. Those two usually name the mechanism. Call 201-450-8280 or use the form below.