Technical Article · Pickling Bath Management
The single most important diagram in pickling chemistry — what it shows, what it means operationally, and how to use it to control bath performance every shift
Most galvanizing operators track either acid strength or iron content in their pickling baths — rarely both at the same time, and almost never in relation to each other. That is the single biggest reason pickling lines underperform: the two parameters interact, and their combined position on the Kleingarn diagram determines whether the bath is working at full efficiency, limping along at 50% capacity, or, in the worst case, physically incapable of pickling at all.
The Kleingarn Curve — published by J.-P. Kleingarn in 1988 based on research into HCl pickling kinetics — maps this relationship. Once you understand what it shows, you will never manage a pickling bath the same way again. This article explains every element of the diagram, the four operating zones, the paradox of the fresh bath, the trajectory of a bath through its entire service life, and how to use it as a daily operational tool.
The Kleingarn diagram is a two-dimensional plot with HCl concentration (g/L) on the horizontal axis and dissolved iron (Fe²⁺) content (g/L) on the vertical axis, calibrated at 20°C. A secondary vertical scale on the right side expresses the same iron content as dissolved FeCl₂ (g/L) — because some laboratories report iron in the ionic form (Fe²⁺) and others in the compound form (FeCl₂). The two scales represent exactly the same physical quantity; always confirm which unit your laboratory reports before plotting your bath readings.
The horizontal axis runs from 0 to 180 g/L HCl and the vertical axis from 0 to about 240 g/L Fe. The acid axis stops at 180 g/L deliberately: that is where the published 50 % line ends, and no galvanizing pickle is run above it. Every point on the chart represents a possible bath composition. The diagram draws boundaries on that space, dividing it into regions of high pickling efficiency, low efficiency, and physical impossibility.
Pickling efficiency is not determined by HCl concentration alone, nor by iron content alone. It is determined by the combination of both — and where that combination falls on the Kleingarn diagram. This is the central insight the diagram encodes.
Four reference lines divide the HCl–Fe space into operational zones. Understanding each line is essential before the diagram can be used as a management tool.
This is the physical hard limit of the bath. Above this line, the solution is saturated with ferrous chloride (FeCl₂) and cannot dissolve any further iron. Pickling does not just slow down above this line — it effectively stops. The saturation line descends from left to right: a bath with low HCl can hold more dissolved iron before saturating than a bath with high HCl. A bath that has crossed the saturation line must be discharged and renewed or externally regenerated; no addition of fresh acid alone will restore it.
This curve marks the combination of HCl and Fe²⁺ that gives the minimum pickling time at 20°C — the fastest, most efficient bath condition. Like the saturation line, it descends from left to right. The optimum line is the target every operator should aim for, but in practice, any operating point within the shaded band on either side of it is acceptable.
The two grey curves bound the practical target zone — the shaded band on the chart. On either of them, pickling takes approximately 50% longer than at the optimum line; the band between them is where a working tank should sit. The lower line is where iron is too low for the acid strength: the catalytic Fe²⁺ has not built up, and below it pickling time climbs steeply. The upper line is where iron has built up too far: pickling slows as the bath heads for saturation, and above it the margin is gone. Notice that the upper line does not reach the left edge of the chart — it begins at about 45 g/L HCl, where it converges on the saturation line; at very low acid the band is capped by saturation itself. Between the upper grey line and the green saturation line there is a visible gap: that strip is not efficient pickling, it is a bath running out of margin.
The saturation, optimum and lower +50 % curves are the exact values from the AGA Pickle Bath Maintenance Calculator. That workbook tabulates only the lower 50 % line, so the upper branch is not an AGA value: it is the same digitisation the HDG Community Pickling Optimizer has always carried, and both tools and this page now draw the identical set. It belongs on the diagram for a physical reason — pickling time rises on both sides of the optimum, so a +50 % contour must have an upper branch; a single lower line would imply that pickling never slows by 50 % until the bath actually saturates, which is not what happens in a tank. Read the two lines as the band's edges, and the green line as the wall beyond them.
Notice that the shaded band narrows toward high HCl concentrations. Measured between the two grey lines it spans about 106 g/L of iron at 45 g/L HCl, where the upper line begins, but only about 64 g/L at 180 — so a plant running strong acid has a tighter operational margin before it drifts outside the efficient zone. The practical consequence: it is easier to hold good pickling conditions in the moderate-HCl, moderate-Fe region of the diagram than at the extremes.
| HCl (g/L) ≈ 110 | Fe Content (g/L) | Zone | Pickling Status |
|---|---|---|---|
| Reading the zones at a fixed HCl of ~110 g/L | |||
| ~110 | Below ~47 | Low Iron / Excess Acid | Below the band — 50%+ longer than optimum |
| ~110 | 47–104 | Inside band, below optimum | Improving as iron builds |
| ~110 | ~104 | On the optimum line | Minimum pickling time |
| ~110 | 104–122 | Inside band, above optimum | Efficient, but iron is now the variable to watch |
| ~110 | 122–145 | High Iron — above the upper +50% line | Margin shrinking. Dilute or regenerate |
| ~110 | Above ~145 | At/above saturation | Pickling stops. Bath must be renewed |
These iron thresholds are approximate and shift with HCl concentration — always read from the diagram at your actual HCl value, not from this table alone.
This is the single most counterintuitive insight that the Kleingarn Curve reveals, and the one most often overlooked in plant practice: a freshly prepared acid bath is not an efficient pickling bath.
When you fill a tank with fresh commercial HCl (typically 30–32% strength, diluted to 140–180 g/L working concentration), the operating point sits at the bottom right of the diagram — high HCl, very low Fe²⁺. That point is below the lower grey boundary, in the zone of extended pickling time. The bath is aggressive, but it pickles slowly.
Pickling efficiency improves as iron accumulates — not despite it. The mechanisms are well understood:
| Factor | Explanation |
|---|---|
| Ionic strength | Elevated Fe²⁺ increases bath conductivity, accelerating the electrochemical pickling reaction at the steel surface. |
| Oxide dissolution | Fe²⁺ participates directly in the reduction of iron oxides — particularly magnetite (Fe₃O₄) — making descaling faster and more complete than with free HCl alone. |
| Hydrogen interference | In very fresh acid, aggressive H₂ gas evolution at the steel surface can physically impede acid contact. A conditioned bath with moderate Fe²⁺ reduces this effect. |
| Electrochemical equilibrium | As Fe²⁺ builds toward the optimum zone, the bath reaches a chemical equilibrium that maximises the dissolution rate per unit of acid consumed. |
The most productive pickling bath carries a controlled, moderate iron load — typically 60–120 g/L Fe²⁺ with HCl held between 90 and 140 g/L. This is why experienced operators never discard mature pickle prematurely, and why cascade tank arrangements — where iron-loaded acid handles first-stage scale removal — extract the most surface area per litre of acid purchased.
A secondary risk of the fresh-acid, very-low-iron zone is relevant for certain steel types: highly aggressive acid with virtually no dissolved iron can cause surface over-attack on reactive steels and excessive hydrogen evolution, creating hydrogen embrittlement risk for high-strength fasteners and cold-worked structural components.
One of the most powerful — and most overlooked — features of the original Kleingarn diagram is a diagonal dashed line running from the lower-right toward the upper-left. This line traces the natural trajectory of a pickling bath through its entire operational life, from first use to exhaustion. Every galvanizing bath follows this path.
This trajectory has a direct operational implication: a tank that is tracking diagonally up-and-to-the-left over consecutive readings is behaving normally. A tank that is tracking straight upward at near-constant HCl — Fe rising without HCl falling — is a signal that poor rinsing is causing pickle to be carried into the rinse, then carried back into the next tank in the cascade. That is a process problem requiring investigation, not just acid addition.
The standard Kleingarn curve is calibrated for 20°C — ambient temperature operation. Many modern galvanizing plants heat their pickling baths to 30–40°C to accelerate throughput. Temperature elevation has two important effects:
| Effect | Mechanism | Operational Consequence |
|---|---|---|
| Saturation line shifts upward | Higher temperature dissolves more FeCl₂ before saturation. The bath can hold more iron before its pickling capability degrades. | The usable operating window expands. The 20°C saturation limit no longer applies — actual limits are higher at elevated temperatures. |
| Pickling rate doubles per 10°C | Arrhenius kinetics: the rate of acid attack on iron oxides approximately doubles for every 10°C temperature increase. | A bath at 30°C pickles roughly twice as fast as the same bath at 20°C. Immersion times can be halved, or throughput roughly doubled, for the same pickling depth. |
Above 25°C, HCl fume generation increases significantly. Above 35°C, fume rates become excessive for most installations — mandatory fume extraction and scrubbing become essential, not optional. Temperature gains in pickling speed must be weighed against ventilation cost, HCl loss from the bath, and operator exposure. Plants operating heated acid must also use temperature-corrected Kleingarn curves, or empirical plant data, as the 20°C standard diagram underestimates the achievable operating window.
The Kleingarn diagram is only useful if operators have reliable, up-to-date measurements to plot on it. Continuous online monitoring is rare in batch galvanizing; most plants rely on a combination of proxy measurements and periodic laboratory analysis.
| Method | What It Measures | Accuracy | Recommended Frequency |
|---|---|---|---|
| Density (Baumé / g/cm³) | Combined proxy for HCl + FeCl₂. A hydrometer reading correlated to the nomogram gives a combined indication of bath strength. | Proxy only — cannot separate acid from iron | Every shift / daily |
| Acid titration | Free HCl concentration (g/L), measured by neutralisation titration in the laboratory. | High — precise to ±2–3 g/L | Daily to twice weekly |
| Colorimetric analysis | Fe²⁺ concentration (g/L) using colorimetric test kits or spectrophotometry. | Medium — adequate for operational decisions | 2–3 times per week |
| ICP / atomic absorption | Precise Fe²⁺ (and trace element) measurement sent to an external laboratory. | Very high — ±0.5 g/L | Monthly or on demand / dispute resolution |
The minimum viable routine for effective Kleingarn-based bath management is daily acid titration and twice-weekly iron analysis, with density as a daily cross-check. A morning reading of both values, plotted on the diagram before the shift starts, takes two minutes and gives a clear operational decision for the day.
In all three of my plants, a printed Kleingarn diagram is mounted on the wall above each pickling line. Every morning the lab reports HCl and Fe concentrations for each tank, and the shift supervisor plots the operating point on the chart. Over a few weeks, the trajectory of each tank tells you exactly what is happening on the line.
A tank tracking diagonally up-and-to-the-left (Fe rising, HCl falling) is in normal, healthy use. A tank tracking straight upward at near-constant HCl is taking on iron faster than it is consuming acid — a strong signal that rinsing is inadequate and pickle is being carried into the rinse and carried back.
The rule across my plants is simple: at the start of every shift, every active pickle tank must plot inside the optimum band. If it does not, it is corrected — by acid addition, cascade rotation, or scheduled regeneration — before new work enters that tank. This single discipline controls pickling time, acid consumption, and surface quality more reliably than any other intervention on the chemical line.
In a well-designed pickling line with multiple tanks, the Kleingarn diagram becomes the strategic guide for cascade management:
When the oldest tank approaches saturation, it is taken out, partially discharged, recharged with fresh acid, and placed back at the end of the cascade as the new "fresh" tank. The cycle continues with no tank ever being fully discarded unless contaminated or physically damaged.
Since every fresh bath starts below the optimum zone, experienced operators use deliberate conditioning to reach the efficient zone faster before processing customer orders:
This example comes directly from my plant operations. It shows exactly how to use the diagram — and a calculation tool — to make a precise, confident bath correction.
This is the discipline the curve enforces: you do not correct a pickle by feel, and you do not discard a bath carrying a healthy iron load. You read two numbers, locate the point on the diagram, and calculate the exact addition that lands the bath back in the band. The Kleingarn Curve turns bath management from guesswork into engineering.
The principles described in this article are fully implemented in the HDG Community Pickling Optimizer — a web-based and installable application developed for galvanizing plant operators. The app performs all mass-balance calculations automatically and displays the resulting bath position on a live Kleingarn diagram, so operators can verify where a planned operation lands before any chemicals are moved.
The app organises calculations into seven modules, each matched to a specific operational scenario:
The "Regenerate (Both Targets)" module is the recommended best-practice approach for plants with a scrubbing or rinse recovery system. It considers all incoming streams simultaneously — spent pickle liquor, rinse water, acidic scrubbing liquid, and fresh acid — and finds the exact combination that lands the bath at a target point inside the optimum zone. The app also checks whether the requested composition is actually achievable from the current bath state before any operation is confirmed.
| Bath Condition | Position on Diagram | Immediate Action | Root Cause Check |
|---|---|---|---|
| ✅ Ideal zone | Inside the shaded band | Proceed. Monitor Fe buildup through the shift. | None required |
| High HCl / Very Low Fe | Bottom-right (below lower boundary) | Allow iron to build naturally through production. Consider bath seeding from a conditioned tank. | Normal for fresh or recently renewed bath. Expected on Day 1. |
| Low HCl / Low–Moderate Fe | Left of band, below optimum | Add calculated volume of fresh HCl. Use "Top Up Acid" module or "Regenerate (Both Targets)". | Normal acid consumption. High throughput or heavy mill-scale load. |
| High Iron (approaching saturation) | Above the upper grey +50% line, below the green saturation line | Dilute (discharge part of bath + add rinse or scrubbing water) or rotate tank out of primary pickling position. | Check rinsing efficiency. Inadequate rinsing carries iron back into the pickling tank, accelerating Fe accumulation. |
| Any HCl / High Fe near saturation | Close to the green saturation line | Do not add more acid. Discharge a significant portion and regenerate. Use "Regenerate (Both Targets)" to plan the operation. | Bath has reached end of service life for this cycle. May indicate cascade mismanagement or irregular tank rotation. |
| Above saturation line | Above the green curve | Full bath renewal required. FeCl₂ is precipitating. No acid addition will restore pickling capability. | Bath was not monitored or rotated before reaching this point. Review monitoring frequency. |
