Two numbers determine the coating weight and crystal structure of a zinc phosphate bath: total acid and free acid. What the point unit means, the titration steps, sludge and iron control, a daily maintenance list and a troubleshooting table.
A zinc phosphate bath is not the same bath on the day it is filled as it is a month later. Every square metre of surface that enters consumes free acid, releases iron into the bath and leaves a certain amount of sludge behind. These three effects feed one another: as the acid balance drifts the crystal structure changes, coating weight moves outside the target band, sludge increases and spray nozzles and heating coils foul up. Phosphate bath maintenance is the work of managing that drift, without dumping the bath, through a handful of measurable parameters.
Foremost among those parameters are total acid and free acid. They carry meaning not separately but through their ratio. In KİMSOL zinc phosphate baths, the way to hit a target of 3–15 g/m² coating weight repeatably at 60–80 °C with 5–15 minutes of immersion runs through tracking these two numbers per shift. Below we cover what the point unit means, how titration is done, how deviations look in the coating and what routine maintenance consists of. For the process as a whole, see what is phosphating.
Free acid represents the unbound free phosphoric acid in the bath. It drives the dissolution of the metal — the acid attack that starts the reaction. Total acid is the overall acidity, covering free acid together with the phosphate salts, zinc and other acidic components in the bath; roughly, it shows how “full” the bath is and how much coating-forming chemical reserve remains.
In the field these two values are expressed in “points”. A point is not an absolute chemical quantity but shorthand for the volume consumed in titration: in the standard method a 10 mL bath sample is titrated with 0.1 N sodium hydroxide solution to the end point, and every 1 mL of NaOH consumed counts as 1 point. Point values are therefore only comparable if the sample volume and titrant normality are the same. If your laboratory takes 5 mL instead of 10 mL, the number you read halves; before comparing points between two plants, make sure the method is identical.
What separates the two is the indicator used. Free acid is titrated with an indicator that turns in the low pH region (bromophenol blue or methyl orange). Total acid is titrated with phenolphthalein to a much higher end point, and is therefore always greater than free acid.
Total acid divided by free acid is called the “acid ratio”, and this single number summarises the character of the bath. In KİMSOL baths total acid is held at roughly 40–60 points and free acid at roughly 5–8 points, which in practice corresponds to an acid ratio between 5:1 and 10:1. The target value varies with process type, temperature and the coating weight required; what is binding is the range given in the TDS of the product you are using.
When free acid rises (the ratio narrows) the bath becomes aggressive. Dissolution at the metal surface speeds up, but the pH at the metal/solution interface never manages to climb to the level at which the coating precipitates. The result: coating starts late, stays thin or does not form at all; grey, dusty, non-adherent areas appear on the surface. As a side effect, excess iron passes into the bath and acid and steel consumption rise. Excessive free acid usually comes from over-replenishing the bath or from acid carried over from the preceding rinse stage.
When free acid falls (the ratio widens) the bath becomes too “soft”. Dissolution slows, but the phosphate already poised to precipitate in solution readily turns into sludge: the bath goes cloudy and accumulation on the tank bottom and heating surfaces accelerates. The crystals forming on the surface coarsen and the coating becomes porous and irregular. A coarse-crystalline surface may look thick at first glance but does not give the homogeneous base needed to carry the soap film in wire and tube drawing; Kimkal soap pick-up weakens in patches.
The analysis is done every shift on a representative sample taken with the bath at working temperature and with agitation or circulation running. The sample should be taken from the flow zone rather than the tank bottom, and filtered to remove sludge. A hot sample is cooled to room temperature.
The replenishment quantity depends on bath volume, the measured deviation and the concentration of the product; there is no universal “add this much” recipe. The correct route is to calculate from the replenishment table in the TDS of the relevant KİMSOL product and confirm the result by titration. Current documents are available from our document centre.
Sludge is the unavoidable by-product of phosphating: it is the part of the dissolved iron and precipitated phosphate that stays in the bath. The problem is not that sludge forms but that it remains in the system. Suspended sludge particles settle on the workpiece, roughen the coating and carry over into the following rinse stages; when they accumulate on heating surfaces they reduce heat transfer, disturb the temperature profile of the bath and generate fresh sludge through local overheating on the coil.
Iron (Fe²⁺) build-up is the bath's invisible ageing indicator. Part of the dissolved iron precipitates into the sludge, the rest accumulates in solution. Rising Fe²⁺ in solution slows crystal formation, coarsens the coating and makes coating weight harder to control. This is why the bath contains an accelerator: it oxidises Fe²⁺, directs it into the sludge and keeps the reaction going. When the accelerator level falls the bath “dies”; coating time lengthens and the surface looks bluish or patchy. Accelerator measurement and its target range are product specific, defined in the TDS, and must be monitored as a separate parameter alongside total and free acid. If the iron level keeps rising and the coating does not recover despite accelerator additions, partial renewal of the bath (decanting) comes onto the agenda.
KİMSOL baths run in the 60–80 °C range with immersion times of 5–15 minutes. Below that range the reaction slows: the coating stays thin, crystals grow larger and extending the time often fails to make up the loss. Exceeding the upper limit is the most expensive mistake; as temperature rises the phosphate-carrying capacity of the solution falls and the bath starts generating sludge on its own. A tank run at 85–90 °C for a shift markedly increases both chemical consumption and the next cleaning load.
The measuring point matters at least as much as the set value. If the thermocouple sits near the heater, the display can be right while the bath is cold. Read the temperature from the zone the workpiece actually passes through, and verify it weekly with an independent thermometer. Parts entering the bath cold and wet also create local temperature drops; the temperature and rinse quality of the preceding stages, especially the degreasing and rinse baths, therefore directly affect the stability of the phosphate bath. An oil film left on the surface blocks the coating, while carried-over alkali consumes free acid.
| Parameter | Frequency | Method | Action threshold |
|---|---|---|---|
| Free acid | Every shift | Titration with 0.1 N NaOH (bromophenol blue) | Outside the TDS range (typically ≈5–8 points) |
| Total acid | Every shift | Titration with 0.1 N NaOH (phenolphthalein) | Outside the TDS range (typically ≈40–60 points) |
| Acid ratio | Every shift | Total / free calculation | Deviation from the target band |
| Temperature | Continuous + daily check | Line gauge and independent thermometer | Outside 60–80 °C |
| Accelerator | Daily | Product-specific method (TDS) | Below the TDS range |
| Coating weight | Daily / on product change | Weigh–strip–weigh | Outside the 3–15 g/m² target band |
| Iron (Fe²⁺) | Weekly | Laboratory analysis | A continuously rising trend |
| Sludge level | Weekly | Visual + bottom measurement | Accumulation reaching the heater or suction line |
| Symptom | Likely cause | Action |
|---|---|---|
| No coating forms or it is very thin; surface bright/bare | Free acid high, acid ratio narrow; insufficient temperature | Confirm free acid by titration; add the corrective or neutraliser calculated per the TDS and re-titrate. Bring the temperature into the 60–80 °C band. |
| Coarse, porous, dusting crystal structure | Free acid low, acid ratio wide; high Fe²⁺; weak activation | Bring free acid into the target band, measure the accelerator, refresh the activation bath. |
| Bath cloudy, sludge production accelerated | Temperature above the upper limit; over-replenishment; scale on the coil | Verify the temperature at the real measuring point, review the replenishment regime, clean the heating surfaces. |
| Coating weight above target, soap consumption up | Long immersion time; total acid high; coarse crystals | Set the time within the 5–15 min band, check total acid and the acid ratio, re-measure coating weight. |
| Patchy or spotted coating on the part surface | Inadequate degreasing or rinsing; carried-over alkali; suspended sludge | Check the degreasing bath (pH 9–13, 40–70 °C) and the rinse stage, rest the bath and remove sludge. |
| Total acid steady, free acid rising rapidly | Acid carry-over from a preceding stage; incorrect replenishment | Examine the rinse stages and drag-out losses; compare replenishment records with the shift form. |
| Coating time lengthening, surface bluish | Accelerator depleted; bath aged (high Fe²⁺) | Measure and top up the accelerator per the TDS; if it does not recover, renew part of the bath. |
The most valuable output of maintenance is not a single measurement but a series. Read together, the total acid, free acid, temperature, accelerator and tonnage processed recorded on the shift form show retrospectively when and why the bath drifted; intervention stops being guesswork. If measurement results on your KİMSOL phosphate coating lines are moving outside the target band and you cannot narrow down the cause, talk to our technical team with your bath analysis records to hand.
A point corresponds to the volume of titrant consumed under standard conditions. In the common method a 10 mL bath sample is titrated with 0.1 N NaOH and every 1 mL of NaOH consumed counts as 1 point. Point values are therefore only comparable if the sample volume and titrant normality are the same; do not compare directly with numbers from a plant using a different method.
First repeat the measurement and make sure the sample is representative. High free acid makes the bath aggressive; the coating thins or fails to form and metal dissolution and iron build-up increase. Correction is applied in stages, in the amount calculated by the method defined in the product TDS, and confirmed by titration after each step. Also check whether acid is being carried over from preceding stages.
There is no fixed schedule; sludge production depends on the surface area processed, temperature control and bath chemistry. In practice a weekly check with bottom removal according to accumulation is enough on most lines. Sludge should be removed before it reaches the heating coil or the suction line; scale on the coil leads to local overheating and still more sludge.
As long as total and free acid can be held in the target band, accelerator replenishment works and coating weight stays repeatable within 3–15 g/m², the bath is kept alive by replenishment. If the iron level keeps rising, the crystal structure coarsens and the coating does not recover despite replenishment, part or all of the bath needs renewing.
Phosphate coating is not a single tank but a connected chain: degreasing, rinsing, activation, phosphating, soap and drawing. This guide summarises each stage end to end — what it does, its typical parameters and how it affects the next one.
Read →Degreasing is the cleaning stage that removes rolling and drawing oil from the steel surface before phosphating. The phosphate bath does not clean a dirty surface; the crystal only grows on a clean one. We explain how degreasing works, the alkaline/neutral difference and how to verify cleanliness.
Read →A zinc phosphate coating is a porous crystalline layer that grows on the steel surface through an acidic bath reaction. This guide explains how the layer forms, what coating weight actually means, and why the layer is indispensable in cold forming.
Read →