Coating weight is the criterion that reduces phosphate coating quality to a single number. The steps of gravimetric measurement, sampling discipline, the likely causes of deviations, and how to monitor the trend.
What you see on a phosphating line is colour: a matt grey, dark grey, sometimes faintly bluish surface. But colour on its own is not a quality criterion; two parts that look identical in colour can differ several-fold in the amount of crystal they carry per unit area. What determines the following process steps — how the Kimkal drawing soap film keys in, the friction during drawing, die life — is not the colour of the coating but its quantity. The criterion that puts a number on that quantity is coating weight, expressed in grams per square metre (g/m²).
Coating weight is the most fundamental acceptance criterion of a phosphating line, and measured correctly it lets you read backwards what is happening in the bath. If the value keeps falling, the bath may be exhausted or surface preparation may have weakened; if it keeps rising, there is chemical waste and a risk of a brittle layer. This article covers how coating weight is defined in KİMSOL zinc phosphate applications, how it is measured gravimetrically, which bath parameters affect it, and where to look when a deviation appears.
Coating weight is the mass of the phosphate crystal layer deposited per unit area of metal surface. There is a practical reason for preferring mass per unit area over thickness (µm): the phosphate layer is not a homogeneous, flat film but a voided crystal lattice. Depending on crystal size and packing density, two layers of the same “thickness” can contain different amounts of material. Mass, on the other hand, can be weighed directly, measured repeatably by a line operator, and shows something close to a linear relationship with bath chemistry.
In KİMSOL zinc phosphate applications the typical working range is 3–15 g/m². Where you sit within that broad range depends on where the part is going: light forming and pre-paint primer applications work near the lower end, heavy cold forming and deep drawing near the upper end. What matters is defining a target band for your own product (say 6–9 g/m²) and staying inside it; a “high” value in absolute terms does not mean a better result.
Deviations on either side of the target band produce different faults, and those faults usually show up not on the phosphating line but at later stations. That is why looking for the cause in the phosphate bath is the last thing that comes to mind.
The reference method for coating weight is gravimetric: the coated sample is weighed, the coating is stripped with a selective solution, the sample is weighed again, and the difference between the two weighings is divided by the coated area. The method looks simple; whether the result is repeatable depends entirely on discipline. The sequence is:
The composition and time of the stripping solution vary with the base metal, so no recipe is given here. The solution used should be validated with a blank sample test showing that it does not measurably attack the base metal: when an uncoated coupon goes through the same treatment, the mass loss must remain negligible. Otherwise what you are measuring is not the coating but base metal loss. For the details of the method and acceptance criteria, refer to the relevant standards and your material specification.
A single sample does not represent the whole line. In immersion baths the parts on the outside of the basket see different solution circulation from those in the middle; on rack lines a difference develops between the upper and lower zones. The sampling plan should therefore work from a known position scheme rather than a fixed point.
Coating weight is not an independent variable but the resultant of bath conditions. The main parameters to keep under control in KİMSOL baths, and their typical working ranges, are:
| Parameter | Typical range | Effect on coating weight |
|---|---|---|
| Bath temperature | 60–80 °C | At low temperature the reaction slows and coating weight falls. Excessively high temperature brings coarse crystals and more sludge. |
| Immersion time | 5–15 min | As time shortens the coating thins; beyond a certain point extra time no longer raises coating weight, it only raises sludge. |
| Total acid | ≈40–60 points | Indicates the concentration of coating-forming material. When it falls, coating weight falls; replenishment is required. |
| Free acid | ≈5–8 points | If high, etching dominates and crystal formation is hindered (thin coating). If low, coarse crystals and a tendency to sludge increase. |
| Total/free acid ratio | Per target band | This ratio is the most sensitive indicator of coating weight. Monitoring total acid alone is misleading. |
| Accelerator level | Process specific | Insufficient accelerator gives a thin, porous coating; excess leads to coarse crystals and sludge. |
| Surface preparation | pH 9–13, 40–70 °C, 3–10 min | Residual oil or oxide locally prevents crystal nucleation; coating weight falls and distribution is disturbed. |
The last row of the table is often the most critical: a significant share of coating weight deviations begins not in the phosphate bath but in the degreasing and activation steps before it. On a surface that has not been fully degreased, you will not hit the target band however well the phosphate bath is tuned.
When coating weight moves outside the target band, follow a path from symptom to cause rather than adding chemistry straight away. The table below summarises the first checks to make at the line.
| Symptom | Likely cause | Check first |
|---|---|---|
| Coating weight below band, surface bright / light grey | Free acid high; etching dominant | Free acid points and total/free acid ratio |
| Coating weight falling slowly, trend downward | Bath exhausted, coating-forming material depleted | Total acid points, replenishment records, area processed |
| Coating patchy, no crystal at all on part of the piece | Inadequate degreasing or contaminated rinse | Degreasing concentration, temperature, time; contamination load of the rinse water |
| Coating weight above band, dusting on the surface | Coarse crystals; free acid low or time/temperature high | Free acid, bath temperature, immersion time |
| Outer parts in the basket correct, middle parts low | Inadequate circulation; parts shadowing each other | Basket loading, rack layout, pump/agitation flow rate |
| Sludge formation and coating weight risen together | Temperature high or accelerator excessive | Bath temperature, accelerator level, sludge removal interval |
| Measurements erratic, no repeatability | Sampling or weighing discipline broken | Drying/cooling time, area calculation, stripping time, balance calibration |
The last row of the table matters especially. Before attributing a deviation to the process, make sure the measurement itself is reliable; a bath intervention based on a measurement error will upset a line that was genuinely stable. For the periodic control and replenishment routine on the bath side, phosphate bath maintenance offers a detailed framework.
A single coating weight value tells you about that part; the movement of values over time tells you about the bath. Stamping a measurement result “pass/fail” and leaving it there therefore throws away most of the data. Record every measurement on the same form: date, shift, sample position, material, m₁, m₂, area, calculated g/m², and the total acid, free acid, temperature and time at that moment.
Collected on a chart, these records show the direction long before an alarm threshold is reached. Draw the lower and upper limits of the target band; if you see measurements inside the band but drifting steadily in one direction, you can intervene before scrap is produced. Overlay the total/free acid ratio on the same chart and it becomes easier to tell whether movement in coating weight comes from bath chemistry or from surface preparation. What a layer that stays too thin or voided means during storage is covered in why phosphated wire rusts.
You cannot control a parameter you do not measure; and you cannot interpret a measurement you do not record.
In the phosphating and drawing soap applications Kimfosan has run since 1982, we find that most line problems come not from the choice of chemistry but from gaps in measurement and record-keeping discipline. To build a target coating weight band and a control plan for your own line, get in touch with our technical team.
There is no single right value; the target is set by the process the part is going into. In KİMSOL zinc phosphate applications the working range is 3–15 g/m². The lower end is preferred for light forming and pre-paint work, the upper end for heavy cold forming and deep drawing. What matters is defining a narrow target band for your own product and staying inside it.
Thickness gauges can give a quick indication, but because the phosphate layer is a porous crystal structure the measured thickness does not always correspond to the real mass per unit area. The gravimetric method is taken as the reference. If you do use a thickness gauge, you must validate it regularly against gravimetric results taken from your own parts.
No. Replenishment comes after the cause of the deviation has been confirmed as bath concentration. First validate the measurement itself (area calculation, drying, weighing); then check the degreasing and rinsing steps, then temperature, time and the total/free acid ratio. Answering a surface-preparation deviation with replenishment only wastes chemistry.
At least one measurement per shift is enough as a routine. In addition, repeat the measurement when the bath is freshly made up, after replenishment, when material or part geometry changes, and when friction, dusting or die wear is reported from the drawing line. Plotting results and monitoring the trend is more valuable than increasing frequency.
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.
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.
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