Which zinc-free pigment can replace zinc phosphate in a primer?

This guide covers anticorrosive primers for steel and aluminium that use zinc phosphate as the inhibitive pigment. It does not cover zinc-rich primers, which protect by metallic zinc dust, or zinc phosphate conversion coatings applied as a pretreatment, and it names pigment chemistries, not products approved for any specification.

What does zinc phosphate do in an anticorrosive primer?

Zinc phosphate in an anticorrosive primer is an inhibitive pigment: water that enters the film dissolves a small amount of it, and the dissolved phosphate helps passivate the metal under the coating. The pigment works only while some of it dissolves, which is why its low solubility is both its strength and its limit.

Researchers who compared it with calcium phosphates described zinc phosphate as having led to "contradictory results, its main difficulty being low solubility" (Vetere and Romagnoli, 1994). A more soluble calcium tripolyphosphate puts "a higher concentration of the passivating anion in the pore solution", which gave better protection of the steel in their tests (Vetere et al., 2001). In a layered aerospace primer, the pigment sits in a reservoir layer and is "leached out only if the coating is damaged by a scratch or scribe" (Yin et al., 2007).

Formulators look for a replacement mainly because of the zinc. The EU harmonised classification lists zinc phosphate as H400 and H410 for aquatic toxicity (PubChem, 2026), which can carry into the classification and labelling of the paint, and some customers set zinc-free targets of their own. A replacement has to reproduce the same job: slow release of an inhibiting ion at a rate the binder tolerates.

Which conditions decide the zinc phosphate replacement?

Five conditions decide which zinc phosphate replacement is worth trialling: the binder, the substrate, the film build, the pigment loading and the performance target. Write them down before you ask a pigment maker for samples, because each maker's data sheet answers a different subset.

• Binder: oxidatively drying alkyd, two-component epoxy, waterborne systems and baked coil primers react differently to a more soluble pigment, as the binder section below shows.

• Substrate: steel, aluminium and galvanized steel corrode differently; aerospace aluminium alloys are a separate case with their own inhibitor results (Yin et al., 2007).

• Film build: thin primers need a fine pigment; one maker recommends a grade with a mean particle size under 5 micrometers for thin-film primers (Pigmentan, 2026).

• Pigment loading: the replacement must fit inside the same pigment volume concentration (PVC) against the critical pigment volume concentration (CPVC) as the current formula, or barrier properties change.

• Performance target: the test hours, scribe creep and adhesion the primer must meet today, and whether a customer specification or approval lists zinc phosphate by name.

The last condition stops many projects early. If a qualified primer is approved with a named pigment, any change may need requalification, whatever the bench results show. Check the approval terms with the customer before the trial, not after it.

Which zinc-free phosphate pigments can replace zinc phosphate?

Four families of zinc-free phosphate pigment can replace zinc phosphate in a primer: calcium phosphates, calcium phosphates modified with magnesium, strontium aluminium polyphosphates and calcium aluminium polyphosphate silicates. They differ mainly in solubility and in the binders their makers have tested them in.

• Calcium phosphates: calcium acid phosphate was "better than zinc phosphate" in a phenolic chlorinated rubber binder (Vetere and Romagnoli, 1994), and calcium tripolyphosphate inhibited steel corrosion in alkyd and epoxy paints (Vetere et al., 2001).

• Calcium-magnesium phosphate: one maker describes a zinc-free "calcium phosphate complex which has been modified with an electrochemical active magnesium compound" for universal use (Heubach, 2026).

• Strontium aluminium polyphosphate: the same maker positions a modified grade for "coil coatings and aircraft primers" (Heubach, 2026). Tripolyphosphates replaced orthophosphates in demanding uses such as coil coatings (Vetere et al., 2001).

• Calcium aluminium polyphosphate silicate: reported by its maker to give good results "in water based 2K epoxy resins and also other waterborne coating systems" (Heubach, 2026).

Phosphosilicates sit lower: one comparison found polyphosphosilicates inferior to polyphosphates (Kalendová, 2003). Read each candidate's safety data sheet as well, because a zinc-free pigment can carry hazard classes of its own.

Which zinc phosphate replacement fits your primer and binder?

The table below decides which zinc-free pigment to trial first for each primer type, and what the trial must look at. Each row is a starting candidate drawn from maker data and published studies, not a qualified substitute.

The row that most often needs care is the alkyd primer: calcium tripolyphosphate matched zinc phosphate at 10% by volume of the pigment, but more pigment made the alkyd film worse (Vetere et al., 2001). Start an alkyd trial low and step up, rather than copying the zinc phosphate level. Primers for galvanized steel are not in the table because none of the sources cited here reports a zinc-free phosphate result on zinc surfaces; treat that case as untested and run the full trial plan below.

For aircraft primers, also keep a zinc phosphate control in the trial, because published work on aerospace aluminium found zinc phosphate the strongest inhibitor in a silane primer (Yin et al., 2007).

Does the resin system change which zinc phosphate replacement works?

The resin system does change which zinc phosphate replacement works, because a more soluble pigment interacts with the binder and with water in the film. A pigment that protects well in one binder can raise permeability and blistering in another at the same loading.

The clearest published case is calcium tripolyphosphate. In alkyd paints, raising it from 10% to 30% by volume of the pigment led to a deterioration of the paint film, which the authors linked to the pigment's relatively high solubility, enhanced by acid-base interaction with the binder (Vetere et al., 2001). Epoxy paints with the same pigment "seemed not to be largely affected by the pigment content", and the authors read the epoxy as more tolerant of the pigment's acid-base character (Vetere et al., 2001).

The practical reading for formulators, an inference from these results rather than a general rule, is that oxidatively drying binders with acid groups need lower loadings and more checking of water resistance, while crosslinked epoxies give more room. Waterborne primers add a further risk: a soluble pigment can affect can stability and flash rust, so check both in the trial; one maker offers a grade it reports for waterborne 2K epoxy (Heubach, 2026). Baked coil primers add cure: common coil primers are polyester resins heat cured with amino resins and acid catalysts (Pigmentan, 2026), so the pigment must not disturb that cure, which only a cure test at your line conditions shows.

Can you replace only part of the zinc phosphate in a primer?

You can replace only part of the zinc phosphate in a primer, and a partial step is often the safer first trial when a full swap is not yet proven. Run it as a ladder: the current formula, a half replacement and a full replacement, all at the same PVC/CPVC ratio.

Keep the ladder on a volume basis. Pigment densities differ, so an equal-weight swap changes the pigment volume. For example, replacing a zinc phosphate grade of 3.3 g/cm3 with a pigment of 2.1 g/cm3 at equal weight raises that pigment's volume by about 57% (illustrative densities; read yours from each data sheet). That shift moves PVC towards CPVC and can open the film to water, independent of the pigment's inhibiting effect.

Partial replacement also tells you something a full swap hides. If the half step holds performance and the full step does not, the replacement is working but not at the same rate as zinc phosphate, and the next trial should adjust loading or add a second inhibitor. Avoid filling the gap with zinc oxide if the goal is zinc-free, even though zinc oxide improved an alkyd paint in one study (Vetere et al., 2001). For the general method of swapping one ingredient in a working formula, see how to substitute a raw material in an existing formulation.

Why do published claims on 1:1 zinc phosphate replacement disagree?

Published claims on 1:1 zinc phosphate replacement disagree because they come from different binders, substrates, test methods and loading bases. A maker's claim and a laboratory study can both be accurate for their own conditions and still point in opposite directions for yours.

On one side, a pigment maker describes its product as a "direct (1:1) substitute for basic Zinc Phosphate" (Pigmentan, 2026), and a calcium acid phosphate beat zinc phosphate in a phenolic chlorinated rubber binder (Vetere and Romagnoli, 1994). On the other, zinc phosphate was "the most effective" inhibitor in a silane-based primer on aerospace aluminium (Yin et al., 2007), and in a study of condensed phosphates a zinc phosphosilicate comparison pigment showed high efficiency (Kalendová, 2003).

Four differences explain most of the gap:

1. Binder: chlorinated rubber, alkyd, epoxy and silane primers respond differently to soluble pigments.

2. Substrate: steel results do not transfer to aluminium alloys.

3. Test: seawater immersion, salt spray and electrochemical tests rank pigments differently, and ISO 9227:2022 says salt spray is not meant for ranking materials (ISO 9227:2022).

4. Basis: a 1:1 claim by weight is not 1:1 by volume, so the PVC changes.

Treat any 1:1 claim as a hypothesis for your binder and substrate, and let your own side-by-side trial decide.

Which tests confirm a zinc phosphate replacement?

A zinc phosphate replacement is confirmed by a side-by-side trial against the current zinc phosphate primer, with neutral salt spray to ISO 9227:2022, scribe creep to ISO 4628-8:2012 and pull-off adhesion to ISO 4624:2023, and with pass and stop criteria written down before any panel goes in.

The plan below fixes those criteria; the limits in it are illustrative and should come from your own specification.

ISO 9227:2022 sets no exposure period for a product, so choose one long enough for the control to show measurable creep (ISO 9227:2022). Run several panels per formula, as many as your specification requires, and code the panels so the rater does not know which is which. The control row decides the trial: a candidate passes by matching the control in the same cabinet run, not by meeting a number from another lab.

In Lubinpla, the formulation design task reviews replacement candidates against resin compatibility and process and designs a bench trial with pass and stop criteria fixed before results, returned as one cited document.