What is a rust converter, and how does it work on steel?

How does a rust converter turn rust into a stable layer?

A rust converter turns rust into a stable layer by reacting tannins or phosphoric acid with the iron oxides in the rust, forming an insoluble iron compound that stays bonded to the steel. Tannins are plant polyphenols whose neighboring hydroxyl groups chelate iron ions; the ferric tannates they form are dark blue, highly insoluble and act as electric insulators between cathodic and anodic sites on the metal surface (Pardini et al., 2001). A 2019 tannin study names the rust phases that react: tannins complex lepidocrocite, goethite and magnetite into ferric tannates, and in its infrared tests lepidocrocite converted most readily, then magnetite, then goethite (Abdulmajid et al., 2019). Phosphoric acid takes a different route. A phosphoric acid converter data sheet says rust is changed to iron phosphate, an inert, hard substance that turns the metal black (Rust-Oleum, 2016), although laboratory work finds ferric phosphate forms only above 8 M acid (Pardini et al., 2001). Most readily available products are a water-based converter and primer in one, pairing tannic acid with an organic polymer that dries to a protective primer layer (Promain, 2018). The conversion is partial. Tannin penetrates rust to varying depths, and usually about half of the rust is affected by the treatment (Pardini et al., 2001). A rust converter therefore seals the remaining rust in place under the converted layer; it does not remove rust from the steel.

What is rust converter made of?

Rust converter is made of a converting agent, tannic acid, phosphoric acid or both, in water, often with a film-forming polymer and alcohol co-solvents. A published laboratory formula shows the kind of ingredients used: tannic acid 15 g/L, sodium molybdate 4 g/L, citric acid 1 g/L, and isopropyl and tert-butyl alcohol at 10 g/L each in distilled water, with phosphoric acid tested from 5 to 200 g/L (Li et al., 2021). Phosphoric acid converters are strongly acidic; one data sheet gives pH 1 to 2 and labels the product a severe eye and skin irritant, so read the product's safety data sheet (SDS) before use (Rust-Oleum, 2016). The table below compares rust converter chemistries from data sheets and studies: what each forms, how long it needs before coating, and where it stops working.

The row that most often changes a schedule is the cure window: before the topcoat, a phosphoric acid converter needs one night (Rust-Oleum, 2016), while a tannic acid converter needs at least 48 hours under an oil-based paint (Promain, 2018).

How long does rust converter take to turn rust black?

Rust converter starts turning rust black within minutes on light rust, but the converted layer needs hours to days before it can be painted. On lightly rusted, wire-brushed steel, a latex converter-primer data sheet says the first coat starts to develop a black color within minutes, with a recoat time of 60 to 120 minutes and at least 24 hours to dry before painting (Henkel, 2017). A phosphoric acid converter changes rusted metal overnight into a paintable surface (Rust-Oleum, 2016). A mixed acid and tannin converter needs a reaction time of 24 to 48 hours (WEICON, 2026). Temperature and humidity shift these times. The latex data sheet limits application to surface and air temperatures between 10 °C and 30 °C, notes that the reaction is slower at lower temperatures, and says high humidity slows drying but assists rust conversion, and rules out application in fog, dew, rain or heavy salt spray (Henkel, 2017). A tannic acid converter supplier advises applying above 10 °C and avoiding relative humidity above 85% (Promain, 2018). The black color shows that the reaction has started, not that the layer has cured, and uneven color indicates the need for additional coats (Henkel, 2017). In a laboratory test with rust powder immersed in a tannin solution at pH 3, about 95% of the rust was transformed within 24 hours and 95.8% after 3 days (Abdulmajid et al., 2019); on a coated surface, the liquid reacts only with the rust it reaches.

Why does rust converter need rust to work?

Rust converter needs rust to work because its tannins and acids are made to react with iron oxide; on sound paint there is nothing to convert, and on bare steel a supplier says it adds no rust prevention beyond its film. A tannic acid converter supplier states that the product will adhere to non-rusted or painted surfaces but does not provide any additional rust prevention on them, and advises against removing all of the rust, because rust converters chemically bond to the rust and seal it in (Promain, 2018). The same supplier lists aluminum, copper, stainless steel and galvanized metal as unsuitable (Promain, 2018). Rust that is thick or loose fails for the opposite reason. A latex converter data sheet requires loose or flaky rust to be removed and states that only conversion of firmly bonded rust will result in durable protection (Henkel, 2017). A 2001 coatings study reports better results on slightly rusted surfaces with an oxide layer under 100 µm thick (Pardini et al., 2001). Contaminants also block the reaction: the latex data sheet requires oil, grease, old paint, mill scale and chlorides to be removed so the rust converter can react with the surface (Henkel, 2017). Where steel has been exposed to salt spray or de-icing salts, the tannic acid converter supplier advises rinsing it with clean water and letting it dry before application (Promain, 2018).

Rust converter vs primer: what is the difference?

A rust converter chemically changes the rust it touches, while a primer is a paint film that bonds to a prepared surface and carries the topcoat; many rust converters combine both roles. A latex converter-primer is described by its maker as a coating applied to rusted metal to stop rust, protect the surface and act as a primer for the finish coat (Henkel, 2017). Other rust converters are only a pretreatment: one mixed acid and tannin product is washed off with water after its reaction time, and the final coating goes on once the surface is dry (WEICON, 2026). Neither type replaces a full paint system in harsh service. A 2001 coatings study, reviewing earlier tests, reports that tannins could not be employed alone and that, under severe exposure, an anticorrosive paint in the paint system is mandatory (Pardini et al., 2001). The rust converter's data sheet also limits the topcoat. A tannic acid converter supplier recommends oil-based or epoxy paints and advises against paints with hot solvents such as xylene and against water-based paints (Promain, 2018); the latex data sheet lists high-build epoxies among the topcoats for moisture and salt spray (Henkel, 2017). When the converter, primer and topcoat come from different makers, their data sheets have to agree on compatibility and cure times, and Lubinpla can work through such a case from your own TDS and SDS and return one cited document for review.

Related terms

• Ferric tannate: the dark, insoluble iron and tannin complex that tannin-based rust converters form.

• Iron phosphate: the inert compound that phosphoric acid rust converters form from rust.

• Lepidocrocite, goethite and magnetite: common iron oxide phases in rust, which react with converters at different rates.

• Rust remover: a product that dissolves rust from the steel instead of converting it in place.

• Converter-primer: a rust converter with a film-forming resin that also serves as the primer coat.