Concept · Coatings and corrosion

Zinc-rich primers

How zinc protects steel galvanically, why zinc silicate needs moisture to cure, and how inorganic and organic zinc primers differ.

Zinc-rich primers carry enough zinc dust for the particles to touch each other and the steel. Zinc is less noble than steel, so where the film is damaged it corrodes first and protects the steel galvanically.

The why: zinc as a sacrificial path

Zinc is more active than steel. When water reaches a scratch, the zinc next to it becomes the anode of a galvanic cell and gives up electrons. The bare steel becomes the cathode and stays free of rust. The zinc is used up in its place.

The cell needs a continuous path for the current, so the particles must touch each other and the steel. ISO 12944-5 defines a zinc-rich primer, Zn (R), as one with at least 80 % zinc dust by mass in the dry film. With less zinc, the chain of contact can break and the galvanic path is lost.

Two field rules follow. Zinc dust is heavy and settles, so the paint in the pot needs continuous agitation. A zinc primer also conducts current, so a holiday detector cannot test it alone. Test after a non-conductive coat covers it.

Inorganic zinc (IOZ, ethyl silicate): why it needs moisture

The binder of an IOZ is ethyl silicate. It cures in two steps. First the solvent leaves, and the film can feel dry in 30 minutes or less. Then the binder reacts with water from the air. This reaction is hydrolysis. It gives off ethanol and links the binder into a hard silica network that holds the zinc dust. Some makers activate the binder with a little water before packing, so the film only has to finish the reaction.

Dry air therefore slows the cure. The FROSIO book says zinc ethyl silicate needs more than 50 % relative humidity. The data sheet gives the minimum for the product. A heated winter shop can have air that is too dry. A film that feels dry can stay uncured for days.

IOZ is hard, very protective and heat-resistant, up to 400 °C (540 °C for short periods). It needs a high standard of blast cleaning. It shrinks as it cures, so a film above its maximum DFT cracks like dried mud (mud-cracking). ISO 12944-5 gives a nominal dry film thickness of 40 to 80 µm for a Zn (R) primer. The data sheet gives the maximum.

Organic zinc (zinc epoxy)

The binder is an organic resin, usually epoxy, so the film cures like any two-pack epoxy and needs no moisture. It is more tolerant of surface condition and application, and easier to overcoat. The organic binder fails in high heat, so it does not suit a hot stack.

In the field

1. Read the data sheet. Note the humidity and temperature limits, the nominal DFT and the maximum DFT.
2. Measure the air temperature, the steel temperature and the relative humidity before and during application.
3. Make sure the pot is agitated all through spraying.
4. Measure the dry film at the end, with extra readings in corners and on edges.
5. Before topcoating, test the cure. Agree the required rating first. Rub a cloth wet with MEK over the film with moderate finger pressure, to ASTM D4752. Count double rubs and rate the result from 0 to 5.
6. Remove dust and overspray. Make sure the mist coat goes on before the full coat.

A rating of 5 means no zinc on the cloth after 50 double rubs. It usually allows topcoating, but confirm it with the coating maker. A rating of 0 means the primer came off to the steel in 50 double rubs or fewer. See ASTM D4752 and cure and recoat.

The mist coat

A cured IOZ film is porous. A full wet coat on top soaks into the pores. The solvent and the trapped air build pressure, and the air escapes through the wet film as bubbles and pinholes. A heavier first coat makes this worse.

A mist coat seals the pores first. It is a thin coat of the next paint, thinned as the data sheet says. One maker limits the film to 30 to 35 µm and allows up to 25 % thinner for high-solids epoxy. After 10 to 15 minutes the full coat follows. A water mist, which some makers allow to help cure in dry air, is a different step.

Worked example

An IOZ has a nominal DFT of 70 µm and a data-sheet maximum of 120 µm. A corner reads 150 µm.

Check: ISO 12944-5 says the maximum should not exceed three times the nominal, which is 3 × 70 = 210 µm. The data-sheet maximum is lower, so it governs. The corner is 150 − 120 = 30 µm over. Expect mud-cracking and report it.

What the inspector records

Record the product and batch, the climate at application and during cure, and the pot agitation. Record the DFT readings, the MEK rating with the rating agreed in advance, and the time between application and the test. Record that the mist coat went on.

Exam traps

• Trap: A film that is dry to the touch is not cured.
• Trap: A low MEK rating shows slow cure, not a failed primer. Hold the topcoat and test again.
• Trap: A heavier first coat over zinc silicate does not seal the pores. It traps more air.
• Trap: A holiday detector cannot test a zinc-rich primer alone, even at the correct voltage.
• Trap: Zinc phosphate protects by inhibition. Zinc dust protects galvanically.
• Trap: A rating of 0 is the worst MEK result, not the best.

How the exams treat it

The CIP 1 and CIP 2 guides list ASTM Volumes 06.01 and 06.02, where ASTM D4752 sits. FROSIO does not publish its practical stations. One training body lists a zinc silicate cure station in its FROSIO course.

Use it: Concept: galvanic corrosion

37 practice items in the members area mention this topic (CIP 1: 15 · CIP 2: 12 · FROSIO written: 10).

Related

• Standards: ISO 1461 · ASTM D4752 · ASTM A123/A123M · ISO 29601 · NACE SP0198 · ASTM A780/A780M
• Defects: Mud cracking · Pinholes · Solvent entrapment and popping · Zinc salts (white rust) · Outgassing
• Coatings and corrosion: Protective coating types · Corrosivity categories (ISO 12944-2) · Galvanic corrosion · Cathodic protection basics