Sa 2½ Blasting and Zinc Silicate Primer: A Dew Point and Salt Contamination Decision
A fabrication yard is blasting and priming the internal surfaces of new steel box girders for a highway bridge. The specification calls for blast cleaning to Sa 2½ and a zinc-rich primer, with the coating system chosen for corrosivity category C4. Work is planned for early morning: dry-bulb air temperature is 34 °C, relative humidity 70%, and the steel, which stood outside overnight, reads 26 °C on a contact thermometer. The blasting crew wants to start at 07:00.
A soluble salt test on a blasted test panel returns a chloride contamination of 35 mg/m². The specification limit is 2 µg/cm². The primer is a two-pack inorganic zinc silicate with a stated overcoating window of 4 to 24 hours at 30 °C and a pot life of 8 hours.
The inside of a box girder is an enclosed space, and the readings above were taken outside the girder, at the yard gate.
1. Work out the dew point margin from the given figures and decide whether blasting and priming may proceed. What would you do next?
Read the worked answer
Dew point calculation. At 34 °C the saturation vapour pressure is about 53 hPa. At 70% RH the actual vapour pressure is 0.70 × 53 ≈ 37 hPa. The temperature at which air at 37 hPa is saturated is about 27.5–28 °C, so the dew point is roughly 27.5 °C.
Margin. Steel at 26 °C is below the dew point by about 1.5 °C, so a film of condensation is already on the steel. The working rule is that the substrate must be at least 3 °C above the dew point: 27.5 + 3 ≈ 30.5 °C minimum. The steel is 4.5 °C short. Painting is a no-go.
Why it matters. Blasting a wet surface produces flash rust within minutes and drives moisture into the anchor profile; a zinc silicate applied over a damp surface will fail by blistering and intercoat delamination, and wet blasting also spreads soluble salts.
Next steps. Take the readings inside the girder, not at the yard gate - enclosed steel spaces are cooler and often more humid than the open yard, so the real margin is probably worse. Warm the steel with hot-air blowers and forced ventilation through the box, or wait until the steel temperature rises with the sun, and re-measure every one to two hours. Record dry-bulb, RH, dew point and steel temperature at each shift and keep the sheet with the job file. When the margin is met, blast and prime the same day with no overnight hold, and re-check before every coat.
2. The salt test reads 35 mg/m² against a 2 µg/cm² (20 mg/m²) limit. Explain the units, the consequence of ignoring it, the test you would specify, and the remedy.
Read the worked answer
Units. 1 µg/cm² = 10 mg/m², so the limit of 2 µg/cm² is 20 mg/m². The reading of 35 mg/m² is 3.5 µg/cm² - about 75% above the limit, not a marginal overshoot.
Consequence. Soluble chlorides and sulphates under a coating create an osmotic cell: water migrates through the film to dilute the salt layer, and blisters form. Corrosion is accelerated at the same time, and a zinc primer suffers undercutting and premature consumption of the zinc. For a C4 bridge girder with a 15–20 year life this is a durability failure built in at day one, and it is invisible until blisters appear.
Test. The Bresle patch method: a adhesive patch of known area is filled with pure water, the surface is agitated for the specified time, and the extract is tested by conductivity (or a titration kit), with the result converted to µg/cm². Note where the panel was taken - the bottom of the girder and weld areas trap salts from the original mill scale, from wet blasting water, or from the abrasive itself.
Remedy. Do not paint over it. Re-blast, or high-pressure fresh-water wash and re-blast, then re-test until the reading is under the limit - typically two cycles on salt-contaminated steel. Also check the source: chloride content of the abrasive, cleanliness of the compressed air (blotter test), and whether the abrasive has been reused. Record the re-test result with the patch reference and location.
3. State the controls for applying the inorganic zinc silicate and what must be done if the 4–24 hour overcoating window is missed.
Read the worked answer
Mixing. Inorganic zinc silicate is a two-pack product with a strict ratio; mix mechanically for the stated time, and allow the required induction period before spraying. The pot life quoted as 8 hours is at 30 °C - at 34 °C it is shorter, and the material thickens at the bottom of the pot, so keep it agitated and mix only what can be used within the pot life. Do not add thinner unless the datasheet allows it.
Application. Apply to the specified dry film thickness in one or two passes with your gun strokes crossing at right angles; wet film checks and dry film gauge readings on test panels. Avoid dry spray: zinc silicate overspray is powdery and cannot be re-dissolved, so it becomes an intercoat failure. Spraying inside a box girder means confined-space entry procedures, forced ventilation, air-fed or cartridge respiratory protection, gas monitoring and a stand-by man.
Curing. Inorganic zinc silicate cures by reacting with moisture, so 70% RH is an advantage here and the film must not be baked dry. It also needs to be overcoated or it will suffer white rust (zinc salts) in a humid atmosphere.
If the window is missed. Once the primer is fully cured and the 24 hours has passed, a topcoat will not bond. The correct remedy is mechanical keying - sweep blasting or abrading the surface, washing off any zinc salts with fresh water first - then re-check the film thickness and recoat inside the new window. A 'mist coat' or thinner tie coat may help in some systems but is not a substitute for mechanical preparation; check with the coating manufacturer and record the decision.