SECC, the electrogalvanized cold-rolled steel of JIS G3313, protects the substrate with a thin zinc layer whose quality decides how long the product survives in service. Because the coating is thin, small differences in coating mass, passivation and surface condition have a large effect on corrosion resistance. This makes testing essential: a structured set of laboratory and shop-floor tests, with written acceptance criteria, separates good material from weak material. This guide explains the standard tests, how to run them, and what the results mean.
Neutral Salt Spray Test per ISO 9227
The salt spray test is the most widely used corrosion check for electrogalvanized steel. The neutral salt spray procedure of ISO 9227, equivalent to GB/T 10125, exposes specimens to a 5 percent sodium chloride solution at 35 degrees Celsius with a pH of 6.5 to 7.2 and a deposition rate of 1 to 2 mL per hour per 80 square centimeters. Exposure times are chosen by application, commonly 24, 48 or 96 hours. Two corrosion products are evaluated: white rust, the loose zinc oxide and hydroxide that signals early corrosion of the zinc layer, and red rust, iron oxide that appears only when the zinc is locally consumed and the steel is attacked. A common acceptance for chromated SECC is no white rust within 48 hours, a white rust area of 5 percent or less at 96 hours, and no red rust within the agreed period. For fingerprint-resistant SECC-N, the organic top film delays white rust further, typically eliminating it within 96 hours and holding the white rust area to about 10 percent or less at 168 hours. Premature red rust, for example within 24 hours, indicates a coating mass below specification, missing plating or failed passivation.
Damp Heat Test
The damp heat test simulates condensation and high humidity in a controlled chamber. Typical conditions are 40 degrees Celsius plus or minus 2 degrees and 95 percent plus or minus 3 percent relative humidity for 72 or 168 hours. A high-quality SECC surface should remain free of white rust, mold and blistering of the zinc layer. Localized accumulation of white rust points to defects in the zinc or passivation layer, such as pinholes or scratches, and if the zinc detaches, poor adhesion of the coating is the likely cause. The test is fast, cheap and especially relevant for products used in warm humid interiors.
Fingerprint Resistance Test for SECC-N
Fingerprint-resistant SECC is validated by two practical checks. In the fingerprint residue test, a sweaty finger is pressed onto the surface and the panel is placed in a hot humid environment, for example 40 degrees Celsius for 24 hours; good SECC-N shows no corrosion at the fingerprint site, and the mark wipes off with alcohol. In the solvent resistance test, the surface is wiped 100 times with alcohol or isopropyl alcohol; the fingerprint-resistant film must not peel off, because a damaged film loses its protective function and the corrosion resistance drops to that of the bare zinc.
Spot and Immersion Tests
Two traditional chemical tests give a fast shop-floor check. The copper sulfate spot test drips a 10 percent copper sulfate solution onto the surface and measures the time until the zinc is displaced by copper, visible as a red deposit; high-quality SECC with an adequate coating and passivation should resist for more than 60 seconds, while reddening within 30 seconds indicates a thin or damaged zinc layer. The lead acetate immersion test immerses the specimen in a 5 percent lead acetate solution and watches for black lead sulfide precipitate, a byproduct of zinc corrosion; noticeable precipitation within 2 hours indicates weak corrosion resistance. These tests are simple, but their results should always be confirmed by salt spray data for contractual decisions.
Visual Inspection: The First Warning
Much can be learned with the eye and a magnifying glass. The chromate passivation layer should be a uniform pale yellow; rainbow-colored patches or exposed zinc-colored areas indicate uneven passivation and locally weak corrosion protection. Pinholes, caused by bubbles in the electroplating bath, and deep scratches that reach the zinc become starting points for corrosion, so their density and depth are part of the acceptance inspection. A supplier that cannot document coating mass, passivation type and salt spray results for SECC should be treated with caution, because these three items define the corrosion performance of the product.
Frequently Asked Questions
Q1. What does white rust mean on SECC?
White rust is the corrosion product of the zinc layer itself, zinc oxide and hydroxide. In small amounts it is normal after testing or wet storage and is mostly cosmetic; in large areas it means the zinc is being consumed and the protection life is shortened. Red rust, by contrast, means the steel substrate is already attacked.
Q2. How long should SECC pass in a neutral salt spray test?
The requirement is agreed between buyer and supplier and depends on the application. A common acceptance for chromated SECC is no white rust within 48 hours and no red rust within the agreed period, for example 96 hours; fingerprint-resistant SECC-N typically shows better performance with no white rust at 96 hours.
Q3. Is SECC-N really more corrosion resistant than plain SECC?
In most cases yes. The organic fingerprint-resistant film adds a barrier on top of the passivation layer, which delays moisture contact with the zinc and improves salt spray performance. Its main purpose is appearance, but the corrosion benefit is real and measurable.
Q4. Can I judge corrosion resistance by the coating color alone?
Only as a first screen. The pale yellow color of the chromate passivation is a sign that passivation was applied, but it says nothing about coating mass. The zinc layer thickness or mass must be verified by measurement, and the corrosion performance by the agreed tests.
Q5. What is the typical zinc coating mass of SECC?
Electroplated zinc coatings on SECC are thin by design, typically in the range of about 10 to 60 g/m² total over both surfaces depending on the specification, compared with 120 to 275 g/m² or more for hot-dip classes. The low coating mass is exactly why SECC is specified for indoor dry service.