Corrosion Resistance Testing of High-Silicon Cast Iron

High-silicon cast iron is a hypereutectic iron-silicon alloy containing roughly 14 to 16 percent silicon, selected for service with sulfuric acid and other aggressive media. Its resistance depends on a thin, silicon-rich passive film that forms at the surface; loss of this film leads directly to attack. Corrosion resistance testing of high-silicon cast iron verifies film stability and quantifies metal loss under defined laboratory conditions. This article sets out the scope and electrochemical principle, coupon preparation and surface control, and immersion weight-loss testing per ASTM G31. It then covers potentiodynamic polarization with electrochemical impedance spectroscopy (EIS), neutral salt spray exposure per ASTM B117, and the corrosion-rate metrics applied at acceptance. Results issued by an accredited laboratory are used to qualify material for acid-handling pumps, valves, piping, and impressed-current anodes, and to accept production batches during procurement and manufacture.

Scope and Principle

The scope covers sand-cast components and machined coupons of high-silicon cast iron. Typical articles include pump bodies, valve seats, pipe sections, and anode castings for acid-handling service. Testing addresses the base material rather than a coating; the protective layer is generated by the alloy itself. At silicon contents near 14 to 16 percent, surface oxidation produces a dense, adherent film of silica and iron silicate. This film suppresses anodic dissolution in oxidizing acids, notably sulfuric acid over a wide concentration range. It remains brittle and is weakened by hydrofluoric acid, fluorides, sulfurous acid, thermal shock, and mechanical impact. Testing therefore combines three complementary dimensions. Gravimetric immersion measures net metal loss in a defined medium. Potentiodynamic polarization and EIS resolve anodic and cathodic kinetics and the resistive character of the passive film. Neutral salt spray examines behavior under chloride-bearing atmospheric exposure. Read together, the methods separate film-forming stability from localized attack such as pitting or intergranular corrosion.

Sample Preparation and Surface Condition

Coupon geometry commonly follows rectangular or cylindrical flats with a known exposed area; edge area is either included in the calculation or masked. Sectioning requires abrasive cut-off with continuous coolant. Dry or overheated cutting alters the near-surface microstructure and must be avoided. Remove burrs and sharp edges with fine abrasive paper so that edge attack does not distort the result. Degrease coupons in acetone or ethanol, rinse with deionized water, then dry and weigh on an analytical balance to the nearest 0.1 mg. Record the exact exposed area before exposure. Surface condition is a controlled variable in this alloy. An as-cast skin carries a thicker silicon-rich layer; a machined surface presents a different initial state. State the chosen condition in the report and match it to the service question. Prepare at least three replicate coupons per condition. Handle coupons with gloves or tongs only, and store them in a desiccator before exposure.

Immersion Weight-Loss Testing (ASTM G31)

ASTM G31 covers laboratory immersion corrosion testing of metals in chemical media. Coupons are suspended in the selected reagent using inert holders, glass hooks, or PTFE thread, with no metallic contact between specimens. The solution volume per unit area is kept high, commonly at least 20 mL per square centimeter, so that the medium is not exhausted during exposure. Temperature is held constant, either at ambient laboratory conditions or in a thermostatic bath, and is recorded throughout. For this alloy, sulfuric acid across dilute and concentrated ranges is a frequent medium; nitric acid and chloride-bearing solutions may be specified separately. Exposure runs from 24 hours to several days or weeks, with inspection of the film and solution at intervals. After exposure, remove corrosion products by gentle brushing or by chemical cleaning per ASTM G1, rinse, dry, and reweigh. Compute mass loss after correcting for any cleaning blank. Retain one unexposed control coupon to verify the cleaning procedure.

Electrochemical

Polarization and EIS Analysis
Electrochemical testing uses a three-electrode cell connected to a potentiostat. The coupon is the working electrode; a saturated calomel or silver/silver chloride electrode serves as the reference, and platinum or graphite acts as the counter electrode. The cell holds the same acid or chloride medium as the gravimetric test, at the agreed temperature. First, record the open-circuit potential until it stabilizes, which indicates passive-film formation. Potentiodynamic polarization follows the practice of ASTM G5. The scan yields a corrosion current density through Tafel extrapolation. Linear polarization resistance, per ASTM G59, gives a less invasive estimate of the same quantity. EIS is then applied over a frequency range from the millihertz to the kilohertz region. Nyquist and Bode plots are fitted to an equivalent circuit; the charge-transfer resistance and the film resistance quantify the barrier behavior of the silicate layer. Keep the anodic scan limited, since aggressive scanning can rupture the passive film and alter the surface under study.

Neutral Salt Spray Exposure Testing

Neutral salt spray exposure follows ASTM B117 in a fog chamber. The working solution is sodium chloride at five percent by mass, with a pH between 6.5 and 7.2 at collection. Chamber temperature is maintained at about 35 °C and the fog is delivered continuously. Coupons are angled so that condensed droplets do not run from one face to another, and they must not touch each other or the chamber hardware. Exposure duration is set by the governing specification; common checkpoints fall between 24 and several hundred hours. This method does not reproduce acid service directly. It ranks relative resistance to chloride-bearing atmospheres during storage, shipment, and coastal installation, which is relevant for anodes and equipment held in inventory. After exposure, rinse gently, remove loose products without scratching the surface, and record appearance, discoloration, pitting, and mass change. Photograph each face before and after so that the report can document film condition objectively.

Rate Metrics and Acceptance Criteria

The primary gravimetric metric is the corrosion rate. It is computed from mass loss, exposed area, exposure time, and alloy density, and is expressed in millimeters per year or mils per year. Mass-transfer units such as milligrams per square decimeter per day serve short exposures. Electrochemical corrosion current density is converted to a penetration rate through Faraday's law, using the alloy's equivalent weight. For EIS, the fitted charge-transfer resistance is compared against a reference material or against historical data for the same grade. Acceptance thresholds are not universal. They are set by the purchaser's specification or by the standard that governs the component. Criteria commonly combine a maximum penetration rate with qualitative requirements: no pitting beyond a defined depth, no cracking, and no intergranular attack on a sectioned and etched coupon. Replicate scatter is reviewed; a result is accepted only when replicate agreement falls within the spread stated in the governing document.