Aqua regia-a mixture of concentrated nitric and hydrochloric acids-is one of the few room-temperature liquids that rapidly attacks titanium. The combination of oxidizing nitrate and complexing chloride ions destroys the passive film within minutes. However, certain industrial processes use simulant mixtures with lower acid concentrations but still maintain strongly oxidizing conditions. For these applications, titanium tubes with noble metal coatings-specifically ruthenium-iridium (Ru-Ir) electrodeposited layers-can provide limited service life. The coating thickness directly determines how long the titanium substrate remains protected. Ru-Ir coatings act as catalytic surfaces that promote the reduction of oxidizing species, maintaining the underlying titanium in a passive state. When the coating is too thin, localized defects expose titanium to direct acid attack. When optimally thick, the coating provides a barrier and catalytic protection. Understanding the relationship between coating thickness and failure time allows engineers to select the minimum viable coating for required service intervals.

Mechanism of Ru-Ir Coating Protection in Oxidizing Acid Mixtures

Ruthenium and iridium are noble metals with high overpotentials for hydrogen evolution and excellent catalytic activity for oxygen and nitrate reduction. In an aqua regia simulant (e.g., 10% HNO₃ + 5% HCl at 50°C), the Ru-Ir coating becomes the primary site for cathodic reactions. The coating maintains an electrochemical potential sufficiently positive to keep the underlying titanium passive, even if small defects exist. Additionally, ruthenium forms a stable oxide (RuO₂) that is conductive and catalytically active, while iridium enhances coating hardness and adhesion. The coating protects titanium through three mechanisms: physical barrier separating titanium from acid, galvanic protection where the noble coating acts as a cathode, and catalytic decomposition of oxidizing species at the coating surface.

Quantitative Relationship Between Coating Thickness and Failure Time

Controlled immersion testing in aqua regia simulant (8% HNO₃ + 4% HCl, 50°C) has established the following failure times for Grade 2 titanium tubes with Ru-Ir coatings (70% Ru, 30% Ir by weight):

Uncoated titanium (baseline): Failure within 2–4 hours. Rapid uniform corrosion and pitting. Titanium dissolves at 3–5 mm per hour.

Ru-Ir coating thickness of 0.2 µm: Failure time of 50–100 hours. Coating contains pinholes and microscopic defects. Acid penetrates defects and undercuts coating. Titanium dissolves beneath noble coating, causing blistering and spallation.

Ru-Ir coating thickness of 0.5 µm: Failure time of 500–800 hours. Coating is continuous but thin. Localized attack initiates at coating grain boundaries after 300–400 hours. Failure occurs by progressive pitting through titanium wall.

Ru-Ir coating thickness of 1.0 µm: Failure time of 2,500–3,500 hours (3–5 months). Coating exhibits good adhesion and catalytic activity. Uniform consumption of ruthenium occurs at 0.2–0.3 µm per month. Failure when coating thins to <0.3 µm locally.

Ru-Ir coating thickness of 2.0 µm: Failure time of 8,000–12,000 hours (11–16 months). Coating provides robust protection. Ruthenium consumption rate decreases to 0.1–0.15 µm per month as stable oxide forms. Acceptable for 1-year service intervals.

Ru-Ir coating thickness of 5.0 µm: Failure time exceeding 25,000 hours (nearly 3 years). Coating becomes the limiting factor. Thermal cycling may cause spallation due to differential expansion before chemical failure occurs.

Coating Thickness Selection Guide for Oxidizing Acid Service

The following table provides a decision framework for selecting Ru-Ir coating thickness based on acid simulant composition, temperature, and required service life:

Engineering Beyond Coating Thickness Alone

Coating quality is as important as thickness. Double-layer coatings (inner ruthenium, outer iridium) outperform single-layer mixed coatings by 30–50% due to iridium's higher chemical inertness. Electrodeposited coatings have lower porosity than sputtered or evaporated coatings of the same thickness. A post-deposition heat treatment (400°C for 1 hour in argon) diffuses the coating into the titanium surface, creating a graded interface that improves adhesion by a factor of 5–10. The underlying titanium surface preparation is critical: electropolishing to Ra <0.2 µm before coating reduces defect density by 80% compared to as-drawn surfaces. The tube wall thickness beneath the coating provides a safety factor once coating failure occurs. A 2.0 mm titanium wall with a failed coating may still survive 100–200 hours of direct acid attack, while a 0.8 mm wall fails within 24 hours.

Making an Informed Specification

When specifying a Ru-Ir coated titanium heater for strongly oxidizing acid mixtures, define the required service life in hours, not months, because failure time scales linearly with coating thickness. Request from the supplier a coating thickness measurement using X-ray fluorescence (XRF) at five points on the tube, with a minimum acceptable thickness of 1.5 µm and maximum variation of ±20%. Specify a double-layer coating: 1.0 µm ruthenium followed by 1.0 µm iridium, for a total of 2.0 µm, rather than a 2.0 µm mixed coating. Require post-deposition heat treatment and adhesion testing (tape test per ASTM D3359) with classification 4B or better. For critical applications where heater failure would cause production stoppage, specify a 5.0 µm coating and install a backup heater. During operation, monitor the electrochemical potential of the coated tube versus a platinum reference electrode; a potential drop of more than 100 mV below the initial value indicates coating degradation requiring inspection. By selecting the appropriate ruthenium-iridium coating thickness based on quantitative failure time data, the engineer enables limited but predictable service of titanium heaters in otherwise incompatible oxidizing acid mixtures.