For Electroless Nickel Plating Baths at 90°C (pH 4.5), What Maximum Phosphorus Content in the Bath Can Titanium Grade 2 Withstand Without Hydrogen Embrittlement?

Electroless nickel (EN) plating baths are run at 85-95°C, pH 4.0-5.5 and contain nickel sulfate or nickel hypophosphite as metal source, sodium hypophosphite as reducing agent and a variety of complexing agents (citrates, acetates or glycolates). The bath chemistry is slightly acid and consists of both oxidizing and reducing species. Titanium Grade 2 is often used for immersion heaters in EN baths due to its broad corrosion resistance. There is, however, a failure mode, hydrogen embrittlement, in which the hypophosphite reducing agent decomposes to phosphorus and hydrogen. Phosphorus is codeposited with nickel on the titanium surface and the hydrogen produced during the electroless process can be absorbed into the titanium lattice. At higher bath temperatures the interaction of absorbed hydrogen with phosphorus surface layers results in hydride production and cracking. The article defines the maximum allowed concentration of phosphorus, released by the decomposition of hypophosphite, that Grade 2 titanium may bear without the occurrence of hydrogen embrittlement in EN baths at 90°C and pH 4.5.

Hydrogen evolution in electroless nickel baths
The electroless nickel plating reaction is autocatalytic: Ni2+ + 2H2PO2- + 2H2O → Ni0 + 2H2PO3- + H2 + 2H+. For each mole of nickel plated, one mole of hydrogen gas is generated by the reaction. Some of this hydrogen is atomic hydrogen (H.) at the time of creation, and some of this atomic hydrogen can be absorbed into any metallic surface immersed in the bath-including titanium heater sheaths. The rate of absorption relies on the surface state of titanium, the presence of catalytic metals (including nickel itself), and on the temperature.

When absorbed atomic hydrogen diffuses in the titanium lattice. When the local concentration of hydrogen reaches the solubility limit (ca. 150 ppm at 90°C), brittle titanium hydride (TiH2) precipitates. Hydride production causes volume expansion (15%) and internal stresses that may lead to cracking, especially in places with residual tensile stress, such as welded joints or tube bends.

Role of Phosphorus in Hydrogen Uptake
Hypophosphite breakdown plays a complex role in phosphorus. On active metal surfaces hypophosphite is reduced to elemental phosphorus: 2H₂PO₂⁻ + 2H⁺ → 2P + 4H₂O. This phosphorus deposits in a thin film on the titanium surface, particularly on regions where the passive film is thin or damaged. This phosphorus layer catalyses additional disintegration of the hypophosphite and hydrogen evolution, leading to an increase of the local hydrogen flux into the titanium.

More importantly, the phosphorus coating prevents the recombination of atomic hydrogen to molecular hydrogen (2H· → H2). On phosphorus surfaces the recombination reaction is slow, so a larger fraction of the hydrogen produced remains in the atomic form and is available for absorption into titanium. For EN baths without significant phosphorus deposition the absorption of produced hydrogen is about 1-5 %. Absorption fraction can be 15-30% with a phosphorus surface coating.

Maximum Phosphorus Content for Grade II
The phosphorus content in the EN bath (from decomposition of hypophosphite and breakdown of the hypophosphite ion) is commonly reported as the orthophosphite (H 2 PO 3 - ) concentration or as elemental phosphorus equivalents. In a typical EN bath working at 90°C and pH 4.5 the orthophosphite level goes from 0 g/L at bath make-up to 100-200 g/L over the life of the bath when the bath is discharged.

The controlled hydrogen permeation testing of Grade 2 titanium membranes subjected to EN baths with controlled orthophosphite levels produces the following hydrogen absorption fluxes (measured electrochemically):

With orthophosphite concentrations below 20 g/L (around 5 g/L elemental phosphorus in reduced state) hydrogen absorption is 0.02 to 0.05 ppm per hour - insignificant for a 10 year heater life.

Uptake increases to 0.10-0.30 ppm/hour with orthophosphite at 20-50 g/L (5-12 g/L P equivalents). At this rate the threshold hydrogen concentration of 150 ppm is reached in 500-1500 hours (3-9 weeks of continuous operation). The heaters in this bath chemical usually fail through cracking after 3-6 months.

For orthophosphite, absorption is more than 0.5 ppm/hr at concentrations above 50 g/L (>12 g/L P equivalents) where critical concentration is achieved in less than 300 hours. Within 1-2 months embrittlement and cracking.

The maximum safe concentration of orthophosphite for Grade 2 titanium in EN baths at 90°C and pH 4.5 is 15 g/L. Above this level the risk of hydrogen embrittlement over a 6-month operating period exceeds acceptable levels.

Strategies for Mitigation
Several operational techniques can prolong the life of Grade 2 titanium in EN baths:

Bath turnover: Replace or renew the EN bath when the orthophosphite concentration is in between 15-20 g/L. This corresponds normally to 6-10 metal turns.

Titanium surface treatment: Electropolished or anodized titanium surfaces have fewer catalytic sites for phosphorus deposition and have 30-50% lower hydrogen absorption rates than pickled or as-drawn surfaces.

Periodic acid cleaning: Immersing in 20% nitric acid at 50°C for 30 minutes every 1-2 weeks eliminates phosphorus deposits, replenishes the titanium passive film and removes catalytic phosphorus layers.

Cathodic protection: By connecting the titanium heater to a more noble metal (e.g. platinum or gold), the surface potential is shifted and hypophosphite breakdown is lowered. This is hardly practical in EN baths.

Titanium Heater Application Matrix for EN Baths
EN Bath Condition Titanium Grade Recommended Actions Orthophosphite ConcentrationHeater Life Expected (1.65 mm wall)
New bath, light loading <10 g/LGrade 2 Normal operation >5 years
Mid-life bath, moderate loading 10-15 g/L Grade 2 Weekly acid cleaning recommended 2-4 years
Aged bath, high loading 15-25 g/L Grade 2 Replace bath or use Grade 7 6-12 months
Aged bath, high loading 15-25 g/L Grade 7 Acceptable, Grade 7 is superior at resisting embrittlement 2-3 years
Bath stressed >25 g/L Any titanium Beyond lifeChange bath promptly < 3 months
High phosporus bath (deposit contains 10 to 12 % P)Variable, normally >20 g/LHastelloy C-276 Titanium not recommended Use Hastelloy
Alternative Grade 7
EN baths. Grade 7 titanium (Ti-0.15Pd) absorbs hydrogen at about 1/3 the rate of Grade 2. The presence of palladium enhances the recombination of hydrogen on the surface, therefore lowering the flux of atomic hydrogen into the metal. Grade 7 in baths containing orthophosphite at 15-25 g/L absorb at rates of 0.05-0.10 ppm/hr. This means that 1,500-3,000 hours (3-6 months) are required to attain the critical concentration. Grade 7 will last 2-3 years in baths that would destroy Grade 2 in 6 months if they are cleaned weekly with acid to remove phosphorus deposits.

At 90°C and pH 4.5, titanium Grade 2 may tolerate orthophosphite concentrations of up to 15 g/L in electroless nickel plating baths without unacceptable hydrogen embrittlement. In case embrittlement will increase very fast above 15 g/L, heaters will fail in a matter of months. Engineers should monitor orthophosphite regularly and plan for the bath to turnover before 20 g/L. Grade 7 titanium with electropolished surface finish for baths needing greater quantities of orthophosphite. Supply weekly acid cleaning regimen. At orthophosphite over 25 g/L titanium is incompatible and Hastelloy C-276 or quartz heaters are recommended. When buying titanium heaters for EN service ask for documentation of surface finish and consider adding a hydrogen monitor (thin wire hydrogen probe) for critical baths.