Titanium Hydriding | Materials And Corrosion Control

Damage Description

·         Critical Factors to consider are metal temperature, solution chemistry and alloy composition.

·         It occurs in specific environments at temperatures above 165°F (74°C) and at a pH below 3, pH above 8 or neutral pH with high H₂S content.

·         Galvanic contact between titanium and more active materials such as carbon steel and 300 series stainless steels promotes damage. However, hydriding can occur in the absence of a galvanic coupling.

·         Embrittlement occurs over a period of time as hydrogen is absorbed by the component and reacts to form embrittling hydride phases. The depth and extent of hydriding will continue to increase until a complete loss of ductility results.

·         Hydriding has also occurred in some chemical environments as a result of the corrosion of iron which has been accidentally embedded into the surface of titanium during fabrication. Corrosion of iron and iron sulfide scale in the process streams brought in from upstream units can result in hydrogen pickup.

·         The solubility of hydrogen in pure titanium and alpha-beta alloys is limited (50 – 300 ppm) and once this is exceeded, hydride is formed. Beta alloys, on the other hand, are more tolerant of hydrogen and 2000 ppm can be tolerated.

Materials & Equipment

Titanium and zirconium alloys.

Equipment

·         Primarily in sour water strippers and amine units in the overhead condensers, heat exchanger tubes, piping and other titanium equipment operating above about 165°F (74°C).

·         Equipment in hydrogen atmospheres at temperatures >350°F (177°C), especially in the absence of moisture or oxygen.

·         Cathodically protected equipment with protection potentials
<-0.9 v sce.

·         Heat exchanger tubes that have become embrittled may remain intact until disturbed by removal of the bundle for inspection. The tubes crack as the bundle flexes.

·         Cracking can occur if there is an attempt to re-roll tube ends that have become embrittled.

·         Another possible damage mode that has occurred is ignition and fire of titanium tubes. Metallographic examination of titanium tubes from a bundle that burned showed significant amounts of hydride, especially in the vicinity of the metal that had been melted.

Control Methodology

·         In equipment susceptible to underdeposit corrosion (and hence, high H₂ pickup), it is preferable to use Titanium Grade 12 versus Titanium Grade 2.

·         Where galvanic contact has promoted hydriding, the problem can be avoided by using all titanium construction or by electrically isolating the titanium from non-titanium components. Eliminating the galvanic couple may not prevent hydriding in alkaline sour water environments.

Monitoring Techniques

·         A quick test for embrittlement is a bend test or a crush test in a vice. Unaffected titanium will be crushed in a ductile fashion while embrittled components will crack and/or shatter with little or no sign of ductility.

·         Specialized eddy current techniques are reported to have been able to detect hydriding damage.

Inspection Frequency

KPIs

Reference