Frequently Asked Questions
1. Q. I am operating offshore with copper based alloy pipework and stainless steel equipment. Now I am advised that titanium pipework is to be introduced into the system. Where can I get advice about possible galvanic corrosion problems when all of these metals are coupled together.
A. Titanium is galvanically compatible with a range of austenitic and superduplex steels in sea water. Care will be needed to protect the copper based alloys from accelerated attack, but protection may already be in place to avoid problems with the existing stainless steel equipment. For full details contact the Titanium Information Group and ask for their Data Sheet No.6 'Connecting Titanium to Other Metals'.
2. Q. I have to stress relieve a titanium alloy structure after fabrication and welding. I am told I must descale and pickle the items to remove the 'alpha case'. What does this mean in practice?
A. Alpha case is a hard, brittle, oxygen rich layer which forms progressively by diffusion at elevated temperature on titanium and its alloys. This layer should be removed from all alloy structures especially where service stress levels are likely to be high. Stress relief operations up to one hour and up to 500C only thicken the existing oxide film and post heat treatment pickling might only be required for cosmetic reasons. Minimal metal removal (typically less than.025mm (.001 inch)) by light pickling in nitric/hydrofluoric acid is suggested for treatments up to 700C. More substantial stock removal by shot blasting and acid or salt bath descaling is typical after hot working, e.g. forging, which involves heating at much higher temperatures. For more detailed information consult ASM Materials Properties Handbook Vol 4 'Titanium Alloys'
3. Q: Why does titanium work as an FGD plant lining? Which grade of titanium should I use?
A. Grade 2, commercially pure titanium sheet is the basic standard for lining, but where aggressive conditions, cold wet/hot dry interfaces exist, such as in the inlet quench area, then one or other of the palladium containing alloys – Grades 7,11, 16, 17, 33, 34 should be considered. Palladium enhances the resistance to reducing acid chlorides particularly against crevice and under deposit attack at higher temperatures.
One of the first and persistent questions asked about the suitability of titanium as an FGD plant lining is its resistance to fluorides. Fluorides are present in the flue gas of coal burning power plants. The mistake is to assume that these fluorides are present as hydrofluoric acid. The Drax Power Plant in Yorkshire, England, the largest FGD installation in Europe, has 24,000 sq. metres of commercially pure titanium flue lining, and has provided valuable data in confirmation of the performance of titanium in earlier installations. Ash samples taken from Drax Power Station confirmed that as with most plants, the fluorides are not active but complexed and therefore harmless to titanium. The second concern in view of flue gas temperatures (Table 1) point to the possibility of sulphuric acid condensation.
Table 1. Drax Flue Gas Temperature Ranges
Note : No measurements have been taken when running mixed gas, i.e. one boiler on FGD, one off, the assumption is that the gas temperature range will be approx. between the above ranges.
| Gas conditions -
|| Normal Operation -
|| Extremes of Range
|| 120 – 125°C
|| 110 - 135°C
|| 88 - 93°C
|| 80 - 95°C
The composition of the ash (Table 2) is such that any condensing acid will be neutralized. The ash also acts as a buffer so that locally the pH of a wet deposit never drops to a value where the fluoride complexes could break down and free active fluoride be released to attack the titanium.
Table 2. Composition of Flue Ash from Drax Power Station
|| Location in Flue
|| Base of stack - || Level 5 (110m) -
|| Level 10 (220m) -
|| Top of Pot
| Aluminium %
| Iron %
| Silica as Si %
| Calcium %
| Fluorine %
| Water soluble F
| Water soluble %
| PH 1:10 soln %
| Al/F Ratio %
As regularly reported, there have been and are no known operational problems with titanium FGD plant linings arising from fluoride attack. As at Drax, fluorides frequently up to high concentrations are present in the flue gas, ash, and scrubber liquors, but there is no record of significant or sustained attack on titanium by hydrofluoric acid or acidic fluorides from these sources. Early in the FGD process cycle fluoride species, potentially corrosive to titanium if active, are spontaneously complexed by aluminium, iron, silicon and calcium – all present in the FGD environment. An aluminium/fluoride ratio of 0.7 effectively neutralises the fluoride activity. The stable complexes formed are harmless to titanium. As can be seen in table 2 there is aluminium substantially in excess of the requirements of this ratio to prevent fluoride attack on the titanium at Drax. If this complexing and inhibitive mechanism were not wholly effective, then titanium would neither be qualified nor acceptable for service as a liner at Drax or any other FGD plant.
Samples of Grade 7 (Ti-0.15%Pd) titanium have also been exposed in the inlet quench and demister outlet sections of the RWE Karnap Municipal Waste Combustion gas scrubbing plant (near Essen Germany). Here the expectation of high fluoride content from combusted plastics was seen as a significant challenge for titanium. In the event samples sustained three years of trial exposure without corrosion. Despite this entirely satisfactory record the titanium industry continues to insist on a review all existing and proposed installations, and will not recommend the use of titanium where it can be shown that operating conditions will permit sustained exposure to active uncomplexed acidic fluorides in flue gas or condensates.
4. Q: Is Grade 5 (Ti6Al-4V) suitable for sour service? It is not listed among the approved titanium alloys in MRO-175
A. Correct, - the most widely available ‘workhorse’ alloy Grade % is not currently listed, but is none the less frequently used as a standard material for drilling and non magnetic instrument housing applications in sour down hole environments. The relatively short exposure times of measuring and logging during drilling operations are a positive factor in the successful use of the alloy.
Any suitable titanium alloy, (and there are many besides the seven listed in MRO-175), may be release tested as satisfactory for operation under proposed sour service conditions for example in accordance with the guidelines for testing laid down in European Federation of Corrosion Handbook EFC17. See TIG Data sheet 15 for further details on this topic.
5. Q: There appear to be many titanium alloys - over 30 are listed in the ASTM specifications - why then am I usually offered a choice of either commercially pure titanium Grade 2 or Ti-6Al-4V alloy Grade 5?
The answer here is that between them these two are, as you have found, most widely available, and because they are produced in considerable quantities by every producer they are available in all normally specified engineering forms and the least costly alloys in the titanium family. In addition these two reliably fulfil the principal requirements for selecting titanium - commercially pure offers outstanding resistance to a corrosion by a wide range of aggressive media. Ti-6Al-4V with strength in the region of 900 -1000MPa satisfies most engineering applications where weight saving is a factor. Both of these basic compositions do have modifications either to enhance corrosion resistance or better to suit them to specific applications. Other alloys have typically been developed for specific applications and this may limit the forms available as well as availability in small quantities. More complex alloying requirements frequently impact adversly on price as does limited production volume and/or more stringent requirements in manufacture and processing such as may apply to alloys developed for aerospace applications. Suppliers can advise further and you can also obtain a free copy of the Titanium Information Group's data disc, 'Titanium and Its Alloys'.