* J.H.P. Ra Èmo È, M.E.T. Sillanpa Èa È University of Oulu, Laboratory of Water Resources and Environmental Engineering P.O. Box 4300, FIN-90014 University of Oulu (Finland) K.T. Saarinen VTT Industrial Systems, P.O. Box 1704, FIN-02044 VTT (Finland) Uniformcorrosionoftitaniuminalkalinehydrogen peroxide conditions: influence of transition metals and inhibitors calcium and silicate Gleichma Èûige Fla Èchenkorrosion von Titan unter alkalischen Wasserstoffperoxid- bedingungen: Einfluss der U È bergangsmetalle und der Inhibitoren Kalzium und Silikat J. Ra Èmo È*, K. Saarinen and M. Sillanpa Èa È Uniform corrosion of titanium was studied in alkaline hydrogen peroxide environments simulating pulp bleaching conditions. Cor- rosion rates of unalloyed Grade 2 and alloyed Grade 5 were deter- mined as a function of hydrogen peroxide anion (HOO  ) concen- tration. Influences of calcium and silicate inhibitors and iron and manganese were investigated. Without inhibition titanium corroded at HOO  content of 200 mg/l: Grade 2 0.4 mm/y and Grade 5 1.4 mm/y. Addition of calcium (Ca 2 ) and silicate (SiO 3 2 ) dimin- ished the corrosion of Grade 2 to critical anion level 400 mg/l, but could not protect Grade 5 even at the HOO  concentration of 300 mg/l. Presence of iron and manganese raised the critical levels of the both grades. High HOO  anion level was observed as a no- table potential difference between titanium and platinum. Gleichma Èûige Fla Èchenkorrosion von Titan wurde in alkalischen Wasserstoffperoxidbedingungen, die Papierbleichbedingungen si- mulieren, untersucht. Die Korrosionsgeschwindigkeiten von unle- gierter Gu Èteklasse 2 und legierter Gu Èteklasse 5 wurden als Funktion der Wasserstoffperoxidanionen(HOO  )konzentration bestimmt. Einflu Èsse von Kalzium- und Silikatinhibitoren sowie von Eisen und Mangan wurden untersucht. Ohne Inhibierung betrug die Kor- rosion von Titan bei einem HOO  -Gehalt von 200 mg/l bei der Gu È- teklasse 2 0,4 mm/Jahr und bei der Gu Èteklasse 5 1,4 mm Jahr. Die Zugabe von Kalzium (Ca 2 ) und Silikat (SiO 3 2 ) verminderte die Korrosion der Gu Èteklasse 2 auf ein kritisches Anionenniveau von 400 mg/l; konnte aber Gu Èteklasse5selbstbeieinerHOO  -Konzen- tration von 300 mg/l nicht schu Ètzen. Die Anwesenheit von Eisen und Mangan erho Èhte die kritischen Niveaus fu Èr beide Gu Èteklassen. Hohe HOO  -Anionengehalte wurden als betra Èchtliche Potential- differenz zwischen Titan und Platin beobachtet. 1 Introduction Environmental concerns are leading to the abandonment of chlorine gas in pulp bleaching and movement towards modern elemental chlorine-free (ECF) and even total chlorine-free (TCF) processes. In both these processes, alkaline hydrogen peroxide stages are important parts of the sequence. Bleaching lines have mostly been designed to operate in chlorine envir- onments, and changes in processes to incorporate hydrogen peroxide may be harmful to existing bleaching towers, which traditionally are made of titanium. Investigations of real pro- cess conditions are needed to assess the nature and extent of the risk. Corrosion rates of titanium and titanium alloys increase markedly with hydrogen peroxide concentration and pH of the solution [1]. The dissociation of hydrogen peroxide to HOO  and H  ions is a pH dependent acid-base equilibrium: HOOH $ HOO   H  1 It has been proposed that HOO  ion forms a peroxide com- plex with titanium: Ti  OOH   3OH  $ TiOH 2 O 2  H 2 O  4e  2 The Ti(OH) 2 O 2 complex hydrolyses to form hydrogen per- oxide (HOOH) and titanium hydroxide (Ti(OH) 4 ), which de- tach from the metal surface [1]. The Ti(OH) 2 O 2 complex may also react with the HOO  ion to form soluble HTiO 3  ion [2]. According to reaction (2), the HOO  ion is the corroding spe- cies. Knowing the HOO  concentration levels critical for ti- tanium is thus important, and determination of these was the purpose of this work. A critical hydrogen peroxide ion concentration of 0.008  0.008 M has been reported for reagent grade 80 8C hydrogen peroxide solution containing magnesium sulphate and sodium chloride [3]. In other studies, calcium and silicate have been found to inhibit the corrosion of titanium in hydro- gen peroxide bleaching environments [4±6], and their influ- ence on the critical HOO  levels was of interest here. The presence of transition metals, mainly iron (Fe) and manganese (Mn) has a strong impact on the behaviour of hydrogen per- oxide, mainly through catalysing its decomposition by differ- ent mechanisms [7]. These metals were also studied. The ex- 898 Ra Èmo È, Saarinen and Sillanpa Èa È Materials and Corrosion 53, 898±901 (2002) 0947-5117/02/1212-0898$17.50.50/0 Ó 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim