Hydrogen Embrittlement in Titanium Steam Surface Condenser Tubing Truths, Myths & Misnomers
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Hydrogen Embrittlement In Titanium Steam Surface Condenser Tubing Truths, Myths & Misnomers Dennis J. Schumerth Titanium Tubular Consultants 4811 E. La Palma Ave Anaheim, CA 92886 USA E-mail [email protected] [email protected] Thomas Demers System Specialist, NB Power Point Lepreau Generating Station 122 County Line Road Maces Bay, New Brunswick Canada E5J1W1 [email protected] ABSTRACT With more than 600 million feet of Grade 2 passivated by circulating water, (CIRH20) and titanium steam surface condenser tubing in the operating temperatures remain relatively service around the world operating without a low, the solubility limit of hydrogen is rarely single reported corrosion event since 1972, threatened. Indeed, the mere existence of few if any competing materials can make such hydrogen embrittlement in a surface condenser a dramatic claim. However, without proper environment has been infrequent reportedly design, operation and maintenance of the occurring only several times over the past 40+ cathodic system envelope, embrittlement or years of powerplant condenser tube service. hydriding of titanium surface condenser tubing Conversely, industries such as the CPI can occur when solubility limits of nascent (chemical process industry), which can expose hydrogen are exceeded. The results of such the heat exchanger to elevated temperatures, an excursion can, in the case of titanium and pH extremes and high levels of hydrogen other materials, induce the formation of brittle charging without the benefit of passivation hydrides. This corrosion event can ultimately have reported hydrogen embrittlement lead to a loss of structural integrity of the damage. material. Notwithstanding the infrequency of this form of Additional contributing factors may include corrosion in powerplant condenser service, elevated temperature, pH extremes and other recently identified activity has surfaced that parameter anomalies. Since the typical warrants further investigation of this powerplant steam surface condenser phenomenon. This paper will identify and tube/tubesheet interface is continually research several case studies associated with 1 hydrogen embrittlement. An in-depth An inability to properly control SCE voltage, investigation of each will provide a practical use of improper sacrificial materials and benchmark for future lessons learned challenges in responding to changing operating experience. Since there appears to water/salinity conditions have been directly be a common denominator to most if not all of connected to embrittlement damage. It should the reported hydrogen damage, the paper will also be noted however that if the improper attempt to clear the air in terms of apparent operating anomalies are discovered early on, confusion surrounding merely the innocuous corrective action can and has, successfully hydrogen charging of certain materials vs. mitigated damage. As we shall see during an actual embrittlement damage. examination of Case Study 1, historical precedence exists demonstrating this action CP Titanium - R50400 [6] Key Words: hydriding, titanium tubing, has not only halted the corrosion but allowed powerplant surface condenser, impressed the equipment to successfully operate to this current systems (I-C), saturated calomel day. electrode, (SCE), silver/silver-chloride electrode (Ag/AgCl), cathodic protection Hydriding 101 systems (CP), sustained load cracking. The study of hydriding or hydrogen INTRODUCTION and BACKGROUND embrittlement associated with power plant surface condensers is a study of the galvanic The phenomenon of hydrogen absorption or environment that exists within the surface charging can occur in many materials but is condenser steam cycle. However, the term typically more common to aluminum, carbon hydriding can be a misnomer. More correctly, steel, titanium and ferritic stainless steels. The and given the enormous difference between yellow metals and austenitic stainless series hydrogen charging or absorption (which is appear less susceptible. Much of both more common) and hydrogen embrittlement purported and actual corrosion or where actual cracking and/or fracture can embrittlement damage (Figure 1) has been occur, care must be exercised when callously attributed to improper design and/or miss- assigning hydriding as a “catch all term” for this operation of the cathodic protection (CP) or activity in the condenser. impressed current (IC) protective systems. These systems have been historically Hydrogen absorption is a common occurrence problematic appearing to be the singular and and exists naturally within many condenser leading cause of embrittlement damage. circuits. Merely the discovery of classic hydride needle formation within the titanium Figure 1 matrix is not an indictment of the integrity of CP Titanium Hydride Needle the material. However, hydrogen embrittlement can and has occurred with damaging results in the chemical process industry (CPI) where heat exchangers are routinely exposed to elevated temperatures, pH extremes and high levels of hydrogen charging without the benefit of passivation. As expected, much of the hydrogen absorption occurs in high conductivity sea or brackish water conditions. In these environments, hydrogen can be produced at the cathode by galvanic coupling to a dissimilar metal such as zinc or aluminum which are very active (low) in the galvanic series. Coupling to carbon steel or other metals higher in the galvanic series generally does not generate hydrogen in neutral solutions even though corrosion is 1. The pH of the solution is less than progressing on the dissimilar metal. It is 3 or greater than 12 (this would not speculated that the presence of hydrogen typically exist in a steam surface sulfide, which acts as a recombination poison, condenser environment). apparently increases the absorption of hydrogen on titanium and stainless steel 2. Impressed potentials are materials. significantly more negative than - O.75V (vs. SCE). With the powerplant surface condenser tube/tubesheet interface continually passivated 3. The temperature is above 176°F in circulating water (CIRH20), the solubility limit (80°C) (this phenomenon would of hydrogen is rarely threatened. Laboratory not typically exist in a surface work 7, 8 has demonstrated that the presence of condenser). Below this as little as 2% moisture in hydrogen gas temperature only surface hydride effectively passivates titanium so that films will form which, experience absorption does not take place. (Table 1) indicates, do not seriously affect the properties of the metal. Table 1 Failures due to hydriding are rarely Effect of Moisture Absorption encountered below this Of Hydrogen temperature. 4. There must be some mechanism for %H O Hydrogen Pickup 2 (ppm) generating hydrogen. This may be a 0 4.480 galvanic couple, cathodic protection 0.5 51,000 by impressed current, corrosion of 1.0 700 titanium or dynamic abrasion of the 2.0 7 surface with sufficient intensity to 3.3 10 depress the metal potential below 5.3 17 that required for spontaneous 10.2 11 evolution of hydrogen. 22.5 0 37.5 0 Within the range of pH 3 to 12, the oxide film 56.2 0 on titanium is generally stable and presents a 600oF (316oC @ 800 psi – 96 Hours Exposure good barrier to penetration by hydrogen. Efforts at cathodically charging hydrogen into As noted earlier, the mere existence of this pH range have been unsuccessful in short- hydrogen embrittlement in a surface condenser term tests at voltages more positive than - environment is exceedingly rare. Furthermore, O.75V (vs. SCE). If pH is below 3 or above 12, the surface oxide film on titanium acts as a the oxide film is believed to be unstable and highly effective and tenacious barrier to can breakdown, permitting easy access of penetration by hydrogen. Disruption of the available hydrogen to the underlying titanium oxide film however, may allow penetration by metal. Mechanical disruption of the film (i.e., hydrogen. As noted earlier, when the solubility iron smeared into the surface) allows entry of limit of hydrogen is exceeded, hydrides begin hydrogen at any level provided the to precipitate. Excessive absorption of temperature is above 176oF (80oC). hydrogen results in possible embrittlement and the possibility of cracking and fracture under Case Study 1 conditions of stress. Laboratory experiments [2] have shown that four (4) conditions noted The 1987 thru 1989 window produced possibly below usually exist simultaneously for the first and certainly, at the time, the most hydriding to occur. comprehensive benchmark study of powerplant surface condenser tube hydriding. Two papers identified significant titanium- tubed surface condenser tube cracking at the St. Lucie unit followed a similar path in 1979 Florida Power and Light (FP&L)1 nuclear units replacing the Westinghouse 4 - installed, OEM St. Lucie 1 (840 MW) (Figure 2) and Turkey aluminum brass tubes again with Gr. 2 Point 3 (666 MW) (Figure 3). titanium. At the time, both units were retrofitted with manually-controlled, impressed Figure 2 current (IC) rectifiers generating - 900mV/- St. Lucie Nuclear Power Station .90VSCE (saturated calomel electrode). In 1986/87, eddy current (EC) testing suggested possible localized tube wall loss and cracking located near or in proximity to the tubesheet face. However, no tube joint leakage was observed and subsequent removal and flattening, tensile, hydrostatic and hoop stress testing of sample tubes demonstrated little effect on as-manufactured Figure 3 mechanical properties [*] and