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17 and more than 50% less catalytic activity and passed all The main drawback of Cd-plating is the high toxicity asso- AMS 1435A deicing criteria, including the proposed corro- ciated with Cd and its compounds. Cadmium can accumulate sion criteria (Boeing Method) (Walker 2007). in the human body with acute or chronic exposure, and even- tually lead to softening of the bones and kidney failure in humans and many animals ("Metals as Toxins--Cadmium" Knowledge Gaps 2007). In addition, the Cd-plating process often involves the use of toxic cyanide baths and the process itself can weaken Although the fundamental mechanisms of catalytic oxidation steel components through hydrogen embrittlement if post- by PDPs are well understood in well-controlled laboratory application precautions are not taken. settings and advances in technologies for its prevention and mitigation have been made in the last decade or so, the prob- Despite the disadvantages of Cd-plating and a large body lem appears far from solved. Action in the following areas of research on its alternatives (Smith 1992; Baldwin and Smith may be beneficial for further advances. 1996; Thomson 1996; Zhirnov et al. 2003), the desirable qual- ities of Cd-plating have yet to be matched or exceeded in a There is still a need to establish a comprehensive PDP cat- single alternative. alytic oxidation test protocol. To this end, a test protocol has been in development in the SAE G-12 Working Group since Field reports increasingly suggest that the contact with early 2003 and is currently being refined for inclusion to AMS modern PDPs (such as potassium acetate- and formate-based 1431 and 1435. Incorporation of such a test protocol (includ- products) promotes damage to aircraft components, including ing a conductivity test as suggested by several ACRP survey those that are Cd-plated. In April 2002, the Aerodrome Safety respondents) into AMS 1431C and 1435B will provide nec- Branch under Transport Canada issued an Aerodrome Safety essary guidance for developing the next generation of PDPs Circular, recommending that airport operators refrain from and C/C aircraft brakes. The proprietary nature of PDP and using deicing fluids containing KF on airside movement areas. C/C aircraft brake technologies may hinder the development The recommendation was based on a Boeing Service Bulletin of such a test protocol, and the ever-changing nature of these indicating that all B737-600, -700, -700C, -800, and -900 air- technologies may entail continued efforts in updating the test plane models were prone to suffer KF-promoted corrosion of protocol. electrical connectors located in the wheel well. In August 2004, Transport Canada cancelled the Circular, based on new Furthermore, more research is needed to better understand evidence that the problem appeared to be limited to the relationships between brake design, AO treatment, and PDP Boeing 737, but suggested that airport operators inform air contamination as factors in catalytic oxidation. PDP devel- carriers serving their airport of the PDPs used on airside move- opment is still an active field, and new products will continue ment areas (Transport Canada . . . 2004). In September 2005, to be introduced to the market. AO treatments designed to the FAA updated an existing airworthiness directive that mitigate catalytic oxidation by PDPs are still immature and applies to all Boeing B737-600, -700, -700C, -800, and mostly proprietary. -900 airplane models. The existing directive required "either determining exposure to runway deicing fluids containing KF, CADMIUM CORROSION or performing repetitive inspections of certain electrical con- nectors in the wheel well of the main landing gear for corro- Cadmium (Cd) had been the standard for protection of steel sion and follow-on actions." The amendment was prompted by parts on aircraft wheels and brakes even before the 1980s. Cd- anecdotal evidence showing similar corrosion effects of KAc- plating is the most popular surface treatment technology for based PDPs and added a new inspection requirement and corrosion protection of aircraft steel parts (e.g., airframe com- related corrective actions. The goal was to "prevent corrosion ponents and fasteners), which is of great importance to flight and subsequent moisture ingress into the electrical connectors, safety and aircraft durability. This is attributable to the unique which could result in an electrical short and consequent incor- combination of its excellent corrosion protection properties in rect functioning of critical airplane systems essential to safe traditional service environments and its other service charac- flight and landing of the airplane" ("Airworthiness Direc- teristics. Cd-plating serves as a highly effective barrier coat- tives . . ." 2005). The U.S. Air Force has found that KAc-based ing, especially in the marine environments often experienced runway deicing fluids caused numerous problems with its air- by aircraft. It also serves as a sacrificial coating to protect steel craft components, mainly electronics (e.g., failure of switches and features nonvoluminous corrosion products. Cadmium and wire harnesses), likely owing to high conductivity of the offers better corrosion resistance and a greater immunity deicers ("Runway Deicing . . ." 2007). domain than zinc (Badawy and Al-Kharafi 1998). Cadmium is also galvanically compatible with aluminum alloys Cadmium corrosion has been observed in Continental Air- (Baldwin and Smith 1996). Other attractive properties of Cd- lines (CO) and Scandinavian Airlines System 737-NG and CO plating include its good conductivity and surface lubricity, high EMB-145 MWW (main landing gear wheel well) electrical ductility, solderability, and potential to be repaired in the connectors, MWW components, and air conditioning bay field. packs. Aluminum corrosion has been observed in CO 737-NG

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18 and other airline 737-Classic MWW and wing aluminum rosion test protocol involving cyclic immersion in the PDP hydraulic lines. The PDPs were also suspected to cause the for 31 days (instead of ASTM F1111, as discussed later). premature corrosion of landing gear joints, accelerate the de- With the first round of tests since 2005, participating Euro- gradation of electrical wire harness insulation, and promote pean and U.S. laboratories observed generally consistent the corrosion of aluminum belly skin (Duncan 2006). Similar mass gain patterns in Cd-plated coupons in the presence of effects have also been observed on ground support equipment KF (Nicholas 2007), and the experiment was redesigned and (GSE) units. Among them, the foremost concern has been the the second round of tests began in 2007. The material used effect of modern PDPs on Cd corrosion, although the other was changed from F1111 Cd-plated steel to AMS QQ-P- problems are more anecdotal and are more easily mitigated 416B Cd-plated steel. The test environment was adjusted to through better aircraft design or maintenance practices. 90F (32.2C) and 30% relative humidity, and the test dura- tion was reduced from 31 to 14 days. The number of brush As such, the rest of this section synthesizes the information strokes on the specimen was regulated at 12 stokes per side. on the validity and nature of the effect modern PDPs (mainly Methanol was used to dry the specimen to ensure moisture alkali-metal-salts) on Cd corrosion, describes the related stan- removal. Data indicated a very reliable test within the same dards and test protocols, discusses ways to prevent and miti- laboratory, but variations between laboratories. The latter gate such effect, and identifies pertinent knowledge gaps. was likely derived from deviations in detailed procedures (Nicholas 2007). The only anomalous weight gain (see Table 9, highlighted in bold) was attributed to an unidenti- Validity of the Effect of Modern Pavement Deicing fied deviation in procedure. The next round of testing was Products on Cadmium Corrosion planned for late 2007 and was to be conducted under signif- Until recently, the principal evidence connecting alkali-metal- icant changes in experiment design to further reduce vari- salt-based PDPs with Cd-plating corrosion has been a trend of ability between laboratories and to incorporate a fluid to increased reports of the latter occurring simultaneously with simulate KAc-based PDPs (along with a KF-based fluid and the introduction of the former (ACRP survey; Duncan 2006). urea as control) into the testing scheme. In the United States, the introduction of alkali-metal-salt-based PDPs in recent years coincided with a rise in the number of Nature of the Effect of Modern Pavement Deicing reported cases of failed or replaced aircraft components result- Products on Cadmium Corrosion ing from Cd corrosion (Duncan 2006). A majority of the rele- vant reports involved mechanical and electrical connectors Because corrosion of metals is an electrochemical process, accessible to runway deicers through spraying or splashing, the thermodynamics of metallic corrosion is generally gov- such as main landing gear wheel wells and air conditioning erned by the combination of pH of the electrolyte and electro- bays. Scandinavian and Northern European airports saw wide- chemical potential of the metal in the electrolyte. For the Cd- spread corrosion of Cd-plated components and electrical fail- water system at 25C, its potential-pH equilibrium diagram ures on 737-NG aircraft in the 2000/2001 winter season, which was established as early as the 1960s (Deltombe et al. 1966). Boeing attributed to their exposure to KAc- and KF-based Such theoretical predictions of corrosion and passivity were PDPs (Hunter 2005). It is interesting to note that all PDPs experimentally validated by testing the dissolution of Cd in used at these airports passed the Cd-corrosion test required solutions of pH 115. Consistent with the potential-pH equi- by AMS 1435 and AMS 1431. The test protocol--ASTM librium diagram, the immersion test results indicated signifi- F1111--involves the continuous immersion of Cd-plated cant corrosion of Cd in both acid (pH < 7) and highly alkaline steel specimens in the PDP for 24 h, which was later consid- (pH > 12) solutions and an intermediate region of a low cor- ered unable to simulate the field exposure conditions. rosion rate of Cd near a pH of 12 (Tomlinson and Wardle 1975). It can be reasonably assumed that Cd in the first two Boeing initiated round-robin testing based on ASTM F1111 solutions dissolved to Cd2+ and HCdO2-, respectively, whereas Cd-plated steel specimens and a custom-designed Cd cor- in the intermediate region it formed a passive layer of TABLE 9 WEIGHT CHANGE OF CD-PLATED STEEL AFTER ROUND-ROBIN TESTING IN KF, SPECIMEN SIZE 25.4 MM 50.8 MM 1.22 MM Specimen Type Weight Change of Specimens (g) Lab A Lab B Lab C Lab D Lab E ASTM F1111-02 0.0114 0.0919 +0.057 0.0077 0.0618 AMS QQ-P-416B 0.0109 0.0707 0.0155 0.0132 0.0955 Adapted form Nicholas (2007).

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19 Cd(OH)2. The formation of Cd(OH)2 or CdOH2O in neutral Institute might shed some light on the PDP effect on Cd-steel and alkaline solutions has been confirmed and this passive corrosion (Fay et al. 2007, with expanded dataset). The corro- film may be unstable in highly alkaline solutions through the sion behavior of mild steel (ASTM A36) and galvanized steel following reactions (Badawy and Al-Kharafi 2000): (highway guardrail) was studied using various deicer products and analytical-grade KAc. For deicers diluted at 3% by weight Cd ( OH )2 + OH - Cd ( OH )3 - (6) or volume (for solid and liquid deicers, respectively), electro- chemical testing of their corrosion to mild steel and galvanized Cd ( OH )3 + OH - Cd ( OH )4 - 2- (7) steel showed that the acetates and formates (except the solid analytical KF) were much less corrosive to mild steel than the In addition to pH, both the dissolved oxygen and tempera- chloride-based deicers. Steel is considered to be passive when ture are expected to have an influence on the kinetics of metal- its corrosion current density icorr < 0.1 A/cm2, and active cor- lic corrosion. The corrosion of Cd in water at pH 8.310.55 rosion occurs when icorr > 1.0 A/cm2. As such, it can be con- was found to proceed under cathodic control through the cluded that the acetates and formates tested (except the solid reduction of oxygen. The corrosion rate of Cd was reduced analytical KF) were noncorrosive to mild steel, whereas the substantially by limiting the available oxygen in water or chloride-based deicers were very corrosive (as shown in increasing concentrations of OH- and CO32- species (Posselt Table 10). Nonetheless, the galvanized steel in the acetates and and Weber 1974). In another study, however, the presence of formates (except the solid NaF-based product) was found to be oxygen was found to passivate the Cd surface in neutral and corroding at comparably high rates, as seen in the chloride- alkaline solutions (Badawy et al. 1998). The electrochemical based deicers. The corrosion potential (Ecorr) data shown in impedance spectroscopy data indicated the presence of two Table 10 indicate that acetates and formates (except the solid phase maxima in neutral solutions, signifying two consecutive analytical KF) significantly shifted Ecorr of mild steel to the charge transfer reactions with different time constants occur- noble direction, but failed to do so with Ecorr of galvanized ring at the Cd/electrolyte interface (Badawy et al. 1998). Sim- steel. The latter might be attributed to the presence of sacrifi- ilar to most chemical reactions, the rate of Cd corrosion usu- cial zinc in the galvanized steel, which likely changed the ally increases with temperature. potential-pH equilibrium of the steel and moved the metal from a passive state to an active corrosion state. It can be Very little research has been conducted to investigate the assumed that the sacrificial Cd in the Cd-plated steel plays a mechanism of Cd corrosion or Cd-steel corrosion in the pres- similar role to the sacrificial zinc in the galvanized steel when ence of alkali-metal-salts (e.g., KF and KAc), partly owing to exposed to alkali-metal-salt deicers. the high toxicity associated with Cd and its compounds. It is known that the corrosion properties of Cd resemble those of A few mechanisms may now be proposed that may be re- zinc in the range pH 811, except for the higher corrosion sponsible for the effect of modern PDPs on the corrosion of resistance of Cd (Posselt and Weber 1974). As such, a recent Cd-steel aircraft components. First, PDP residues may be laboratory study conducted at the Western Transportation highly concentrated on localized areas of aircraft, owing to TABLE 10 ELECTROCHEMICAL ANALYSIS OF DEICER EFFECT TO MILD STEEL (ASTM A36) AND GALVANIZED STEEL (Guardrail) Deicer Corrosion Rate (MPY) Ecorr(mV, vs. SCE) icorr(A.cm2) MgCl2 (liquid) 2.7 1.1 616.0 1.8 6.0 2.5 Mild Steel Salt/Sand 2.5 0.6 764.3 6.0 5.4 1.3 KAc (liquid) 2.5E-03 9.1E-05 155.3 30.2 5.5E-03 2.0E-04 KAc (solid, analytical) 2.5E-03 3.0E-04 132.3 13.3 5.5E-03 6.0E-04 NaAc (solid) 7.1E-03 4.1E-03 204.3 68.6 6.8E-02 9.3E-02 NaF(solid) 2.5E-03 2.1E-04 199.5 12.0 5.5E-03 6.0E-04 KF (solid, analytical) 8.5 11.5 598.0 316.5 19.8 23.9 MgCl2 (liquid) 1.7 0.2 1037.5 5.0 3.5 0.6 Galvanized Steel Salt/Sand 0.8 2.0E-02 1047.5 5.0 1.6 0.1 KAc (liquid) 1.7 0.6 1032.5 5.0 3.0 0.9 KAc (solid, analytical) 1.8 1.2 1050.0 0.0 4.4 1.3 NaAc (solid) 0.9 0.2 1035.0 19.1 1.8 0.3 NaF(solid) 5.2E-02 5.9E-02 1003.3 8.3 0.2 9.1E-02 KF (solid, analytical) 1.6 0.9 1060.0 0.0 4.1 0.6 Note: 1 MPY = 1 milli-inch per year = 0.0254 mm per year.

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20 the hydroscopic nature of these PDPs (e.g., KAc and NaF). The Boeing standard limits the Cd weight loss owing to This may lead to localized high alkalinity that can disrupt the corrosion to no more than 0.03 mg/cm3, equivalent to no more passive film of Cd(OH)2 by means of the reactions in Eqs. 6 than a 0.0077 g loss from a single test coupon. Because the and 7. Second, the potential corrosion products (CdAc and majority of Cd weight loss occurs in the first 14 days of test- CdF) are highly soluble in water, which may facilitate the cor- ing, Boeing reduced the test duration from 31 days to 14 days rosion of Cd-steel. Third, Cd serves as a sacrificial anode to and plans to establish a new weight loss threshold level based protect the steel components and the corrosion of Cd may be on the revised test method. accelerated by the high conductivity derived from its con- tamination by PDPs and the increasingly smaller area ratio Currently, the Boeing Cd corrosion test protocol is being of anodic sites (Cd) to cathodic sites (steel). Finally, the pres- modified. As discussed earlier, round-robin testing data in- ence of PDPs might promote the hydrogen embrittlement of dicated good consistency within the same laboratory, but Cd-plated steel. revealed large variations between different laboratories. These variations highlight the need to develop reliable, stan- It should be cautioned no conclusions should be drawn dard testing procedures that can be used to evaluate the cor- about the corrosivity of PDPs without stating the specific rosivity of PDPs to Cd-plated steel, which would allow better deicer product and its concentration, the test protocol used practices for preventing or mitigating such corrosion. Details (SAE, ASTM, National Association of Corrosion Engineers, of the test protocol such as the Cd-plated steel material, dimen- or electrochemical test), and the type of metal tested. sions, and configuration; specimen pretreatment and post- treatment; and testing environment (relative humidity, temper- Standards and Test Protocols ature, etc.) should be well-defined and controlled to ensure repeatable and reproducible results. In May 2002, ASTM Committee F07 on Aerospace and Air- craft published an updated version of ASTM F1111-02, Standard Test Method for Corrosion of Low-Embrittling Prevention and Mitigation Cadmium Plate by Aircraft Maintenance Chemicals. As discussed earlier in this chapter, the testing parameters of There are potential opportunities for all stakeholder groups to ASTM F1111-02 have been considered insufficient for dis- collaborate to prevent and mitigate the effects of PDPs on air- criminating PDPs for their corrosivity, likely resulting from craft components from aspects of aircraft and component the relatively mild temperature (95F or 35C) and short, design, aircraft operations, and airfield maintenance. Installa- continuous immersion period (24 h). In response to this defi- tion of TR (engine thrust reverser) cascades with improved ciency, Boeing developed its current Cd corrosion test (dis- design, replacement of Cd-plated connectors with stainless cussed later) and integrated it into the broader Boeing test steel and anodized aluminum connectors, and application protocol. Similar to those found in ASTM F1111-02, the of corrosion-inhibiting compounds (CICs) are suggested by plating specifications contained in the Boeing Cd corrosion Boeing (Duncan 2006). Frequent inspection or online moni- test are intended to be used only for evaluation purposes and toring of corrosion-prone components, although costly, is differ from those in AMS QQ-P-416 B, the accepted standard another way to mitigate corrosion of Cd-plating and aluminum for electrodeposited Cd-plating in aerospace applications. corrosion. AMS QQ-P-416 B references separate ASTM and NASA standards for determining corrosion resistance to salt spray, In the domain of CIC technologies there is still great but does not set forth parameters for general corrosion test- potential for improvement. At this stage, the research has to ing such as those specified in ASTM F1111-02 or the Boeing build on the existing knowledge base of inhibiting Cd cor- protocol. rosion in the absence of PDPs. For instance, some quino- Boeing Document D6-17487, Evaluation of Airplane line derivatives (quinaldic acid, oxine, 2-methyloxine, and Maintenance Materials, contains a section (20) specific to oxine-5-sulfonic acid) were found to form stable chelate measuring the corrosivity of runway deicers to Cd-plating. compounds with Cd and thus inhibit the general corrosion The Boeing test sample specification was derived from the of Cd (Kato et al. 1973). Quinoline, however, is a known ASTM F1111 specification using a specimen size of 25.4 mm hazardous air pollutant. Precoating the Cd surface with CO32- 50.8 mm 1.22 mm. The Boeing test protocol uses a or sodium metasilicate was reported to greatly reduce its 31-day cyclic immersion of Cd-plated steel in the electrolyte, corrosion rate (Posselt and Weber 1974). Triazoles are known instead of a 24-h continuous immersion used in the ASTM to be effective CICs, but they have been banned in most specification. Using this protocol, Cd-plated steel specimens Northern European countries owing to toxicity concerns were exposed to eight runway de-icing fluids and three con- (Duncan 2006). trol fluids, with three replicates in each solution. The test pro- tocol was demonstrated to be capable of distinguishing the Methanol, ethanol, isopropanol, and n-propanol were found corrosivity of PDPs to Cd-plated steel, with a KAc-based to inhibit Cd corrosion in aqueous solution, and the electro- PDP and a KF-based PDP being the most and the least corro- chemical testing indicated their corrosion inhibition effi- sive, respectively (Hunter 2005). ciency at 0.1 M to be 29.9%, 37.8%, 39.3%, and 98.6%,

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21 respectively. The inhibition mechanism validated by surface The U.S. Air Force suggested adding the solution conduc- analysis was proposed as follows. The alcohol molecules tivity test (ASTM D1125) to AMS 1431 and AMS 1435 as a absorb on the active Cd sites by means of functional -OH required test for PDPs. In addition, other tests such as wet arc groups and stabilize the passive film, with their hydrophobic propagation resistance (SAE AS4373, Method 509), immer- alkyl tails limiting the access of electrolyte to the metal sur- sion volume swell (SAE AS4373, Method 601), bend (SAE face. A concentration of 0.075 M of n-propanol in an alka- AS4373, Method 712), and voltage withstand (SAE AS4373, line solution (pH = 13) achieved 97% reduction in weight loss Method 510) were suggested to be added to AMS 1431 and after a 180-min exposure. It should be noted that the experi- AMS 1435 if possible (USAF Research Laboratory 2007). ment employed highly polished, spectroscopically pure Cd rods rather than AMS- or ASTM-compliant Cd-plated steel (Badawy and Al-Kharafi 1998, 2000). Knowledge Gaps Although the fundamental mechanisms of Cd corrosion in Commercially available CICs for application to fasteners water are relatively well studied, the link between alkali-metal- and other exposed metal are available in wipe-on, brush-on, salt-based PDPs and Cd-plating corrosion has yet to be exper- and spray-on types (Groupe Meban 2007). Most employ an imentally validated and thoroughly investigated. Action in the adsorption mechanism with an active ingredient(s) similar following areas may be beneficial for further advances in miti- to n-propanol as described earlier, which blocks the active gating the effects of PDPs on Cd-plating and aircraft alloys sites on the metal surface. As a secondary effect, the CIC base may then harden or dry on the metal surface, forming in general. a temporary but durable physical barrier to salt spray, cleaners, and other contaminants. There is scant--if any-- First, there is still a need to establish a comprehensive metal- relevant research available to the public on the effectiveness lic corrosion test protocol for PDPs. To this end, a Cd corro- of these CICs in mitigating Cd-plating corrosion, especially sion test protocol has been in development in the SAE G-12 in the presence of PDPs. Working Group since 2003 and is currently being refined for inclusion to AMS 1431 and AMS 1435. Incorporation of such In lieu of a comprehensive prevention solution to Cd- a test protocol (including a conductivity test as suggested plating corrosion or a satisfactory Cd-plating replacement, by several ACRP survey respondents) into AMS 1431C and shop-level mitigation practices such as additional and en- AMS 1435B will provide necessary guidance for developing hanced maintenance and inspection should help reduce the the next generation of PDPs and aircraft components. The pro- effects of PDPs on corrosion-prone Cd-plated steel aircraft prietary nature of PDP and aircraft components may hinder the components. Such best practices would also minimize the development of such a test protocol, and the ever-changing impact of PDPs on other aircraft components. nature of these technologies may entail continued efforts in updating the test protocol. For instance, the need for an accept- In addition, the corrosion of aircraft components (e.g., Cd- able alternative to Cd-plating has led to extensive research on plating and aluminum parts) can be mitigated by utilizing less this subject and several promising alternatives such as Zn- corrosive PDPs on airfield pavements. U.S. patents were Ni-P (Veeraraghavan et al. 2003), Zn-Sn-P (Zhirnov et al. granted in 2001 (USP 6287480), 2003 (USP 6623657), and 2006), and Zn-Ni (Thomson 1996; Claverie and Chaix 2007). 2005 (USP 7063803) for deicing formulae based on potas- sium succinate, succinic anhydride, and succinct acid. At least Second, more research is needed to better understand the one potassium lactate formula has been marketed in the interactions among the aircraft component design, the CICs United States, but only to military facilities (Shi 2007). Such used, and the contamination of PDPs in the processes of metal- deicing compositions are claimed to be suitable and effective lic corrosion. This is further complicated because the use for airport applications in which corrosion of aircraft alloys of Cd-friendly PDPs is still in the burgeoning stage and new and concrete runways are of concern, but they have not been products will be continually introduced to the market. Simi- tested at commercial airports. In addition, no testing in rela- larly, CICs designed to mitigate Cd-plating corrosion by PDPs tion to C/C catalytic oxidation is known to have been con- are still immature and mostly proprietary. Furthermore, field ducted on these formulae. corrosion of metals may be affected by component design and exposure conditions, and various other mechanisms that are PDPs with low electrical conductivity have been sug- unique to the operational environment (e.g., galvanic corro- gested to potentially pose less risk for aircraft components as sion, pitting corrosion, crevice corrosion, stress corrosion well as C/C composite brakes. PDPs based on betaine (a bio- cracking, corrosion fatigue, erosion corrosion, and microbially based freezing point depressant and corrosion inhibitor with influence corrosion). low electrical conductivity) have been developed in Finland and are qualified for AMS 1435A certification owing to low Finally, there is still a lack of academic research data from corrosivity. In addition to its high cost, however, betaine's controlled field investigations regarding the aircraft metallic relatively high BOD and nitrogen content may present some corrosion by PDPs, which would help differentiate the con- challenges, as discussed earlier (Duncan 2006). tribution of PDPs to such corrosion from other possible