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CHAPTER FIVE
CONCLUSIONS
Responses representing approximately 100 airports were gath- KF on airfield pavements leads to premature oxidation of
ered from the recent (2006) EPA questionnaire, which indi- C/C composite brake components. As the brake frictional char-
cated that potassium acetate (KAc) and sand are most widely acteristics are changed by the use of alkali-metal-salt-based
used at U.S. airports for snow and ice control of airfield pave- deicers on airfield pavements, airline operators are concerned
ments, followed by airside urea, sodium acetate, sodium for- about the adverse effect of these PDPs on the braking perfor-
mate, propylene glycol-based fluids, ethylene glycol-based flu- mance and safety of aircraft.
ids, and other. Responses representing 12 U.S. airports and 3
non-U.S. airports were gathered from the ACRP synthesis sur- There are potential opportunities for all stakeholder groups
vey distributed in this project to the 50 busiest U.S. airports to collaborate to address the catalytic oxidation issue of C/C
among others. From the ACRP survey results, the selection of aircraft brakes, with respect to aircraft and component design,
pavement deicing products (PDPs) by airport staff was based brake testing, aircraft operations, airfield maintenance, etc. In
on many factors, including cost, effectiveness, environmental the domain of brake technologies, the combination of chem-
impact, risk of corrosion, and electrical conductivity. "Effec- ical modification of C/C with structural changes or defect
tiveness" was ranked as the most important criterion and "elec- elimination seems to offer promising solutions to mitigating
trical conductivity" as the least. The effectiveness criterion also catalytic oxidation. Catalytic oxidation of C/C brakes may
exhibited the lowest standard deviation, with "corrosion risk" also be mitigated by utilizing more carbon-friendly PDPs on
being the highest. Interestingly, the challenges and dilemmas airfield pavements.
faced by the airports pertinent to snow and ice control were
highlighted because no airport selected "unimportant" or "not · Cadmium Corrosion
very important" for any of the criteria options in the survey.
Cadmium (Cd) plating is the most popular surface treatment
Alkali-metal-salt-based PDPs such as KAc and potassium technology for corrosion protection of aircraft steel parts
formate (KF) entered the European market to a significant (e.g., airframe components and fasteners). Field reports in-
extent in the mid- to late-1990s. A few years later, these mod- creasingly suggest that the contact with modern PDPs pro-
ern PDPs entered the U.S. market. In both cases, these salts motes damage to aircraft components, including Cd-plated
were introduced as alternatives to urea and glycols used in components. Until recently, the principal evidence connect-
traditional PDPs for freezing point depression, to mitigate the ing alkali-metal-salt-based PDPs with Cd-plating corrosion
environmental concerns related to airfield deicing and anti- has been the increasing number of reports of the latter occur-
icing operations. It became apparent soon after their intro- ring concurrently with to the introduction of the former.
duction that these new deicers presented new challenges, to
both the aircraft and airfield infrastructure. There are potential opportunities for all stakeholder groups
to collaborate to prevent and mitigate the effects of PDPs on
· Catalytic Oxidation of CarbonCarbon Composite aircraft components, from aspects of aircraft and component
Brakes design, aircraft operations, and airfield maintenance. In the
domain of corrosion-inhibiting compounds, there is still great
Thermal oxidation is the primary design specification govern- potential for improvement when it comes to mitigating the
ing durability of aircraft carboncarbon (C/C) composite effect of PDPs on aircraft frames and components. Little aca-
brakes. Catalytic oxidation of C/C composite brakes resulting demic research on interactions between alkali metal salts and
from airfield PDPs has become a growing concern that needs Cd-plating is available, and still less is available on inhibition
to be monitored in the ever-changing operation environment. of these interactions. In lieu of a comprehensive prevention
In recent years, as non-traditional chemical contaminants, solution to Cd-plating corrosion or a satisfactory Cd-plating
modern PDPs may be responsible for the more rapid structural replacement, shop-level mitigation practices such as addi-
failure of C/C composite brakes. To avoid potential safety tional and enhanced maintenance and inspection should help
implications, this concern has to be mitigated through more reduce the effects of PDPs on corrosion-prone, Cd-plated
frequent proactive maintenance and inspection activities incur- steel aircraft components. Such best practices would also min-
ring high direct and indirect costs. A growing body of field evi- imize the impact of PDPs on other aircraft components. In addi-
dence from airline operators suggests that the use of KAc and tion, the corrosion of aircraft components (e.g., Cd-plating and
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aluminum parts) can be mitigated by using less corrosive PDPs both aggregates and asphalt mixes. Concurrent to the use of
on airfield pavements. acetate and formate-based deicers in the 1990s, asphalt pave-
ment in Europe saw an increase in pavement durability prob-
· Interaction with Aircraft Deicing and Anti-Icing lems. At some Nordic airports, these problems emerged as
Fluids degradation and disintegration of asphalt pavement, softening
of asphalt binders, and stripping of asphalt mixes occurring
Thickeners used in modern aircraft deicing and anti-icing flu- together with loose aggregates on the runways. Such problems
ids increase viscosity through chargecharge interaction; were not identified before the airports changed from urea to
organic salts such as KAc and KF are known to disrupt this KAc- and KF-based deicers. According to laboratory and field
interaction and cause a measurable reduction in their viscos- investigations conducted under a joint research program--the
ity. Not only do alkali-metal-salt-based PDPs accelerate the JÄPÄ Finnish De-icing Project--the damaging mechanism
precipitation and buildup of thickener residues, but under the of asphalt pavement by modern PDPs appeared to be a combi-
right conditions, they may also encourage greater moisture
nation of chemical reactions, emulsification, and distillation, as
uptake by the thickeners.
well as the generation of additional stress inside the asphalt
mix.
Although interaction between runway and aircraft deicers
is inevitable, there are opportunities to control the effects of
To prevent or mitigate the effects of PDPs on asphalt pave-
the interaction by means of enhanced operational practices.
Nonetheless, challenges such as financial and environmental ment, the first and most important countermeasure is to fol-
constraints remain for such operational practices in commer- low best possible practices in asphalt mix design and con-
cial aviation. In addition, spray from PDP pools is unpredict- struction. Responses to the ACRP survey for this project
able during aircraft take-off and landing. Interaction with pointed toward adoption of some of these preventive mea-
Type IV aircraft deicing and anti-icing fluids has been seen sures: one European airport reduced asphalt pavement air
to rapidly promote rough, persistent residue on wing leading void to 3.0%; another European airport indicated using
edges with unfavorable aerodynamic properties. polymer-modified binder; and one U.S. airport changed the
asphalt binder to PG 76-32, citing current FAA specifications.
· Impact of Pavement Deicing Products on Concrete Nonetheless, the JÄPÄ Project research showed that the resis-
Pavement tance of asphalt pavement to deicers can be improved only
partially by mix design. According to the laboratory results,
The last decade has seen an increase in the premature deteri- binders with high viscosity or polymer-modified binders were
oration of airfield portland cement concrete (PCC) pavements recommended when formate/acetate-based deicers were to be
with the use of alkali-metal-salt-based PDPs. Such PDPs have used. High-quality (sound) aggregates could also improve
been used more extensively and for more years in European the durability of asphalt pavements in the presence of such
countries for winter maintenance than in the United States. deicers, and so did the aggregates with higher pH. The void
The degree of distress in the PCC pavements of European contents of the asphalt mixes were recommended to be kept
facilities ranged from mild to severe in terms of surface crack- low enough to limit deicer solution in pores.
ing, repair, and rehabilitation efforts needed. Limited exist-
ing laboratory studies indicated that alkali-metal-salt-based · Impact of Pavement Deicing Products on Other
deicers could cause or accelerate alkali-silica reaction (ASR)
Airfield Infrastructure
distress in the surface of PCC pavement by increasing the pH
of concrete pore solution.
Other airfield infrastructure that comes into contact with PDPs
includes ground support equipment, signage, and lighting and
To prevent or mitigate the effects of PDPs on concrete
pavement, the first and most important countermeasure is to other electrical systems. Empirical evidence exists indicating
follow best possible practices in concrete mix design and con- that PDPs are responsible for damaging such infrastructure.
struction. ASR has been conventionally controlled by limit- However, no academic-peer-reviewed scientific information
ing alkali content in cement and selecting aggregates of good could be found to corroborate these empirical observations.
quality. Furthermore, efforts have been made to mitigate ASR
by adding various supplementary cementitious materials or · Looking to the Future
chemical admixtures such as lithium compounds.
When it comes to airfield pavement deicing and anti-icing
· Impact of Pavement Deicing Products on Asphalt there are no simple solutions to the competing, and sometimes
Pavement conflicting, objectives of aircraft safety, environmental reg-
ulatory compliance, materials compatibility, and operational
In addition to the effects of PDPs on PCC pavement, their implementation viability.
effects on asphalt pavement are also of increasing concern. A
laboratory study found that the use of PDPs (sodium chloride, The ACRP survey distributed for this project provided a
KAc, and sodium formate, as well as urea) was damaging to forum to describe knowledge gaps and research needs, as well
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TABLE 13
SUMMARY OF PDP EFFECTS, KNOWLEDGE GAPS, AND ONGOING RESEARCH
PDP Impact What Is Known What Is Unknown Ongoing Research
1. A growing body of field evidence from
The SAE G-12 Carbon Oxidation
airline operators suggests that the use of 1. There is still a need to establish a
Working Group is in the process of
KAc and KF on airfield pavements leads comprehensive PDP catalytic
Catalytic refining a carbon compatibility test
to catalytic oxidation of C/C composite oxidation test protocol.
oxidation of protocol.
brake components.
carboncarbon
composite 2. More research is needed to better
2. Existing research in the laboratory has The SAE A-5A Brake Manufacturers
brakes understand relationships between
demonstrated the catalytic effects of Working Group is in the process of
brake design, AO treatment, and
potassium, sodium, and calcium on carbon developing an oxidation test method
PDP contamination as factors in
oxidation. for AO-treated coupons.
catalytic oxidation.
1. There is still a need to establish a
1.Until recently, the principal evidence
comprehensive metallic corrosion
connecting alkali-metal-salt-based PDPs
test protocol for PDPs.
with Cd-plating corrosion has been a
trend of increased reports of the latter 2. More research is needed to better
Corrosion of understand the interactions among A Cd-corrosion test protocol has been
occurring simultaneously with the
aircraft alloys the aircraft component design, the in development in the SAE G-12 Cd
introduction of the former.
(with a focus CICs used, and the contamination Corrosion Working Group since 2003
2. Very little research has been conducted
on cadmium of PDPs in the processes of and is currently being refined for
to investigate the mechanism of Cd
plating) metallic corrosion. inclusion to AMS 1431 and 1435.
corrosion or Cd-steel corrosion in the
presence of alkali metal salts (e.g., KF and 3. There is still a lack of academic
KAc), partly owing to the high toxicity research data from controlled field
associated with Cd and its compounds. investigation regarding the aircraft
metallic corrosion by PDPs.
1. The contamination effects of
ADAFs by runway deicing fluids
have been well-observed but not
Interaction 1. Recent laboratory data appear to
yet thoroughly quantified.
with aircraft corroborate anecdotal reports of increased The SAE G-12 Fluid Residues
deicing and rates of thickener residues in environments Working Group is leading research
2. Further research is needed to
anti-icing where alkali-metal-salt-based PDPs have efforts in this field.
better understand the interactions
products been used.
between ADAFs and PDPs, as new
ADAFs and PDPs are continually
introduced to the market.
1. There is a need for research data
1. The last decade has seen an increase in
from controlled field investigation IPRF Project 05-7: Performance of
the premature deterioration of airfield
regarding the effects of alkali-metal- Concrete in the Presence of Airfield
PCC pavements with the use of alkali-
Salt-based PDPs on concrete Pavement Deicers and Identification of
Impact of metal-salt-based PDPs.
pavement. Induced Distress Mechanisms and
PDPs on
IPRF Project 06-5: Role of Dirty
concrete 2. Limited existing laboratory studies
Aggregates in the Performance of
pavement indicated that alkali-metal-salt-based
2. There is a need to unravel the Concrete Exposed to Airfield
deicers could cause or accelerate ASR
specific mechanism by which alkali Pavement Deicer, both conducted by
distress in the surface of PCC pavement,
metal salts cause or promote ASR. Clemson University.
by increasing the pH of concrete pore
solution.
1. Although it was observed in some
1. There is a need for research data
Nordic airfields that exacerbated asphalt AAPTP Project 05-03: Effect of
from controlled field investigation
deterioration occurred with applications of Deicing Chemicals on HMA Airfield
regarding the effects of alkali-metal-
alkali-metal-salt-based PDPs, there is thus Pavements, conducted by the
Impact of salt-based PDPs on asphalt
far little observation reported in U.S. or Advanced Asphalt Technologies.
PDPs on pavement.
Canadian airports.
asphalt
2. There is a need to unravel the RITA Project: Mitigation of Moisture
pavement 2. Significantly accelerated deterioration
specific mechanisms by which and Deicer Effects on Asphalt Thermal
of asphalt pavements was found in
alkali metal salts and other PDPs Cracking through Polymer
laboratories when exposed to
(e.g., bio-based deicers) deteriorate Modification, conducted by Montana
acetate/formate-based deicers.
asphalt pavement. State University.
Impact of 1. Empirical evidence exists indicating 1. No academic-peer-reviewed
PDPs on other that PDPs are responsible for damaging scientific information could be
N/A
airfield other airfield infrastructure (GSE, signage, found to corroborate these
infrastructure lighting and other electrical systems). empirical observations.
AO = anti-oxidant; CICs = corrosion-inhibiting compounds; ADAFs = aircraft deicing/anti-icing fluids; ASR = alkali-silica reaction;
N/A = not available.
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as potential challenges for the future of airfield pavement deicers with airfield infrastructure and airframe materials, as
deicing and anti-icing. Some of the key findings from the sur- well as standards for environmental effects of deicers. There
vey and the literature review are summarized in Table 13. was a strong call for best practices and new test methods
with pass/fail criteria, but also skepticism about the actual
A dominant theme throughout all the responses provided impacts of PDPs and whether scientific data are truly avail-
by airports was the challenge of needing environmentally able to confirm these impacts. One suggestion noted was to
benign products that are simultaneously safe for aircraft, keep precise records of PDP use to develop the knowledge
pavement, and electrical systems. Several respondents indi- base needed to determine if PDPs are damaging aircraft and
cated the need for standards concerning the compatibility of airfields.
