Available - NATO Research & Technology Organisation

T-2
Each NATO country identified restrictions on cadmium and chromium as their top concern and
the technology specific presentations echoed that concern.
2.2 Cleaning
Papers were presented from Canada and the USA. A general concern about the
substitution of flammable solvents for non-flammable ODS solvents was described in both papers.
Many effective substitutes for ODS solvents are developed and implemented. Most of these are
organic wipe solvents that consist of oxygenated hydrocarbon blends or terpene/parafinnic
hydrocarbon mixtures. Although these substitutes are effective, tradeoffs must be made between
evaporation rate (relating to vapor pressure and user acceptance) and flammability. Non-liquid
substitutes were also discussed but are in the preliminary research and development (R&D) phase.
These include ultraviolet light with carbon dioxide pellet blasting, ultraviolet light in an elemental
oxygen atmosphere as well as CO, and excimer laser processes.
2.3 Surface Preparation
Several technologies are under evaluation as substitutes for chromate based adhesive bond
pretreatments, conversion coatings and de-oxidizers. Wet chemistry processes are the standard
for surface preparation and papers from Germany, the United Kingdom, and the USA presented
the latest status and summary of results of materials substitutes and new processes under research
and development. Cerium formate, nickel metavandate and tetra-ethyl ammonium thiotungstate
were evaluated as alternatives for strontium chromate corrosion inhibition. Phosphoric Sulfuric
Acid (PSA) anodizing, additions of cerium and potassium permanganate to standard sulfuric acid
anodize baths were presented as promising substitutes for chromic acid anodize while, ion beam
enhanced deposition, sol-gel deposition, excimer laser oxidation, plasma polymerization and
plasma spray techniques were discussed as innovative techniques that were explored as
alternatives for chromated adhesive bond pretreatments.
Replacements for chromate anodize operations were presented by Germany and the
United Kingdom. It was concluded that the PSA anodize process could be used for applications
that do not require a hot water seal. The process requires a shorter immersion duration and has
some potential economic benefits, however, the need to operate a recycling facility to keep the
content of dissolved aluminum within the desired limits offers a challenge to large production
scale-up. Chemical additions (at 7.5 g/l) to the standard sulfuric acid anodize process were
evaluated against the neutral salt fog results of chromic acid anodize. The addition of cerium (111)
or cerium (IV) provided high (but not yet comparable) levels of corrosion protection. More work
needs to be pursued in this area to vary concentrations, study alternative additions and define the
fatigue and salt fog/SO, performance of other aerospace alloys that undergo these novel
processes.
Among the new adhesive bond pretreatments discussed, the sol-gel technology showed
particular promise since it appears to be the easiest, most economical process to scale-up. Sol-gel
materials also provided the best materials performance properties (wedge and lap shear tests of
adhesive bond strength) as reported by the USA.