Chromate-Free Coating for Aircraft Fasteners

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Solution Overview

Problem

Existing coatings for dissimilar metal assemblies, such as titanium and aluminum in aircraft structures, face issues with galvanic corrosion, inadequate toughness, and high production costs due to short shelf-life and toxicity concerns with chromate-based coatings, while also experiencing galling problems in threaded fasteners.

Innovation Solution

A chromate-free corrosion-resistant coating composition containing 4-8% inorganic salts and 2-15% alkaline polyamine fatty acid salt (PFAS) suspended in a phenol-formaldehyde thermosetting resin, providing both passive and active corrosion resistance and reducing galling by forming a hydrophobic layer that prevents water permeation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If chromate-based coatings are used for corrosion protection, then corrosion resistance is improved, but toxicity and environmental hazards increase

Engineering Contradiction:

Improvecorrosion resistanceVSAvoidtoxicity

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

The patent removes chromate from the coating formulation entirely, extracting the toxic component while retaining corrosion protection functionality through alternative mechanisms using zinc phosphate, zinc molybdate, and other non-chromate corrosion inhibitors

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent changes the chemical composition parameters by substituting chromate with alternative corrosion inhibitor salts (zinc phosphate, zinc molybdate, strontium aluminate) and adjusts the resin system to epoxy, polyester, or acrylic bases, achieving equivalent corrosion protection without toxic hexavalent chromium

Inventive Principle:

Principle #35Parameter changes

2Reliability

If cadmium plating is applied to fasteners, then galvanic corrosion protection is improved, but embrittlement of titanium and high strength steel occurs

Engineering Contradiction:

Improvegalvanic corrosion protectionVSAvoidtoughness

Core Design Contradiction:

ReliabilityVSStrength

Solution Approach 1:

The patent eliminates cadmium from the coating system, removing the embrittling agent while maintaining galvanic corrosion protection through zinc-based inhibitors and proper coating formulation that does not induce brittleness in titanium or high strength steel substrates

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent uses zinc phosphate, zinc molybdate, and other sacrificial corrosion inhibitors that provide protection through controlled corrosion of the inhibitor itself rather than the substrate, avoiding the embrittlement issues associated with cadmium

Inventive Principle:

Principle #27Cheap short-living objects (Disposable)

3Device complexity

If organic or inorganic coatings are applied as passive barriers, then coating simplicity is improved, but substantial corrosion protection is insufficient

Engineering Contradiction:

Improvecoating simplicityVSAvoidcorrosion protection

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The patent creates composite coating formulations combining organic resin matrices (epoxy, polyester, acrylic) with inorganic corrosion inhibitor salts (zinc phosphate, zinc molybdate, strontium aluminate) and pigment particles, achieving both simplicity of application and substantial active corrosion protection through the synergistic composite structure

Inventive Principle:

Principle #40Composite materials

Solution Approach 2:

The coating formulation incorporates self-healing corrosion inhibition through zinc phosphate and zinc molybdate that can actively inhibit corrosion at defect sites and provide cathodic protection, allowing the coating to protect itself and the substrate without additional external intervention

Inventive Principle:

Principle #25Self-service

4Strength

If interference-fit fasteners are used, then assembly strength is improved, but coating toughness and adherence requirements increase

Engineering Contradiction:

Improveassembly strengthVSAvoidcoating tolerance

Core Design Contradiction:

StrengthVSManufacturing precision

Solution Approach 1:

The composite coating structure with flexible polymer matrices and dispersed inorganic inhibitor particles provides both the toughness to withstand interference-fit forces and the adherence to maintain coating integrity, accommodating the mechanical demands of interference-fit assemblies

Inventive Principle:

Principle #40Composite materials

Solution Approach 2:

The patent adjusts coating formulation parameters including resin selection, crosslink density, and inhibitor particle size to optimize the balance between coating toughness and adherence, enabling the coating to survive the interference-fit installation process while maintaining protective functionality

Inventive Principle:

Principle #35Parameter changes

5Reliability

If wet primers or elastomeric sealants are used during installation, then corrosion protection is improved, but assembly cost and production complexity increase

Engineering Contradiction:

Improvecorrosion protectionVSAvoidassembly cost

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

The coating formulation integrates multiple functions into a single application: corrosion inhibition through zinc phosphate and zinc molybdate, adhesion promotion, and gap filling capabilities, eliminating the need for separate wet primers or elastomeric sealants and reducing assembly complexity and cost

Inventive Principle:

Principle #6Universality (Multi-functionality)

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The coating composition offers equivalent corrosion resistance to chromate-containing coatings, extends shelf-life, eliminates tackiness, and significantly reduces galling in fasteners, with improved frictional characteristics and lubricity, suitable for interference-fit and threaded systems.

Implementation Method 1

making the coating material hydrophobic to thus prevent water-borne corrosive materials to reach the substrate

Methodology Applied

Scientific EffectHydrophobic layer formation: Hydrophobe

Implementation Method 2

the chromate undergoes an oxidation reaction in the presence of water and in between two materials that are dissimilar with respect to galvanic potential. This reaction will typically result in a buildup of an oxide layer on the surface of aluminum

Methodology Applied

Scientific EffectOxidation reaction: Oxidation

Implementation Method 3

The organic and inorganic type coatings typically act as a passive, physical barrier against salt, moisture and the like without providing substantial corrosion protection

Methodology Applied

Scientific EffectPhysical barrier:

Implementation Method 4

significantly reduces galling in fasteners, with improved frictional characteristics and lubricity

Methodology Applied

Scientific EffectLubrication: Lubrication

Data Source

PatentUS10259973B2Anti-corrosion and low friction metal pigmented coating

Publication Date: 2019.04.16 HI SHEAR CORP

AI summary

The anti-corrosion coating material contains corrosion inhibiting inorganic constituents, or a combination of inorganic and organic corrosion inhibiting constituents, suspended in a polymeric resin. The corrosion resistant composition includes a salt of inorganic constituents, and alkaline polyamine fatty acid salt (PFAS) as a corrosion inhibitor, suspended in a polymeric resin remainder. The coating material may be applied to metal parts but not limited to aircraft fasteners, such as aircraft fastener parts, including nuts and bolts, screws, rivets, and sleeved systems.