computational corrosion analysis of airframe structures

Alan Rose, Keith Legg Corrdesa

Contacts: [email protected] 770.328.1346; [email protected] 847.680.9420

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a b

Note that corrosion current in (b) depends almost entirely on the polarization resistance of the cathodic material. Protecting the Al makes very little difference. The way to protect the Al is to increase Rp of the Ti and/or to add an electrochemically insulating layer to either surface. Note that curves cross at the far cathodic end of the Ti and stainless, and at the rapid pitting rise of the Al. That means for accurate corrosion rates we have to get the Al curve correct near OCP and deconvolute the redox branches, and we have to get the noble materials right in the far cathodic region.




In absence of CAD for spars, simple model of bushing repair built from flat plate of Al and typical bushing design

Repairs made by grinding out corrosion and filling oversize holes with stainless steel bushings




Bare 316 stainless

Chromated 316 stainless

Bare Ti6Al4V

Chromated Ti6Al4V

25.6

7.3 5.0

0.1 0.05.0

10.015.020.025.030.0

SS316 bare SS316 1200STi6Al4V bare Ti6Al4V1200S

Corrosion current (µA)

Galvanic corrosion models of repair bushings in 2024 Al show that corrosion rate after a Ti bushing repair is only 20% of the rate after a stainless bushing repair. Chromating a Ti bushing practically eliminates corrosion. Brush anodizing the Al after repair also inhibits general corrosion but will do little to preventing galvanic corrosion.

Al

Bushing

316 SS Ti64




2024 Al Anodize Bare Anodize Bare

316 SS Bare Chromate Bare Bare Ti6Al4V Chromate Bare Bare Bare

Model shows what we would expect: Strong galvanic corrosion around stainless fasteners, very little around Ti. Almost no corrosion around Ti, especially if chromate passivated. Extent of corrosion depends on electrolyte thickness – the thicker the electrolyte the further corrosion extends from fasteners.




Note: Very large cathodic current to stainless, very little to Ti. Depending on electrolyte film thickness, stainless fasteners can affect corrosion rate near Ti.




Careful examination around the fastener (front and back of Al panel) shows the cathodic current density is uniform. But anodic current density is very non-uniform on the Al. Reason: Rp(Ti)>>Rp(Al) + Rp(electrolyte) Hence I~(Ecorr Ti – Ecorr Al)/Rp(Ti) Therefore the effect of distance along the film is negligible on the Ti surface




Same is true on the stainless, but the effect is not as pronounced as with Ti All this says that you can protect the Al by increasing Rp with inhibitors, or by adding a resistive layer to the bolt surface Reducing Ecorr Ti – Ecorr Al (e.g. with ZnNi plating) also helps, but not if it results in increased cathodic current density




250μm film

50μm film

J(mA/m2)

J(mA/m2)

Thicker film (lower impedance) spreads corrosion wider, but peak current density is lower adjacent to fastener (lower corrosion rate and depth of damage) Small labels show local J

Note that the cathodic current density is unaffected by the film thickness, so total corrosion current is the same. Jcathodic controls the total corrosion, but film thickness and resistivity control extent and depth of damage




Cu Fe

Two uncoated adjacent flat plates Equal areas, longer conduction paths in fluid Smaller Ecorr difference than Ti/Al Cathodic material not as dominant




As electrolyte film thickness approaches 25mm, total corrosion current asymptotically approaches bulk calculation value.

For thinner electrolyte layers current is less, but concentrated in a smaller area near the material interface. Peaks at about 0.5mm (0.020”). So, depending on NaCl concentration and materials can get faster local corrosion with lower NaCl concentration or thinner corrodant layer

What this means is that we need to better understand the thickness and variability of corrosion layers around material interfaces. What are the typical thicknesses and concentrations of fluid layers caused by water ingress or condensation? How does fluid accumulate around fasteners, bushings, connectors, etc.?




What we learn from this evaluation of different materials and electrolyte film thicknesses is that controlling the galvanic current of the more cathodic material is the best way to control Al corrosion for small items such as fasteners and bushings

However, this does not necessarily imply it is the best approach for larger items where areas are more equal and path length in the electrolyte is longer We need to do more analysis for these types of applications

It also brings up the interesting question of whether we might be better off using electrical connectors with Ti backshells and cathodic current control, than using ZnNi coatings and trying to get them to pass both corrosion and conductance



computational corrosion analysis of airframe structures

Engineering

total corrosion current

extent of corrosion

general corrosion

strong galvanic corrosion

faster local corrosion

fastener lower corrosion

accurate corrosion rates

ti bushing repair