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A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 5

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USING SIMULATION TO DESIGN

ANTICORROSIVE AUTOMOTIVE

COMPONENTS

Simulation of components and joints made of hybrid materials enables

innovative designs for corrosion protection in automotive applications.

TECHNICAL SPOTLIGHT

ivetsarecommonlyusedtoholddif-

ferent materials together—and can

easily be spotted on bridge support

beams, airplane doors, and car hoods, for

example. Found in metal-bodied vehicles

and support structures across the trans-

portation industry, these rivets usually

go unnoticed despite their critical role in

joining components that must withstand

enormous mechanical stress. Some cars

contain over 2000 of them. As automo-

tive designs trend toward lightweighting

and the use of multiple metals, questions

arise regarding the destructive, invisi-

ble culprit whose handiwork often goes

unnoticed until it is too late: Corrosion.

GALVANIC CORROSION

Galvanic corrosion is a constant

problem that costs the automotive

industry billions of dollars each year.

Caused by chemical reactions between

different metals coming into contact

withone another, this typeof corrosion is

often visible as a white powdery growth

that forms on the surface of metal parts

(Fig. 1, center). Bubbling paint and dete-

riorating aluminum are also signs that

metallic ions are being exchanged and

degrading a metal’s surface.

Various metal combinations react

differently to environmental impacts,

and a number of factors such as joining

techniques, material properties, and

surface roughness affect the chemical

reactions on rivets and the sheets they

bind together. Therefore, understand-

ing the underlying electrochemistry

is essential in order to develop robust

corrosion protection.

Engineers at Helmholtz-Zentrum

Geesthacht (HZG) and Daimler AG, both

Fig. 1 —

Clean rivet (top). Rivet showing

magnesium hydroxide deposit (white

growth) due to corrosion (center). Magnifica-

tion of a rivet in a test sheet (bottom).

in Germany, joined forces to investigate

corrosion prevention using multiphys-

ics simulation. The team sought ways

to streamline rivet design and develop-

ment, minimize physical testing, and

reduce the need for subsequent steps

such as surface treatment.

MODELING OFFERS INSIGHT

INTO CORROSION BEHAVIOR

To study galvanic corrosion kinet-

ics, including material loss, surface

conditions, and the long-term behav-

ior of interacting metals, Daniel Höche,

scientist at HZG, created a simulation

of a steel punch rivet joint using Multi

physics software from COMSOL, Burl-

ington, Mass. The rivet was plated with

an aluminum-zinc alloy that cathodi-

cally protects the steel. Software allows

the electrochemical interactions at the

surface and edges of the rivet to be

analyzed, predicts the decay of joined

sheets, and adjusts the geometry to

minimize corrosion.

This model consists of the rivet,

bonded metal sheets of aluminum and

magnesium, a 0.1% NaCl electrolyte

layer on the surface representing the

outside environment, and a galvanic

couple at the interface between the riv-

et and the sheets (Fig. 2). A corner burr

in the rivet geometry was also added to

simulate the presence of a sharp edge,

which increases gradients in the elec-

trolyte potential. This, in turn, increases

current flow and hastens the electro-

chemical reactions that cause galvanic

corrosion.

As the interface between the rivet

and sheets experiences corrosion, the

magnesium sheet begins to degrade

more rapidly than the other metals. The

chemical reaction produces magne-

siumhydroxide, Mg(OH)

, which forms a

weak barrier filmon the surface. Growth