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Protecting Mg Alloys from Corrosion with PEO
Keronite Whitepaper
Protecting Mg
alloys from
corrosion with PEO
© Keronite 2020 | keronite.com
1
Protecting Mg Alloys from Corrosion with PEO
Contents
2 - Introduction
3 - Developing an advanced coating for lightweight
magnesium components
6 - Further developments in coatings for extreme
corrosion resistance
7 - Duplex PEO coatings in salt-spray testing
© Keronite 2020 | keronite.com
1
Protecting Mg Alloys from Corrosion with PEO
Introduction
As weight becomes ever more important when
considering the design of new products, the
interest in magnesium continues to increase,
particularly in sectors such as aerospace,
automotive, oil & gas. In many ways, magnesium
is the ideal material for applications in these
industries – it has a low density, it is easy to
process, and has high mechanical stability and
strength. However, there are two key challenges
with using magnesium - its low wear resistance
and susceptibility to corrosion.
Keronite’s magnesium coatings offer a solution to
these problems via the conversion of reactive and
soft magnesium to stable and hard magnesium
oxide. Coatings formed on magnesium by plasma
electrolytic oxidation broadly consist of three
layers: a thin but dense interface layer between the
coating and the underlying alloy, a compact and
hard middle layer, and a porous outer layer. These
separate layers provide a magnesium PEO coating
with its characteristic high adhesion, high wear
and corrosion resistance, and excellent binding to
topcoats. Due to the manner in which the coating
forms, (see ‘What is PEO?’) the resulting coating
is also flexible, allowing for increased mechanical
loading without the cracking and flaking associated
with other coating processes.
© Keronite 2020 | keronite.com
Figure 1. Magnesium component
being PEO coated with G3Mag.
Figure 2. Cross section of
G3Mag coating on magnesium
component.
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Protecting Mg Alloys from Corrosion with PEO
Keronite’s coatings don’t just convert the underlying alloy. By using
our proprietary electrolytes and regimes, we are able to incorporate
other elements into the ceramic in order to produce wear and corrosion
resistant surfaces e.g. by forming very hard Al/Mg spinel structures. The
highly conformal nature of Keronite’s process also reduces the problem
of thin and thick patches in a coating associated with spray or electrochemical deposition techniques.
Developing an advanced coating for
lightweight magnesium components
Keronite’s primary magnesium coating is G3Mag,
and has long been used to protect magnesium
components in the aerospace sector from both
mechanical and chemical attack. Highly conformal,
G3Mag is capable of providing extremely dimensionally
precise coatings which allow for the design of
components with the tight dimensional tolerances
required in precision aerospace applications.
Immersion in 5% NaCl at 80°C
Before
After
A coating of only 5μm is sufficient to protect ZE41A
alloy unscathed for 6 hours in 5% NaCl at 80°C for 6
hours (Table 1).
G3Mag is further complemented by the use of
impregnation sealing. The surface of a PEO coating
has numerous pores, formed from the plasma
discharge events. Keronite has developed
© Keronite 2020 | keronite.com
Table 1. Salt corrosion test of 5μm
G3Mag coating on ZE41A alloy.
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Protecting Mg Alloys from Corrosion with PEO
techniques which allow for a variety of materials to be bonded to the
surface. These include glass ceramic, polymer resin, ceramic polymer
matrix, fluoropolymers and silicones.
As can be seen in Table 2, both the unsealed and sealed 8μm coating of
Keronite G3Mag is broadly unaffected by immersion in 5% NaCl at 80°C
for 6 hours.
Unsealed
Glass
ceramic
Polymer
resin
Ceramic
polymer
matrix
Fluropolymer
Silicone
Before immersion in 5% NaCl at 80°C
After 6 hours
Table 2. Salt corrosion test of 8um G3Mag coating on AZ91 alloy using different topcoat sealers.
© Keronite 2020 | keronite.com
4
Protecting Mg Alloys from Corrosion with PEO
Glass
ceramic
Polymer
resin
Ceramic
polymer matrix
Fluropolymer
Silicone
1 min
240 min
240 min
23 min
35 min
Table 3. Acid corrosion test of 8um G3Mag coating on AZ91 alloy using different topcoat sealers.
Even in 15% HCl, – traditionally immediately destructive to magnesium
oxide coatings – several of the PEO and sealer combinations showed
very impressive chemical resistance (Table 3).
© Keronite 2020 | keronite.com
5
Protecting Mg Alloys from Corrosion with PEO
Further developments in coatings for
extreme corrosion resistance
In order to explore more extreme corrosion resistance, particularly
for reactive alloys, Keronite developed G3Mag-C. G3Mag-C combines
multiple developments to improve the corrosion resistance of even
the most reactive of magnesium alloys. A new electrolyte formulation,
advanced electrical regime and co-developed sealer options allow
for the use of reactive magnesium even in applications where it was
previously thought impossible.
While initially developed from Keronite’s previous G2Mag, G3Mag-C
is an entirely different formulation which creates different phases to
adapt to the varied corrosion mechanisms present in different locations
within the ceramic and the underlying metal. As crevice corrosion is
of key concern when using reactive
alloys, G3Mag-C solves this issue
by creating a stabilised layer at the
This advanced corrosion
interface between the ceramic and
protection has already
underlying alloy. This layer inhibits
garnered significant
corrosion at sites adjacent to inter-meinterest, in particular
tallic particles in the underlying
within the energy and
alloy by limiting localised pH swings,
automotive sectors.
essentially forming a localised
passivation effect for those sites.
This advanced corrosion protection has already garnered significant
interest, in particular within the energy and automotive sectors.
© Keronite 2020 | keronite.com
6
Protecting Mg Alloys from Corrosion with PEO
20 hours
G2Mag
G3Mag-C
92 hours
G2Mag
G3Mag-C
164 hours
G2Mag
G3Mag-C
Table 4. Salt fog test of G3Mag-C versus G2Mag.
Duplex PEO coatings in salt-spray
testing
As can be seen in Table 4, this new formulation is incredibly corrosion
resistant, even when unsealed.
Combining G3Mag-C with Keronite’s new fluoropolymer sealer blends
immensely improves the corrosion resistance even on alloys containing
extremely reactive elements such as lithium, as used in the hydraulic
fracturing industry.
Part of the secret of these new “duplex” coatings is eliminating the
formation of pinholes in the top-coat. In solvent based sealers,
carrier solvent and other chemicals must be evaporated during the
© Keronite 2020 | keronite.com
7
Protecting Mg Alloys from Corrosion with PEO
curing process. As this is not a wholly
uniform process throughout the sealer
layer, pinholes form to allow the
vapours to escape. These pinholes
have traditionally been the bane of
the corrosion protection due to the
formation of moisture pockets and
development of a crevice corrosion
redox cell which accelerates the rate of
corrosion. The Keronite process helps
solve this issue due to the formation
of an oxidatively stable ceramic
layer at the base of the pinholes.
Keronite scientists and engineers have
developed blends and curing processes
that eliminate these types of pinholes
altogether. The resulting surface is
both smooth and extremely corrosion
resistant, as shown by Table 5.
Dissolvable Magnesium Alloy
Before
immersion in 5%
NaCl at 80°C
30 minutes
Keronite Coated Dissolvable
Magnesium Alloy
After 6 hours
7 hours
Table 5. Salt corrosion test of dissolvable
magnesium alloy vs G3Mag-C fluoropolymer
Independent testing has shown
duplex coated dissolvable magnesium alloy.
that these new duplex coatings are
capable of withstanding extreme corrosion environments on reactive
magnesium alloys e.g. 95°C 3% KCl for 7 days without failure.
© Keronite 2020 | keronite.com
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Talk to our experts about PEO’s
corrosion resistance potential
on Mg alloys
Advanced surface coatings are creating new opportunities
for manufacturers to exploit the opportunities provided by
magnesium alloys. They are helping reduce weight, improve fuel
efficiency and enhance the performance of all sorts of equipment.
At Keronite, our materials scientists have developed advanced
corrosion-resistant coatings for magnesium. We also have a range
of research projects underway, in partnership with our customers,
to offer uniquely advantageous coatings. To learn more, get in
touch with our materials science experts.
Get in touch
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