Pretreatments and Coatings for Corrosion Protection and Top Coat Adhesion
The prior section outlined the early difficulties encountered by magnesium, demonstrated its
excellent corrosion resistance with respect to ASTM B-117 and other common materials (Steel
and A380 Aluminum) and summarized its modern day usage in vehicle systems. This section will
describe the two most common method of applying pretreatments and the various systems
available.
The application method for the vast majority of treatments is either a TANK or SPRAY
processes.
Tank Line Process
Pretreatments are often applied in tank lines or dipping processes using an overhead
conveyance system and parts suspended on racks or in baskets. The parts are lowered into a
variety of chemical baths and rinse tanks (stages) to convert the surface (so called chemical
conversion processes) to the desired state for corrosion protection or for a top coat application.
The tank solutions can be heated or at ambient temperature and may contain neutral pH, acidic
or alkaline chemistries with other additives to achieve the desired results. Treatment times,
chemistries and the number of stages can vary substantially by substrate material being treated
and the desired end state of the surface. It is common to have rinsing stations between
treatment baths.
One of the strongest advantages of the tank line system is the flexibility to treat many
substrates and various chemistries by programming the conveyance system to use tanks 1, 2, 3,
6-8 and 12 for one part/customer and then tank 4, 5, 9-10 for another part/customer, for
instance.
Spray Line Pretreatment Options
Spray line pretreatments are often used in powder coating and E-coating lines and have the
advantage of being continuous speed and in general handle higher volume projects with
“standardized” or recurring pretreatments. The system uses nozzles to spray the part and
usually contain three to seven stages in the systems. The primary disadvantages to the system
is the limited flexibility in applying various chemistries (limited number of stages) and nozzle
spraying is not always capable of reaching all features on a parts geometry.
Hexavalent Chromium
This section will briefly discuss the use of hexavalent chrome (Cr+6) pre-treatment and outline
the pre-treatment methods available as an alternative.
Hexavalent chrome (Cr+6) has been the pre-treatment of choice for both magnesium and
aluminum alloys as a basic corrosion resistant coating and for an excellent base for paint and
coating adhesion for many decades, followed closely by phosphate systems.
In general as a stand-alone surface treatment on magnesium Cr+6 provides moderate corrosion
resistance in the range of 96-120 hours of ASTM B117 salt fog. Chromate combined with an ECoat or Powder Coat paint system can achieve 200-600+ of hours of ASTM B-117 salt fog
protection depending on the top coat finishing system used. All salt spray test results (SST)
referenced in this Topic are based on the ASTM-B117 unless otherwise noted.
Cr+6 is a known carcinogen and its use is rapidly declining or disappearing as a result of stricter
permitting, restrictive or banning legislation and a growing number of alternative chemical
treatment systems. Cr+6 is banned in China, is not RoHs (EU) compliant and in the US is largely
restricted to legacy programs and military applications.
As a result of the longevity of Cr+6 and the abundance of test data we felt the need to introduce
the chemistry.
We will now explore the commercially available systems along with several newer (2008 and
newer) chemistries that have favorable test data but limited commercial exposure. All
comparison systems are hexavalent chromium free.
Alodine (Henkel) - ALODINE 5200 treatment is a chromium free product and specifically
formulated for treating non-ferrous alloys. Spray or immersion application may be used. The
process provides an excellent base for bonding of adhesives and organic finishes. Alodine 5200
provides minimal SST corrosion resistance (24-48 hours). This pre-treatment system is the
industry leader for most light metal processes that require some type of top coat finish.
Henkel has developed, and is using primarily in Asia, an Alodine M206 formulation designed
specifically for the AZ class of magnesium alloys. It provides limited corrosion (24-48 hours)
resistance and has a more aggressive etching of the surface which is beneficial when cleaning
the Magnesium Oxides that form on the surface of the alloy.
Phosphate(s) based systems (primarily iron and zinc, via multiple suppliers) are also commonly
used to displace Cr+6 systems with an aggressive cleaning that prepares the surface for the
phosphate treatment. Spray and immersion systems are typically used in large scale enclosed
industrial washers and tank lines. Fair to Good top coat adhesion results with a minimal
corrosion resistance when used as a stand-alone treatment with (24-48 hours) of salt spray test
(SST) achieved. Phosphate systems are coming under increasing environmental scrutiny as a
result of the waste water treatment and the fact that phosphates are one of the primary causes
of Eutrophication; more precisely, hypertrophication (algae growth or algae blooms), which is
when the ecosystems response to the addition of artificial or natural substances, such as
nitrates and phosphates, through fertilizers or sewage, to an aquatic system causing a loss of
oxygen in the water system.
Zirconization: developed in the mid to late nineties the process is “relatively” new but is making
significant inroads in displacing phosphate based systems. The following chart illustrates the
benefits of the process with respect to various substrates. At this time there is limited data
with respect to magnesium (see the module on Test Results for E-Coated magnesium with a
Zirconite base coat) however results in Aluminum suggest a (48-96 hours) SST and magnesium
would likely perform with similar results.
Cerium Based Conversion Coatings (CeCC’s). Cerium conversion coatings offer an alternative
to both Cr+6 based systems, Alodine and Zirconization processes and are designed primarily as
tank processes. Development began in the mid 1990’s and has progressed to
commercialization in the last decade or so. Cerium coatings have not meet with broad
commercial implementations primarily as a result of high costs. Cerium based coatings provide
moderate corrosion protection – up to 120 hours and are excellent as a paint base.
Anomag - anodization of magnesium has been available using a Cr+6 DOW17 process and a
non-chromium process DOW HAE that provided a superior corrosion resistance and paint
adhesion than the traditional process of Cr+6 chromate conversion. The Anomag process uses
an electrolytic bath process where the electrolyte is phosphate and ammonium salts that
create a layer on the substrate and is hexavalent chromium free. The thickness of the layer can
be varied over a range of 5 microns to about 30 microns. The thickness is important for
corrosion resistance and resistance to galvanic couples. The figure below shows a depiction of
the cross section of a typical anodized surface. The photomicrograph shows the structure at
very high magnification.
Depiction of Anodization layers and SEM view of the top layer.
Left diagram – Dr. Enzo Strazzi R&D Director Italfinish SpA, Grassobbio (BG) Italy
Right photograph - Materials Transactions, Vol. 49, No. 5 (2008) pp. 924 to 930 - Special Issue on Platform Science and Technology for Advanced
Magnesium Alloys, IV #2008 The Japan Institute of Metals
There are several extraordinary properties of the Henkel Anomag chemistry and the formation
of the compounds as they build up on the part. The 1st is that the structure is sacrificial and
self-healing – that is as it is attacked by the external corrosion it sacrifices itself and has the
ability to “grow” or repair itself for an extended period of time. The 2 nd is that the ceramic
nature of the layers serves to insulate the galvanic potential between the dissimilar metals and
in some cases completely eliminates the potential voltage (depends on the galvanic couple). To
achieve the results shown below a minimum of 10 microns of coating thickness is required.
Tagnite & Keronite – these are additional electrolytic anodization processes that provide
another level of paint adhesion and corrosion resistant beyond Anomag. These treatments are
typically used in Defense, Aerospace, Elite Sports and very high end applications.
Henkel MgC – Trade named by Henkel as Bonderite-MgC, is a plasma electrolytic ceramic
deposition process using pretreatment tanks and a charged tank and part that creates a buildup on the substrate. This process has been in development for three to four years and several
projects are entering serial production in the very near future. Test data is presented in the
Module entitled “Test Results”. This process is similar to Anomag and Keronite but has a lower
operating cost.
Please find below the chart presented in the Overview section as a summary of this section.
Summary of Pretreatment and Coating Processes
Coating Process
Chemistry/Process
SST Hours†
Substrate
(no top
coat)
Hexavalent
Chrome
Cr+6 bath process
96-120
Excellent
NA
Alodine
Non Chromium
conversion coating
24-48
Good to
Excellent
2
Phosphates (Zinc
or Iron)
Zirconite –
zirconium based
MgC - Henkel
Non- Chrome Multi
stage washer or bath
Non- Chrome Multi
stage washer or bath
Electrolytic process
240-320
Fair to
Good
Good to
Excellent
Excellent
Anomag
Electrolytic immersion
312+
Excellent
7
Tagnite/Keronite
Electrolytic immersion
600+
Excellent
9
24-48
24-48
Paint/Top
Coat
Adhesion
Cost*
2
Notes
(Hazardous Substance in
China). Banned in Gmbh.
Not RoHS. REFERENCE ONLY!
Alodine is a Henkel trade
name – other chemistries
from various suppliers are
also available
Generally used in large scale
powder coating & E-Coat
lines
3
Not fully commercialized
5
Not fully commercialized but
test results are shown
Used in marine and military
applications
* Relative coating costs ranked relative to one another. A (10) is highest cost. Estimated.
†Salt spray tests are subjective and related to a pass/fail criteria based on the surface area that shows corrosion according to the table below.
For the purposes of this document we have provided a range that in our experience corresponds to the 9-10 range on the table below.
Salt Spray Test – Failure Ranking Criteria