a brake resistor perspective on corrosion

A brake resistor perspective on corrosion - of 8

A brake resistor perspective on corrosion

By Thomas B. D. Björklund - R&D Tests & Pictures: Jon Otten

Danotherm Electric A/S - Summer 2014

Preface Being an engineer with an electrical and electronic background, it was not clear what we were up against when dealing with environ-mental requirements. A project was therefore initiated with a mechanical engineer student from DTU; Jon Otten - who has been specialised in corrosion mechanisms and metallographic structures. During the project we were able to run tests and inspections, using state-of-the-art equipment at the DTU campus. Our test item was a realistic combination equal to what is running in our production lines, combined with new experimental items. The main issues with brake resistors is the combination of aluminium and other metals for screws and brackets, together with the main purpose of burning off a high amount of brake energy from a motor or generator. This thermal energy drives the housing temperature above 200°C and even up to 375°C in worse case.

During the project, we have been advised by Per Møller, who was the appointed supervisor for Jon. Per is a professor in corrosion and surface technology at the MEK-DTU Section for Materials and Surface Engineering. He has wrote what is now called the bible of corrosion theory; Advanced Surface Technology - A Holistic View on the Extensive and Intertwined World of Applied Surface Engineering, which was very helpful reading, getting insight in the mechanisms of our corrosion situation.


Aluminium corrosion The aluminium profiles are in general protected since it spon-taneously forms a thin but very hard and an effective passi-vation layer of normally 4nm in thick-ness. Aluminium oxide or alumina, Al2O3 is a white solid ceramic which is stable in the pH 4–9 range. It is an electrical insulator but has a relatively high thermal conductivity at 30 Wm−1K−1. The thickness of this oxide layer can be increased by using an anodising process.

Galvanic corrosion occurs due to metallic contact and electrolytic bridging between metals of dif-ferent potential. Here the least noble metal becomes the anode and thereby corrodes. In most combinations, aluminium is the least noble metal and therefore presents a greater risk of galvanic corrosion than most other structural materials. The intensity of the corrosion increases with the conductivity of the electrolyte here the salt in sea water plays a big role. For comparison typical values of various fluids are listed.

Distilled Water 0.5-2 μS/cm

Stored Distilled Water 2-4 μS/cm

Supply Water 50-1500 μS/cm

Sea Water 50,000 μS/cm

Sat. Sodium Chloride 250,000 μS/cm

Sulphuric Acid <800,000 μS/cm

The unit siemens is defined by: 𝑆 = Ω−1 =𝐴

𝑉

Where Ω is ohm, A is ampere, and V is volt.

Ref.: Atlas Steel Technical Note No. 7 Measured in oxygen rich ocean water from Øresund.

On average, seawater in the world's ocean salinity is

between 3.1% and 3.8%. The dissolved salts, consists

predominantly of sodium Na+ and chloride Cl− ions.

The concentration of various salt ions in seawater has

been determined to be: Cl− 55%, Na+ 30.6%, and then

SO42− 7.7%, Mg2+ 3.7%, Ca2+ 1.2%, K+ 1.1%, Other 0.7%

(CT, Br−, BT, Sr2+, F−).

pH value is in the range of 7.5 to 8.4

Magnification 225x

0.2 mm

Picture of oxide layer on resistor profile

Anode (Aluminium)

Cathode (Steel)

Electrolyte (Seawater)

2H2O+O2+4e-4OH

- AlAl3+

+3e-

Al3+

+H2OAl2O3+3H+ 2H

++2e

-H2↑

e-

Al3+

Al3+ OH

- OH-

e-

e-

e-

e-

e-

H+ H

+


HALT and Salt-Spray Making a Highly Accelerated Life Test is a philosophy of increase the intensity of the environment to get the corrosion speeded up. EN ISO 9227 is the most common used salt-spray test and the test is good at showing differences in material and reveal improve-ments in surface treatments, but there is no standard for setting the test parameters compared to real world environments, it all comes down to knowing what a 10 year old component looks like and then treat a new component in this elevated harsh environment until it has the somewhat same appearance. But if you then don’t have this ten year old component you will just have to shoot wild guesses in the dark. A new form of test has been developed, HACT - Highly Accelerated Corrosion Testing, which adds a cyclic process of oxygen rich salt water, and periods with hot dry oxygen rich air. This gets more realistic according to a sea en-vironment, but then again there is no deter-mination of the inside of an offshore windmill or the inside of a marine vessel or offshore power plant. Vibration and thermal stress due to constantly heating and cooling is not taken into account, but here - tests with different preheated items 250°C and 375°C have been subjected to the same salt-spray as a similar test item that have no preheating and big differences are seen in the varying surface treatments of plates in combination with screws of different metals. The constant salt-fog also prevents the protective zinc to produce a passive zinc patina. Hence the zinc degrades more rapidly than in real life. Only equal tilted surfaces can be compared. Vertical and horizontal surfaces do not have the same corrosion rate due to the collection and dripping of dewdrops.

PH-value and Sea-salt Controlling the PH-value is important and easy in a controlled chamber where also the very natural sodium chloride NaCl salt is used, but in real life the pH-value is not controlled and can therefore vary. Pourbaix diagrams is a graphical presentation of the thermodynamic equilibrium states of a metal-electro-lyte system. They show how metals are reacting to the environment. Here is represented pure Al (not an alloy) at 25˚C in aqueous solution. Pourbaix diagrams changes in different environments and with actual alloys and with the higher operating temperature. Oxidizing conditions are described by the top part of the diagram (high positive electrode potential). Reducing conditions are described by the bottom part (high negative electrode potential). Acidic solutions in the left side of the diagram (pH < 6). Alkaline solutions in the right side of the diagram (pH >than 6).

Initial test setup

After 200 hours of 5% NaCl 35°C pH 6,5-7,2


Pitting corrosion and raney nickel Pitting corrosion is an extremely localized corrosion that creates small holes in the surface. The corrosion speeder is the de-passivation of a small area, which becomes anodic and a surrounding area then becomes cathodic, which leads to local galvanic corrosion.

Pitting corrosion is the most common type for aluminium. It occurs only in the presence of an electrolyte. The products of corrosion often cover up the pit hole leaving only a little visible pit to be detected. The oxygen is reduced outside of the pit and the pit or crevice becomes acidic, which increases the corrosion rate, even if the water may initially be alcalic by nature.

Na++e−Na = −2.71v, Is not very likely as a reduction due to high negative value.

2H+ + 2e− H2 = 0.0000v, Is more likely but there are very few H+ ions in water outside the pitting.

Na++ OH- NaOH. These ions will be oxidized/reduced by electrolysis.

Crevice corrosion occurs also commonly at places with gaskets and screw joints where crevice exists. It creates pits similar to pitting corrosion.

Environmental classification and lifetime Customers refer often to corrosion degrees from ISO 12944; C3 Middle - Coastal areas with low salt content, with a Zink weight loss per year on >5-15 g/m2, equal to thickness loss on >0.1 to 2.1µm/year or C4 High - Coastal areas with moderate salt impact, with a Zink weight loss per year on >15-30 g/m2, equal to thickness loss on >2.1 to 4.2µm/year. This could be nice and easy if the corrosion rate, measured in g/m2/year, could the just be transferred to a specification of minimum protective layer added up to the 15 or 25 years of expected lifetime, but then again there is missing data on what kind of salt, and how much rain and wind, and of course at what temperature? Looking at the NEMA 4X category, which is a water and corrosion proof test for protection of electrical enclosures against ingress of water (rain, sleet, snow, splashing water, and hose directed water). Here Aluminium is said to be corrosion free. The test for salt-spray is divided into two categories; Indoor 200h and outdoor 600h. This is commonly accepted but there is absolutely no link to a product lifetime. The lifetime of a mechanical product is dependent on the strength of the structure and when corrosion occurs, it is therefore most severe regarding the thicknesses of the plates. The large surfaces must then be protected first and only on a second concern the sides, but this is not so crucial since it has no direct impact on the strength, but can lead to an increase in corrosion speeding up the process on the surfaces. If the component is kept indoor in an offshore wind turbine, sea salt will blow in and deposit on the surface. This is critical. First of all, the deposited salt will not wash away in the rain when placed indoor. Second, the Sea salt contains a high amount of MgCl2 and CaCl2. The Calcium chloride is highly hydroscopic and is normally used for desiccant bags when moist protection is needed. It will suck water out of the air and thereby continuously keeping the surface wet. This process cannot be stopped by dehumidifier or any climate conditioner, the salt has to be rinsed off with pure water spray. Otherwise it will lead to heavy corrosion in a way that is not predictable since it depends on the amount of CaCl depositing. Another example of a corrosion speeder is when the nickel is left on top and the zinc is diffused into the intermetallic layer. Then the nickel can in rare circumstances develop into Raney-nickel which has an average surface area of 100 m2 per gram. This will lead to increased contact with the electrolyte and therefore the higher corrosion rate.

Pitting corrosion (Aluminium)

2H2O+O2+4e-4OH

-

AlAl3+

+3e-

Al(OH)3

Al3+

OH-

e-

e-

e-

H+

Na+ Na

+

Cl-

Cl-

Cl-

Passive film OH

-

pH 4

pH 9

OH-+Al(OH)3AlO2

-+2H2O

Al(OH)3+3H+Al

3++3H2O

OH-

Anode

Cathode Cathode Cl

-

H++Cl

- HCL H

+


Brackets and plates To protect against corrosion, iron plates are galvanized with alu-zinc or nickel-zinc. But a risk is added when using nickel-zinc as the nickel can occur in massive local formation creating a noble cathode that speeds up the corrosion. If that happens we were better preserved without any protective layer. It is clearly that the pure nickel spot on the nickel-zinc is causing the steel to corrode red-rust at a much earlier state than the surrounding well deposited nickel-zinc this layer is in state of producing white-rust which means that the preservation layer in not penetrated yet and the mechanical strength of the item is kept. The mechanical structures of a break resistor needs first of all to be strong and light weight but also easy and economical to manufacture. Aluminium would be a corrosion choice, but the brackets need to keep the thermal energy from the hot aluminium housing away from the base that it is mounted on. Hence steel is needed. Thermal conductivity (Watt per meter per Kelvin) Aluminum 205.0 W/mK Steel 50.2 W/mK

Hot-dip galvanizing or electroplating Steel needs a protective zinc layer as a barrier against water, oxygen and atmospheric pollutants, to prevent rusting, but also to cathodically protect from coating imperfections, abrasion, cut edges, drill holes and bending cracks, from the shaping process. But the zinc also keeps the corrosion, towards the aluminium, low according to the reactivity series, and the zinc also produce a patina layer that slows the corrosion rate down. Zinc patina is critical in long-term corrosion protection. Within 24 hours a thin ZnO layer is developing from the oxygen in the air. When exposed to moist, the surface reacts to form a porous galantine-type zinc hydroxide Zn(OH)2 in the time frame of 24 hours to 3 months. During dry cycles of exposure, the zinc hydroxide develops a thin adherent film of zinc carbonate 2ZnCo3, this can take from 3 to 12 months to form, depending on the moist in the atmosphere and the duration of the surface wetness. The film is stable in values pH 4-12.

Cathode (Steel) Protected by the Zinc Anode

2H2O+O2+4e-4OH

-

Anode (Zinc Coating)

2H2O+O2+4e-4OH

-

2H++2e

-H2↑

e- e

-

Zn2+ OH

-

H+ ZnZn

2++2e

- H+

Zn2+

Initial test setup Salt-spray 50 hours

Salt-spray 175 hours


Electroplated Zinc After heating

Hot dipped with Zinc

Zinc alloys and heat treatment The plates are coated in a Sendzimir process, used to galvanize a steel strip by using a small amount of aluminium in a zinc bath. The addition of aluminium does no improve corrosion performance, but ensure good coating adhesion during forming of the steel. The sheet is annealed at 650˚C ahead of the pot. Then it is cooled to 470-490˚C before it enters the bath. The zinc melts at 419˚C and usually the bath is usually heated to 465˚C.

When dipped in the bath a diffusion happens which creates an intermetallic compound or and intermetallic alloy. If this is made from pure zinc, the intermetallic layer becomes significant. This alloy is very hard and brittle and can easy cause shear cracks or the coating might even flake off. The intermetallic process will happens anyway over time, but is initially speeded up during the heating. The intermetallic process decays by square root, so the initial process is the worse. When adding 0.15 to 0.19% of aluminium to the molten zinc, something interesting happens to the intermetallic process; a very thin aluminium-iron Fe2Al5 alloy develops extremely fast (within 0.15 seconds) after the steel enters the coating bath. It creates a barrier that slows down the zinc-iron FeZn7 alloying and a ternary alloy develops instead Fe2Al5-XZnX. This process is causing a thick layer of pure zinc which has much better properties when forming the metal sheet. In the drawings below the principle is shown together with general measures of hardness DPN (Diamond Pyramid Hardness) also known as Vickers Hardness.

The Aluminium-zinc is more expensive and not applicable at all galvanizing manufacturers. This is due to the controlling of the zinc bath. If the aluminium contents it too low, galvannealing is happening, giving no corrosion protection. The surface finish is light gray and keeps its lightness longer than zinc. ISO 12944-2 – Class C3 or C4 - Typical life to first maintenance years: Very long >20 years – Hot dip galvanized conforming to ISO 1461 – Mean coating thickness of 55-70µm dependent on the plate thickness 1,5 to 3 mm.

The hot-dipped process develops initial diffusion, where electroplated zinc layer is perfect initially, but quickly diffuse into the steel when heated during the use of the brake resistor, the electroplating process also only provides up to 15µm of thickness. Here the steel is then not protected for very long when scratched, but the cut edges is also electroplated so the steel won’t have to create a protective red rust layer, which gives a better appearance. Heating can develop flakes of zinc though. Here the addition of nickel can be used for better thermal endurance.

Zinc-nickel has been investigated and found better in heating performance, but zinc-nickel also has better corrosion performance when electroplated. Corroding speed according to salt spray test in µm/year decreases with the nickel content until approximately 13% - above this point the corrosion rate increases heavily. The zinc nickel surface has the Stainless Steel Look, but it creates a highly stressed layer that cannot be post process bended and tend to crack and peel in edges. It also creates pitting corrosion due to its surface robustness.

Hot dipped with Al-Zinc

Electroplated Zinc

Before heating

Eta – Zn100 (70 DPN)

Zeta – Zn94Fe (179 DPN)

Delta – Zn90Fe (244 DPN)

Gamma – Zn75Fe (244 DPN)

Base Steel – (159 DPN)

60

to

80

µm

Bar

rier

Pro

tect

ion

Base Steel

8 t

o 1

0 µ

m B

arri

er P

rote

ctio

n

Eta – Zn100 (70 DPN)

Ternary alloy – Al45Fe35Zn

3000X magnification with electron microscope - Before and after 24h heating!


Screws and Gaskets The mechanical function in Brake Resistors unfortunately requires the aluminium housing and endplates to be screw-mounted with steel screws that is self-tapping. This Is due to the nature of extruded profiles, where other solutions would implement very expensive machining added to the product. Aluminium is becoming the offering anode when screw mounted, but the stainless screw (AISI 304) also reacts with the cathodic graphite gasket. This explains the thick oxide layer that had formed on the surface of the screw during the 200h salt-spray test, where it was connected to the graphite packing. The chemical reaction at the anode, stainless steel is:

2𝐶𝑟+3 + 3𝐻2𝑂 → 𝐶𝑟2𝑂3 + 6𝐻+ + 3𝑒−

Where 𝐶𝑟2𝑂3 then is the Chromium (III) oxide layer that is

shown on the pictures below.

The chemical reaction at the cathode, graphite packing is:

4𝑒− + 𝑂2 + 2𝐻2𝑂 → 4𝑂𝐻−

After the salt-spray test a profile was cut open to get the screw out without using any torque. An ordinary demounting would have ruined the corrosion trail on the surface of the screw due to friction. The white residual in the top picture is NaCl from the salt fog. In the picture of the demounted screw, the red box indicates the area that has been exposed to the graphite. At the surface where the screw was connected to the aluminum profile and the surrounding environment, the chromium (III) oxide layer was much thinner. This is also called a cathodic protection of the stainless steel, but the anode has a very little surface, therefore it is a limited amount of electrons that is transported from the aluminum to the stainless steel. This means that even though the aluminum creates at cathodic protection for the stainless steel, the stainless steel still corrodes.

Cut open housing revealing contact from the self-

tapping screw

Screw thread exposed to the graphite gasket Screw thread exposed to the aluminum

Demounted screw


Electrical leak current

The brake resistors also has leak current running from the system out in the mechanical structure of up to 50µA, this means that there will be an overload of electrons in the metal hence the anode will become protected since it no longer has to offer its electrons bound in the metal structure, but a new phenomenon occurs called stray current corrosion. This kind of corrosion can occur at any contact point where the higher potential metal delivers its electrons to the connected ground and hence the metal becomes the offering anode despite the metal potential.

Protective Painting and plastic barriers

It is very common to paint or coat metal pieces, that have to sustain harsh environments, and it is also very useful to put in a plastic barrier between metal parts that has a too high electric potential. However, when dealing with temperatures above 200°C as in Brake Resistors, no coatings can handle the temperature.

Conclusion When dealing with corrosion, it is all about knowing the actual environment and get experienced in; what is actual happening at the location. Putting the right metals together is crucial as an initial prevention and herein lays a binding knowhow on thermal conditions and specification of the optimal surface treatment, but when the overall design demands for materials that are not theoretically correct to join, it all comes down to real life facts.

We can make test such as HALT from now on and for ever, but we will never get a true image of what is happening, the nature has is ways of giving a different and often unpredicted effect. This is why we must learn from old components having several years of real time corrosion, and then of course constantly improve our products for the future.

Reference Reading Books recommended:

Articles and papers found on the web: Durability and Corrosion of Aluminium and Its Alloys: Overview, Property Space, Techniques and Developments N. L. Sukiman, X. Zhou, N. Birbilis, A.E. Hughes, J.M.C. Mol, S.J. Garcia, X. Zhou, G.E. Thompson ISBN 978-953-51-0861-0

Corrosion Protection For Windmills - Onshore And Offshore Karsten Mühlberg, Hempel (Germany) Ltd., Cologne (Germany)

Atlas tech note no. 7 - Galvanic Corrosion - revised Aug 2010 www.atlassteels.com.au

The Basics of Hot Dip Galvanized Steel – Process, Protection, Performance Galvanizers Association of Australia - Edition 1 June 2012

The Role of Aluminum in Continuous Hot-Dip Galvanizing - GalvInfoNote 2.4 www.galvinfo.com/galv_info_notes.htm

Zinc Alloy Plating Solutions www.surfacetechnology.co.uk/protective-plating/zinc-alloy-plating/

a brake resistor perspective on corrosion

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