What is the difference between a Bare Floor Electrode and a Flushable Floor Cell?
With a Bare Floor Electrode, the actual surface of the electrode (I.E. anode for cathodic E-coat paint) is in direct contact with the E-coat paint. With a Flushable Floor Cell, an ion-exchange membrane surrounds the Electrode and the E-coat paint never touches the surface of the Electrode. Both require an electrical connection and the Flushable Floor Cell requires electrolyte supply and return connections.
What is a dry anode?
This is another term for a bare anode (I.E. used with cathodic E-coat paints).
Why use the word ‘electrode’?
Electrode is the opposing pole to the ware being painted and is a generic term. For cathodic E-coat paints, the ware is the cathode and the electrode is the anode. For anodic E-coat paints, the ware is the anode and the electrode is the cathode.
Why use the word ‘electrolyte’?
Electrolyte is an aqueous solution containing ions and is a generic term. For cathodic E-coat paints, the electrolyte is called anolyte. For anodic E-coat paints, the electrolyte is called catholyte.
Why are 316L steel anodes sacrificial in a cathodic E-coat bath?
The electro-chemical reaction at the anode in an aqueous solution (I.E. water-based) is one where water is broken down as follows:
A majority of the liberated electrons come from the breakdown of water. However, some of the electrons also form because the iron in the 316L stainless alloy is also broken down. The oxygen gas is very reactive and creates a tough operating environment for ferrous-based metals. Iron-oxides are formed the entire time electrical current flows off the surface of the anode; thereby resulting in a continual loss in mass of the stainless steel anode.
What happens to the iron degradation by-products that are released into the cathodic E-coat bath from a Floor Electrode?
These generally carry a positive charge and thus are attracted to the ware. Some of the iron oxide is trapped in the E-coat film and the rest remains in the E-coat bath. If there is a significant level, then there can be defects visible in the cured E-coat film.
What is the most common anode material?
316L stainless steel has always been the most economical choice. Even though the material is sacrificial and does wear away, it is the most commonly used. 316L has become the de-facto standard for anode materials, except in cases where the presence of iron in the E-coat paint film can create defects, such as color change.
Are there other anode materials?
In addition to 316L stainless steel, there are more inert materials available for use as anodes. First, there are precious metal oxides coated over titanium. These precious metal oxide materials are more insoluble as an anode, but they too can suffer a short life. Sometimes there is a break in the precious metal coating and the titanium substrate is exposed to oxygen. The titanium surface is then oxidized because the voltage is greater than 12 to 16 volts.
What do you mean by ‘more inert’?
More inert means less sacrificial in the environment of an anode in an aqueous solution. 316L is the most soluble anode material, followed by precious metal oxides.
Why is 316L stainless steel the most common anode material?
The ‘L’ indicates low carbon. The presence of carbon can increase the likelihood of iron loss, especially near the grain boundaries of the stainless alloy. Many stainless steel makers only make 316L now. 316 stainless steel can be used as a substitute. 316L and 316 stainless steel form a ‘protective-passivation’ layer on the surface when current is flowing. The elements that make up this protective layer are the elements found in the alloy itself, such as Cr, Mo, C, etc. and others such as oxygen from the hydrolysis of water. This passivation layer acts to moderate the loss of iron and prolong the life of the anode. However, in some cases 316L can suffer a short life if the Corrosion Factor (CF) is high.
What is ‘CF’?
CF is a term used by UFS to quantify the level of corrosion the anode is exposed to. Under ideal situations, the lowest loss rate is less than 10 micrograms/Coulomb. On the other end of the scale, the loss rate can exceed 250 micrograms/Coulomb. Current density, temperature, and CF all contribute to the final loss rate.
What is ‘Anode Current Density’?
The current passing through a Floor Electrode/Cell divided by the surface area for that Floor Electrode/Cell. For design purposes, with a high painted throughput system (I.E. automotive or appliance) the anode current density should not exceed 35 amps/square meter (3.3 amps/SF). For an overall average, the anode current density should not exceed 50 amps/square meter (4.7 amps/SF).
What if my first Floor Electrode/Cell exceeds the recommended anode current density figures?
Generally, the first Floor Electrode/Cell will exceed the recommended figures and about the only thing to do is move the second one closer. By doing this, the load is shared more between the two. In any event, the first unit will wear out first and need to be replaced sooner.
Can I use a lower grade of stainless steel?
No. Use of 304 stainless steel and lessor grades can lead to extremely fast wear rates, sometimes measured in weeks.
How long can I expect an electrode to last?
In an anodic E-coat paint, the electrode is the cathode and its life is indefinite because the electro-chemical reaction at the cathode is not a tough environment for a metal. On the other hand, with a cathodic E-coat paint the electrode is the anode, which is a severe environment for metals because oxygen is created on its surface. For typical heavy duty cycle applications such as an auto body E-coat system or job shops, the life of the anode can be as short as 6 to 9 months or as long as 1-1/2 to 2 years at the best.
Why should I place the Floor Electrode/Cell perpendicular to the travel of the ware?
A perpendicular orientation presents the greatest amount of the electrode’s surface area to the ware, especially when the ware does not have any or much E-coat film thickness started. In contrast, an electrode that is placed with its long dimension in the same direction as the travel of the ware will suffer because the ends will perform a lot of work and the center will become lazy. Current would always want to flow off from an edge first. In this situation the ends of the Floor Electrode/Cell are the ‘edges’ and thus will always show the greatest wear.
What if I want to place a Floor Electrode/Cell right underneath a hole in the rocker panel?
It is possible to do this. However the first several Floor Electrode/Cells should be placed perpendicular in order to start a film on the entire bottom surface first. The later Floor Electrode/Cells can be placed underneath the location of important access holes. There are some tricks to reduce the ‘end effect.’ Ask your UFS sales engineer for guidance.
When should Floor Electrodes be replaced?
As a general rule, once a Floor Electrode/Cell has lost about 60% of its original mass, it should be replaced in the next 6 to 9 months.
How do I estimate the lifetime of an anode?
The loss rate of an anode can be measured with great accuracy. For the purposes of this question, the method to arrive at an estimate is simplified as to how long an Electrode will perform. Given perpendicular orientation, 2” Schedule 80 316L material, cathodic E-coat paint, less than 50 amps/square meter of anode current density, 19 days/month, and 16 hours/day. If you have a file copy of this document, then use Excel to make changes and see the results. (See the comments in the fields with the red corner ID):
Do Floor Electrode/Cells always have to be in the second half of the E-coat tank?
Traditionally most Floor Electrode/Cells have been in the second half (I.E. exit side) as a matter of practice. However, it is possible to place Floor Electrode/Cells in the first half of the E-coat tank.
What benefits can I expect if Floor Electrode/Cells are placed in the first half of the E-coat tank?
Several positive things happen because the bottom (and other recessed regions such as the rocker panel) of the vehicle is painted sooner rather than later. The first is that the side cells can concentrate on the interior once the exterior verticals are started. In many cases Zone 1, and maybe Zone 2, voltages can be slightly reduced.
Why can Zone 1 and Zone 2 voltages be reduced if Floor Electrode/Cells are moved into the first half of the E-coat tank?
The painting on the bottom surface of the vehicle is started earlier in the process. The majority of the painting is accomplished with the floor electrodes, which are closer to the vehicle bottom than the side cells. Anytime the distance between the electrode and the vehicle is reduced, the voltage is reduced. The voltage in the second half can also be reduced since there will be less work performed.
Moving some of the Floor Electrodes/Cells to the first half of the E-coat tank will probably result in an improvement of the rocker panel E-coat film thickness. However, the opportunity for voltage reduction is not as strong and may even require more voltage in some cases.