Yttrium as a Lead Substitute in Electrodeposition Coatings
Overview
Yttrium
is a chemical substance which is represented by Y symbol and has atomic number
39. Yttrium is a transitional metallic which is silvery in color and resembles
the lanthanides and is grouped as a rare earth element. It occurs in group 3
and period 5 with electronic configuration of [Kr] 4d15S2
in the periodic table. It often exists together with the lanthanides in nature
and also has only one naturally occurring isotope 89Y. Yttrium finds many uses in different disciplines,
for instance, it is used as an additive in alloys, microwave filters of radar,
catalyst in the ethane polymerization, lasers that are employed in cutting the
metals. The Yttrium oxide is used in glasses for making camera lenses to enable
the camera lenses to be shock and heat resistant. The radioactive 90Y is applied in the medical applications like
the treatment of liver cancer. In 2001, the PPG Industries received an award
for their improvement of Yttrium as an alternative substitute for lead-based
coats in the electro-deposition in the automobiles. In their study, they stated
that Yttrium was protecting the materials against corrosion as the same as the
lead inhibitors and it is 100 times safer. The protective coatings and paints
that are employed on the automobiles contain a small percentage of corrosion
inhibitors combined with the paints. The chemicals that are applied to the
metal surface to protect them against the chemical reactions, which occur as a
result of the dissolved water and oxygen which exist in the environment, are
known as corrosion inhibitors. The electrochemical techniques referred to as
the electro-deposition is the best way to cover the motor vehicles with protective
agents together with the paints (Kete et al. 11239).
Problem
In
the early days, commonly employed corrosion inhibitors in the automobile
industries were lead-based substances.
The application of the lead in the paints caused a significant risk to
human health. The risk posed by the
lead-based compounds made the U.S Government to come up with regulations which
aimed at removing the lead-based substances in the residential paints. The low cost of the lead and effective as
anti-corrosive forced the US Government to allow the use of the lead in the
automobile industries (Cann, and Umile 65).
Solution
However,
the PPG Industries found that Yttrium metal can substitute lead in the
protection of vehicles from corrosion. The Yttrium has many pros over the lead
and due to this makes it a suitable substitute for the use of the lead in
automobiles. For instance, it is twice as abundant as lead in the ground,
contains 1/120 toxicity of lead and be easily substituted for lead or
lead-based substances in electro-deposition process. Yttrium as the alternative
green solution for replacing the lead-based inhibitors in automobiles, In this
paper, the yttrium-based inhibitors as the alternative substitute of lead-based
inhibitors for electro-deposition, chemistry of corrosion are addressed.
Background
Various untreated metal objects,
for example, steel automobiles and an untreated sheet of the metal go through a
corrosion process. Corrosion is an electrochemical degradation process, where
the metals are oxidized to substances which are eroded quickly. In America, corrosion is anticipated to have
contributed to $20.91 billion costs incurred by the motor vehicle owners for
changing of automotive parts, automobile depreciation and maintenance (Cann,
and Umile 65). The oxidation of the
metal of the iron with the dissolved water and oxygen is commonly known an
example of corrosion. The reaction between the iron metal and the dissolved
oxygen and water yields the rust (hydrated red-orange iron oxide) which is
represented chemically as Fe2O3.H2O.
The
oxidation chemical substances differ according to their position in the
electrochemical series. The standard
electrode potential (E˚) is the quantitative measure used to determine the ease
by which a substance losses or accept electrons. The electrical energy is needed to cause the
redox reaction to occur. When the potential of the reduction (E˚rex)
is known, and one needs to obtain the potential of the oxidation (E˚oxi,)
then one needs to change the sign of the reduction potential. For instance, the
potential reduction of lead ion is -0.13, the opposite of this become the
possible oxidation (+0.13). Metals are oxidized (corrode) by either one or more
of the following chemical reactions:
2Fe
2Fe2+ + 4e-
……………………………………………..Oxidation
2H2O
+ O2 + 4e-
4-OH……………………………………......Reduction
4-OH+
2Fe
2Fe (OH) 2
2H2O
+ 2H2O + O2
2Fe (OH) 2……………………….......... Overall
reaction
The last reaction is the formation of the rust as
follows:
O2
+ 4Fe (OH) 2
4H2O + 2Fe2O3.H2O
In the early studies, it was found
that water alone can cause corrosion due to its negative electrode
potential. However, water is not a
strong oxidizing agent, but when it is placed in the oxygenated or low pH
environment it oxidizing power increases for a given half-reaction. Corrosion
is more pronounced in the environment containing both the dissolved oxygen and
acid, such environment has strong standard potential (E˚) that is capable of
oxidizing different types of metals (Subba, Anantha, and Venkatesha 3200).
For
one to inhibit the corrosion from taking place, a substance (corrosion
inhibitor) is added to the metal surface.
There are various ways in which the corrosion inhibitors work. For
example, corrosion inhibitor can act as the protective layer occurring on the
surface of the metal. The corrosion inhibitor on the surface of the metal
functions as a barrier making it impossible for the corrosion causative agents
to pass through into the metal. The inhibitors which are readily oxidized than
the metal can be added to the metal surface.
This process of adding corrosion inhibitor on the metal surface is
referred as the scarification of the inhibitor. The corrosion inhibitors that
are used as the scarification process have more positive values of electrode
potential. The passivation is another
way of preventing corrosion from occurring. The passivation is the process in
which the corrosion inhibitors increase the electrode potential of the metals.
The passivation facilitates the formation of a thin layer of the own metal's
insoluble oxide which protects the metal beneath from oxidation.
Discussion
The most commonly used substances as
corrosion inhibitors are the lead-based inhibitors, this due to their low cost
and efficient in the protecting metal against corrosion. Nevertheless, they contribute to human health
risk. In the US, it was reported that
the absorption of the lead into the body can result in the destruction of the
nervous system, stimulates the behavioral and learning problems hence results
in problems of the reproductive system.
In the same study, it was stated that the adults can come in contact
with the lead in two ways; the lead in old homes and the lead that contaminated
domestic water (Cann, and Umile 65). Lead has a greater risk to children as
compared to adults; since children can eat small lead-containing objects and
also the nervous system of the children are still in their developing
stages. Due to these obvious risks
forced the U.S government to eliminate the lead-based containing substances
with the focused on the residential paints, with the exemption on the use the
lead-based corrosion inhibitors for trucks, cars.
A Greater percentage (55%) of the
automobiles is coming from steel.
Electro-deposition technique is used to protect the steel from the
corrosion. Electro-deposition
encompasses, placing and an item in the bath which contains the
electro-coat. The placing of the metal
in the bath is followed by the application of the electrical current on the
metal surface. The application of
electrical current makes paints and the corrosion inhibitors to be absorbed on
the surface of the metal. One example of the electro-deposition is the
protecting steel from corrosion by galvanizing. The galvanizing is the
electro-deposition where the zinc is coated on the metal surface, hence,
forming a layer that prevents the undersurface of the metal from corrosion.
However, the galvanizing is expensive, to meet the high automobile demands by
the automobile industry. Therefore, cheap alternative corrosion inhibitors are
needed. In 2001, yttrium was developed to replace the lead-based coating
materials by the PPG Industries (Cann, and Umile 65). In the study of the PPG industries found that
yttrium gives the best protective layer when it is electrodeposited on the
metal steel. When yttrium is added to
the metal surface by the electro-deposition process, a small percentage of the
yttrium oxide precipitate is formed on the steel surface. The yttrium oxide
formed on the metal surface passivates the metal; this leads to the formation
of the insoluble ferrite on the metal surface thus guarding the underneath of
steel from corrosion.
For the corrosion inhibitors to be used in
coating metal surface, they must meet certain conditions. For instance, during
the electrodeposition, the coating material bath is exposed to long periods of
inactivity, high temperature, compatible with chemicals and high pressures.
Therefore, the corrosion inhibitors must meet all the conditions. The yttrium
oxide satisfies all the required conditions. Since the yttrium occurs as the
yttrium oxide, thus the toxicity is for the oxides but not the yttrium. Studies showed that the yttrium oxides are
non-toxic when ingested. As the same with lead-based compounds, yttrium oxides
also find its way to the surrounding during the degradation of the automobile paints,
but the yttrium is not a common occurrence in nature (Zhernokleeva, Baranovskaya,
and Karpov 1062). Even though health
risks of yttrium have not been studied, it is believed that it is not
dangerous. The yttrium oxide which is
used as corrosion inhibitors in paints is insoluble, and it is not toxic. The
yttrium isotope 90 is applied in the medical field; studies indicate it treats
various kinds of cancer, and this means when the ytrtrium-90 isotope is exposed
to the surrounding it can be of benefit to human.
Research
shows that by introducing the yttrium-based corrosion inhibitors in the automobile
industry can replace approximately one million of the lead being employed in
the automobile industries yearly. The
use of the yttrium-based as coating materials offer greater safety and prevent
unwanted exposure of users to the lead poisoning during the refining and mining
process. The effectiveness of yttrium against corrosion is comparable to the
lead, and since yttrium is of lighter atomic weight (Da 88.9) as compared with
lead (207.2 Da), this makes it possible for less of the amount by weight of
yttrium to be applied as the electro-coat than lead.
Yttrium has another advantage over the lead
as the corrosion inhibitor. For example, lead-based coating materials need the
use of the chromium and nickel at the pre-treatment stage to facilitate
corrosion resistance and adhesion on the metal surface. The employment of
nickel and chromium in the process results in the production of large amounts
of wastes containing chromium and nickel. However, yttrium does not need the
application of nickel and chromium at the pre-treatment stage. This characteristic the yttrium poses
(chromium-nickel free treatments) is paramount in the reduction of the amount
of chromium (50,000 pounds) and nickel(25,000 pounds) utilized in the
automobile industries(PPG)(Cann and Umile 64).
Conclusion
The use of the yttrium oxide as the coating
substance inhibitor to substitute the lead-based compounds in preventing metals
from corrosion has more pros than cons. As discussed in this paper before, the
pros the yttrium has over the lead-based compounds enable one to understand the
urgent need for the increase of the use of yttrium-based compounds in the
automobile industries. These advantages of the yttrium in the
electro-deposition of the runs from the health sector to the treatment process
of the electro-deposition. For instance, the early studies indicate that
isotope 90 of the yttrium finds use in treating various kinds of the cancers
like liver cancer. The yttrium is also considered as safer as compared to lead-based
inhibitors; it is 1/120 toxic as lead. Therefore, it is advantageous to be used
as the electrocoating materials. The chromium-nickel free associated with the
use of yttrium as the corrosion inhibitors reduce the material wastes, reduce
the company production cost and prevent the pollution of the environment. The
yttrium oxide is non-toxic, hence, when yttrium oxide is exposed to the
environment, the yttrium does not cause any risk to human health. This characteristic
of yttrium makes yttrium the best candidate for the substitution of the
lead-based substances which have greater risks to people's health and they
involve in additional steps during the electro-deposition process (addition of
nickel and chromium in the pre-treatment stage (Protsenko, and Danilov 1203).
The abundance of the yttrium in the earth crust is another advantage, yttrium
is twice as abundant as the amount of the lead, and this abundance makes it an
excellent substitute of the lead-based corrosion inhibitors.
Since the introduction of the
yttrium-based substances as the corrosion inhibitors in the automobile industry
in 2001 by the PPG industries, many manufacturing automobile industries like
the General Motors, Volkswagen, Ford, Mitsubishi and Nissan were using the PPG
Industries' yttrium-based corrosion inhibitors.
The research shows that by 2006 thirty-eight million of the motor
vehicles had started employing the utilization of the yttrium-based corrosion
inhibitors in their production process, and this has led to the removal of the
lead-based compounds as the corrosion inhibitors in almost all of the
consumers' applications (Kim et al. 295).
For the yttrium-based corrosion inhibitors
to be effective, then one must understand the chemistry used in the chemicals
applied in the paints, this to ensure compatibility of the paints with the
electro-coat. The electro-deposition process should also be taken into
consideration, for yttrium-based inhibitors, the best electro-deposition that
should be applied is the passivation electro-deposition method. The passivation
technique ensures the formation of the thin layer on the metal surface and
substantially reduces the corrosion rate. Therefore, the yttrium is the best as
the lead substitute in electro-deposition coating since its importance in its
application in the automobile industry outweighs it cons and also it is safer
as compared with the lead-based corrosion inhibitors.
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