Homework VI - Biomaterials Science. Corrosion is a natural process which can be found, not only in metals, but also in ceramics and polymers (instead of corrosion, it is usually called “degradation”). 2 main concerns around this topic include economic and security issues. In fact, big accidents related to corrosion are present in the world’s history... Some involved crashed bridges and sunk ships. Processes to avoid events like this should be carefully chosen, accordingly to our monetary resources, as well as considering the severity of a hypothetical situation where the material can fail (e.g. - if there are any lives at risk). IST - 4th Year - 2nd Semester - Biomedical Engineering.

1. Biomaterials Science – 4th year | 2nd Semester
Luís Rita | 78680
Mechanisms in an Aqueous Solution for the Corrosion of a Metal Alloy
Corrosion
Corrosion is a natural process which can be found, not only in metals, but also in ceramics and polymers (instead
of corrosion, it is usually called “degradation”). 2 main concerns around this topic include economic and security
issues. In fact, big accidents related to corrosion are present in the world’s history… Some involved crashed
bridges and sunk ships. Processes to avoid events like this should be carefully chosen, accordingly to our
monetary resources, as well as considering the severity of a hypothetical situation where the material can fail
(e.g. - if there are any lives at risk).
Corrosion of a Metal Alloy
Ions in solution and pH (uniform corrosion)
Many are the factors that directly influence the rate of corrosion in a material (although they vary between
ceramics, metals and polymers). Focusing on metal alloys, their chemical/physical structure is strongly
influenced by pH of the medium. As soon as a metal is put in contact with a strong acid (sulfuric acid, nitric
acid or hydrochloric acid) or a strong base (caustic soda), it is almost certain it will get damaged, considering
that coating or other protection measures weren’t considered. What about pure water (neutral pH)? The
problem remains! The supply of oxygen will dictate the rate at which corrosion will occur.
Related to the case of acidic environment (sulfuric acid) the following set of reactions are stimulated:
Moreover, certain metals, such as copper or lead are resistant to the attack of sulfuric acid, due to the
reaction product that is formed (CuSO4). It will form a protective surface layer around the metal and confer
it protection.
On the other side, alkalis (caustic soda) cause corrosion through the following steps:
Salt and fresh water (uniform corrosion)
Additionally, both salt and fresh water causes corrosion. 1st
one causes the dissociation of the metal in a
positive ion and a certain number of electrons lost by the initial atom. Iron releases 2 e-
plus Fe2+
and a
negative potential is generated inside the bar (because Fe2+
is emitted to the solution and e-
not - Fig. 1).
Now, if 2 bars (Fig. 3), with 2 different corrosion potentials (Fig. 2), are connected, it’s expected to witness
a flux of electrons from the one with a lower potential to the other (generating an electric current). Iron
will then get corroded and the copper ions will plate onto the copper bar: Assuming
the liquid solution of CuSO4 is replaced by fresh water and oxygen stays in contact with copper (Fig. 4), a
hydrolysis reaction will substitute the previous cathodic:
Is it necessary to be in the presence of 2 different materials (w/ different corrosion potentials) to
observe the corrosion phenomenon? No… Bad news! If one material presents a different exposition to
oxygen at different locations, e.g. bar, then a hydrolysis reaction will predominate at higher concentrations
and, at lower concentrations, the bar will keep being oxidized as before. Releasing ions to the environment.
Selective corrosion
If corrosion events happen in specific locations and associated to local stresses and/or local changes in
microstructure, then the material will tend to fail suddenly. Examples where this can be seen more
frequently are near to welded joints.
Pitting corrosion (attack that occur in small breaks in the protection film, e.g. formed
spontaneously after oxidation events); Intergranular events (due to different chemical properties between grains and its boundaries);
Galvanic attacks (the presence of grains with different compositions can coincide with the presence of ionic solutions in the medium
around and give rise to corrosion cells); Stress corrosion cracking (accelerated action of corrosion because of the breaks caused by
stresses in oxide films near to cracks) and, finally, Corrosion fatigue (related to the accelerated rate at which cracks grow due to a
corrosive environment) are some of the localized corrosion mechanisms.
Fig. 1 – Metal in an ionic solution.
Fig. 2 – Reduction potentials scheme.
Fig. 3 – Flow of e- across 2 different
metals in a CuSO4 solution.
Fig. 4 – Flow of e- across 2 different metals
in water.