Mechanical Technology

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⎪ Structural engineering materials, metals and non-metals ⎪
Recently I asked my students to
write about their experiences
with corrosion, just in general
life where they have come
across problems caused by corrosion
damage. The students are from a wide
range of backgrounds with some growing
up in more rural conditions. Some of the
stories the students related impressed
on me the extent of corrosion and how
widely it influences our lives. The stu-
dents from the more rural backgrounds
related issues with vehicles and support
structures that corroded.
The consequences varied from being
just a hindrance to life threatening. One
student described how a modified ox-
wagon failed after a small modification
did not take corrosion into consideration.
Another student described how the family
put up a small structure to hold a large
water tank and they were quite surprised
when the structure failed unexpectedly
and almost fell on top of another fam-
ily member. After closer examination
they found that the structure failed as
a result of excessive corrosion of one of
the structural members. Another student
reported on a water pipeline that burst.
This was also in the news because it
caused tremendous damage and loss of
life. And several students reported on
the effect of atmospheric corrosion on,
especially, roofing.
This fits in well with research that
we are involved with at the School of
Chemical and Metallurgical Engineering.
One of our students is investigating the
extent of atmospheric corrosion in the
greater Johannesburg Metropolitan. This
is a much-needed study since a gener-
alised corrosion map for South Africa was
set up a number of years ago based on a
limited number of sites. Therefore, one of
our PhD students (Janse van Rensburg)
has decided to focus on studying atmo-
spheric corrosion after a number of years
performing atmospheric corrosion testing
as an Eskom consultant. The investiga-
tion considers close to 60 different sites
In this issue’s Wits on Materials column, Josias van der Merwe of
the School of Chemical and Metallurgical Engineering talks about
corrosion and some of the research being undertaken to combat
its effects.
References
1. Myburg G, Varga K, Barnard W, Baradlai P, Tomc-
sányi L, Potgieter J, Van Staden M: (1998). Sur-
face composition of Ru containing duplex stainless
steel after passivation in non-oxidizing media.
Applied Surface Science, 136 (1-2), 29–35. doi:
10.1016/S0169-4332(98)00326-2.
2. Potgieter JH: (1991). Alloys cathodically modi-
fied with noble metals. Journal of Applied Elec-
trochemistry, 21(6), 471–482. doi:10.1007/
BF01018598.
3. Tomashov ND & Ustinskii EN. (1990). Cor-
rosion behaviour of chromium-ruthenium
alloys in sulphuric acid solutions at active
dissolution potentials. Protection of Metals
(English Translation of Zaschita Metallov),
26(1), 99–102. Retrieved from http://www.
0025480610&partnerID=tZOtx3y1.
Material engineering in practice:
Corrosion is everywhere
that are widely spread over the whole
Johannesburg Metropolitan Area.
The initial results have been very
interesting, describing the effect of in-
dustrial sites that contribute to most of
the increased corrosion rates found, the
effect of wind direction. High rainfall and
seasonal changes were highlighted. What
has been very clear is that atmospheric
corrosion in a region cannot and should
not be described by a single point mea-
surement. These results will be presented
as a corrosion map, a guide that can be
used to compliment climate condition
monitoring.
In the South African context it has
been very important to consider and
implement beneficiation of our mineral
resources. Ruthenium is one of the met-
als forming part of the Platinum Group
Metals (PGM) and is one of the least ex-
pensive metals in this group. It has been
found that ruthenium provides excellent
corrosion resistance to stainless steels
(Myburg et al., 1998; Potgieter, 1991;
Tomashov & Ustinskii, 1990), but is still
too expensive and cannot be feasibly
used as a bulk alloying element. For
that reason we have been investigating
the application of ruthenium rich layers
to stainless steel, specifically for
sulphuric acid service. The im-
provement found in corrosion
resistance is considerable,
with only small ruthe-
nium additions. This is
extremely beneficial as
it makes the use of
ruthenium a viable
option for sulphuric
acid service.
The corrosion re-
sistant layers are ap-
plied by laser alloy-
ing and cladding and
have also been used
for both general cor-
rosion to replace more
expensive materials such
as Hastelloys. In similar
work focused on more
specific plant related
corrosion problems,
we are looking at the
research of stress cor-
rosion cracking as well as metal dusting.
Some of these forms of corrosion are
not found everywhere but only in very
specific industrial processes. When they
occur, their presence can be extremely
limiting and a huge stumbling block to
processes such as those used in refineries
and power generation plants.
Corrosion clearly affects all of our
lives. q
Mechanical Technology — July 2015 35