Volume 22 Preprint 46
GREEN CORROSION INHIBITORS: CORROSION INHIBITION EFFICIENCY OF COLA NITIDA HUSK EXTRACTS
E. O. Fehintola, E.F. Olasehinde, L. Lajide and D. T. Oloruntoba 3
Keywords: Inhibition efficiency, weight loss measurement, Corrosion, Plant extracts, Mild steel, Statistical analysis
As a follow up on our previous studies, Cola nitida husks were collected at Litaye Village near Adeyemi College of Education, Ondo. Cleaned husks were subjected to sun-dry and air-dry processes. Sun-dried and air-dried husks were powdered, sieved and stored in desiccators at room temperature. A known mass of the powdered husks was soaked in ethanol in different containers for 72 hours to obtain inhibitor extract.. Extract was used as inhibitor for mild steel of known composition. Weight loss, inhibition efficiency (IE) and corrosion rate were studied using standard methods. Models that relate concentration of inhibitor and temperature to IE were proposed, established and evaluated using statistical methods. The inhibition efficiency increases with increasing extracts concentration to 91.333 % and 89.27 % at 333K of 1.0 g/l of extracts for the air and sun-dried extracts, respectively. CD, MSC, AIC and SC were in the range of 0.8815 to 0.9658, 1.8 to 3.1, 84.7 to 109.7 and 81.8 to 109.7 for both air and sun dried extracts respectively. The study revealed that the best models for sun and air dried extracts were linear with interaction with MSC (3.1 for both), AIC (81.7 for both ) and SC (84.7 for both), respectively. The worst models for sun and air dried extracts were log-linear without interactions and non- linear without interaction with MSC (1.8 for both) , AIC (109.7 for both) , SC (106.7 for both), respectively. The cost analysis revealed that it is economical to utilise plant husks extract. It was concluded that these two extracts of the present study can serve as effective green corrosion inhibitors for mild steel in acidic media and further investigations to assess the corrosion morphology and to isolate and confirm the active phytochemicals responsible for the inhibition of mild steel corrosion in acidic media are required.
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Summary of Findings on Failed Gas Valves
This note summarises our findings on the failure of gas valves associated with hydrogen
embrittlement of bolts used in the assembly of the valve.
Hydrogen embrittlement (HE) is a well-established phenomenon; while it can occur in many alloys, it
is most frequently a problem with high strength steels. It arises when hydrogen dissolved in the steel
collects at regions of high tensile stress and facilitates cracking. The susceptibility of a steel to HE is
strongly affected by the strength; lower strength steels are essentially immune to HE, while higher
strength steels can be very susceptible. The strength above which HE becomes a problem is
dependent on a number of factors, including the hydrogen concentration in the steel and its
chemical composition and microstructure, but it is typically in the region of 1000 MPa. Steel bolts
are classified by a code of the form xx.y, where xx is a number corresponding to the specified
minimum ultimate tensile strength (UTS) in units of 100 MPa, while .y is the specified minimum yield
stress (SMYS) divided by the UTS and rounded to one digit. Thus class 12.9 bolts have a specified
minimum UTS of 1200 MPa and an SMYS of 1080 MPa (1200 MPa x 0.9), and are potentially
susceptible to HE.
Hydrogen can be introduced into steel by many means, including pickling, electroplating and
corrosion. In the case of electrogalvanised high strength bolts, it is known that hydrogen enters the
steel during pickling prior to electroplating and during the electroplating itself. In order to remove
the hydrogen, treatment specifications require that the bolts are baked after (or sometimes during)
plating to remove the hydrogen. In order to be effective the baking should occur immediately after
plating. There are consequently a number of ways for flaws in the plating process to result in
defective bolts that will fail under stress due to hydrogen introduced during production. Failures due
to hydrogen introduced during production are known as Internal Hydrogen Embrittlement (IHE).
High strength bolts can also fail as a result of hydrogen produced by corrosion in service; in this case
the phenomenon is known as Environmental Hydrogen Embrittlement (EHE).
The zinc coating on electrogalvanised bolts has a number of effects – it leads to the introduction of
hydrogen during the coating process, it acts as a barrier against hydrogen entering or leaving the
steel (since hydrogen diffuses much more slowly in zinc than it does in iron), and it provides
corrosion protection for a period, though once the steel is exposed it will tend to increase the local
rate of hydrogen entry compared to uncoated steel. Hydrogen embrittlement due to hydrogen
present in the bolts from manufacture, IHE, is typically relatively rapid (of the order of days, weeks
or months), although the time to failure will depend on the stress applied to the bolts and the
hydrogen concentration. Thus tests for potential hydrogen embrittlement of fasteners apply high
loads for a short period; ISO 15330 "Fasteners. Preloading test for the detection of hydrogen
embrittlement. Parallel bearing surface method" specifies a minimum test duration of 48 hours. The
failure time for hydrogen entering as a result of corrosion, EHE, depends on the corrosion processes
occurring and can be much longer, especially for galvanised bolts, where little hydrogen will enter
the steel until corrosion penetrates the zinc coating.
The observed failures consist of broken bolts holding the bonnet to the body, or in one case the
gland seal to the bonnet, of a gas valve; if sufficient bolts break the valve will leak. Consequently two
classes of failure can be considered:
bolt failure - failure of an individual bolt without regard to other bolts in the valve
valve failure - failure of sufficient bolts in a valve for leakage to occur.
Reported valve failures are listed in Table 1.
It is generally agreed that the bolt failures are primarily a result of HE, although some bolts may fail
by mechanical overload once the majority of bolts in a valve have failed.
In addition to failed valves, in which all or most bolts have failed, some isolated bolt failures have
been observed during valve inspections, either in service or in unexposed valves in storage.
Bolt failures observed in storage are clearly a result of IHE, since little or no corrosion will occur in a
dry storage environment. Failures occurring in service could be a result of IHE or EHE, although long
times to failure and evidence of corrosion are suggestive of EHE.
Review of valve failure data suggested that a cluster of failures had occurred in valves manufactured
in early 2010. Therefore, available data on failed and unfailed bolts and valves was collected and
analysed in collaboration with Professor Patrick Laycock. The analysis was based on bolt and valve
failure information collected by AVK. Unfortunately the data are somewhat distorted, since valve
failures are essentially self-selecting (i.e. a failed valve reveals itself by leaking) whereas failure of
one or two bolts on a valve (which would not cause valve failure) is revealed only if the valve is
In summary, the statistical analysis, last updated in March 2014, suggests that there were one or
more 'bad' batches of bolts, supplied by one manufacturer (TVS) that had a higher than normal
susceptibility to IHE, and gave rise to a cluster of failures in weeks 11 to 14 in 2010. This is shown in
Figure 1, which plots the estimated probability of bolt failure as a function of the date of valve
production. Three estimates of bolt failure probability are shown:
Raw failure rate - this is simply the number of failed bolts divided by the number of bolts
examined. Owing to the small number of bolts from some periods (especially outside the
known 'at risk' period), this may under-estimate the true failure probability.
Best estimate failure rate - this assumes that the next bolt to be examined will have failed.
This will typically over-estimate the failure rate, especially when no failures are observed.
95% confidence estimate - this provides a very conservative estimate of the possible failure
To a first approximation the probability of multiple bolt failures in a single valve are given by the
product of the probabilities of individual bolt failures. Thus if the probability of a bolt failure is 1%
(0.01), the probability of failure of 3 bolts in a single valve will be 0.0013 or 1 in 1 million. In practice
the situation will be complicated by corrosion effects (which will typically be the same for a given
valve) and the increase in stress in intact bolts that will occur when a bolt fails (which will increase
the probability of failure of intact bolts once one or more bolts have failed). These factors may
explain why no valves have been found with 4, 5 or 7 failed bolts. In statistical terms the effect of
corrosion is to lead to a lack of homogeneity in the failure probability, while the effect of increasing
stress as bolts fail will lead to a lack of independence in the failure probability.
Based on Figure 1, assuming that at least three bolts must fail for a valve to fail, and ignoring
deviations from homogeneity and independence, a bolt failure probability of 1 % leads, as explained
above, to a valve failure probability of 1 in 1 million, which can reasonably be considered as
acceptable. From the data of Figure 1 it can be seen that there is a significantly enhanced probability
of valve failure in weeks 11 to 14, as is found for actual valve failures. Week 16 also has an
apparently high failure probability, but this is based on very limited data, with only one failed bolt in
a sample of 96. This highlights the difficulty of obtaining adequate data for reliable statistical analysis
when the probabilities of failure are low.
Three valve failures occurred in valves manufactured prior to 2010. One of these, S/N 12720,
manufactured on 29 April 2008, used black (i.e. not zinc plated) bolts; these are unlikely to suffer
from IHE, and there was evidence of corrosion, indicating that the probable failure mechanism was
EHE resulting from corrosion in service. Valves S/N 100586 and S/N 103303 manufactured on 25
June and 30 July 2009 respectively, used zinc plated bolts, so IHE was possible. However, in the case
of S/N 100586 there were a number of factors that suggested that the failure was due to EHE
resulting from corrosion rather than IHE:
The broken bolts were situated at the 'corners' of the valve, and were therefore
somewhat more exposed to corrosion than the two unbroken bolts in the middle of the
Of the three bolts that I was able to examine, the two broken 'corner' bolts showed
more corrosion and loss of zinc than the 'middle' bolt.
The time to failure was relatively long.
There is no evidence of any other broken bolts from this period of production (other
than S/N/ 103303).
In the case of S/N 103303 I have less information, but the time to failure and date of production
suggest that this failure was also due to EHE.
Subsequent to these investigations a valve failure was reported for a valve (S/N 123643) that was
produced in July 2010; after the period associated with a high failure probability. This valve had all
but one of the bolts broken, and the remaining bolt displayed signs of cracking at the root of the
unengaged threads. Examination of the bonnet and the failed bolts revealed significant corrosion,
and it is probable that this failure was due to EHE induced by corrosion, rather than IHE.
Another valve (S/N 119460) that was produced in May 2010 failed after a silicone sealant had been
applied following an inspection. This produced acetic acid during the curing process, and the acidity
was considered to be responsible for the production of hydrogen and the subsequent EHE.
Since the main period of my investigations in 2014, there have been two further failures, S/N 103303
(discussed above) and 119890. The latter, manufactured at the end of June 2010, was inspected and
found to be intact in June 2013, but subsequently failed in November 2015. The absence of early
failure and the apparent absence of corrosion protection strongly suggests that this failure was EHE
resulting from corrosion.
In addition to the strong effects of bolt manufacturer and date of production on the probability of
bolt failure, there was also a weaker effect of service environment, with buried valves having a
higher probability of failure. This may be indicative of an environmental effect on failure, but it may
also be the result of a differing proportion of unfailed valves being examined, since the examination
of buried valves is significantly more difficult.
It is clear that there was a faulty batch of bolts in early 2010 that lead to the cluster of failures in
valves produced in weeks 11 to 14 of 2010 (the primary 'at risk' period). This failure was primarily a
result of IHE, although there may have been some environmental effects.
Some failures occurred outside the 'at-risk' period. It is considered to be most probable that these
failures occurred as a result of EHE due to corrosion, although this cannot be proved conclusively.
Professor Robert A. Cottis
Exeter 4 February 2017.
Table 1 - Valve Failures
SSE reported as 4 bolts failed and bonnet lifted and leaked at 2 bar.
Commisioned 16th Nov 2009. Longham Dorset.
Kevin Boland at National Grid reported all 6 bolts failed on LP system
Bolt exchange by Nat Grid near London City Airport. When excavated both
gland bolts found broken and valve leaking. (3.5 bar). Clamp fitted to B/B Joint
(original bolts left in)
Failed in Hamilton. SGN
Failed in Oxford. SGN
(TVS) Reading valve all bolts broken. Hydrogen embrittlement due to plating.
Tunbridge Wells Kent (SSE), Repaired by customers engineer (studs and Bolts?)
under an emergency operation due to gas leak being reported. All bolts
reported as being broken. A Bite and customer engineer returned to the site
and found 1.5 bolts thrown / disguarded on the ground at the site location
these were returned to AVK. Fasteners returned are TVS grade 12.9 plated.
Proper bolt exchange carried out 8/8/12, valve fitted with grade 8.8 &
witnessed by RM/RCS
National Grid Valve (Stoke) Failed on 6.9 bar. 8 bolts fratured, 2 remain intact.
(TVS). Heads of broken bolts not found. Gland bolts unknown
Bord Gais - Outside of date range. See seperate report - Failed after
Reported as bonnet lifted and valve leaks (7 bar) 7 of the 8 bonnet bolts
broken. Repaired by fitting 8.8 bolts. Bolts sent by NG to HSE for investigation.
Liquid coated to CW5 instead of being wrapped
The valve was installed in Church Manor Way, Eritch, South East London ( just
south of the Thames near Dagenham ) [originally listed as failing in 2013)
Valve installed in approx March 2009
The valve was below ground on low pressure and unwrapped. All 6 fasteners
Figure 1 Bolt Failure Rates
Raw Failure Rate
Best Estimate Rate
Upper 95% Confidence Rate