Technical Presentations at the April 2007 Meeting
1.1
'Some
Marine Coating and CP Failures',
Chris Googan, Anticorrosion Engineering
The
talk emphasised that carbon steel was the material of choice for
construction in the marine environment.
Moreover, where suitably protected from corrosion, it usually gives
satisfactory service. However,
there are all too many occasions where the coating or the CP system used
for protection give unsatisfactory performance.
In these instances the coating, or CP, is deemed to have
“failed”.
Irrespective
of the nature of the project, the causes of such failures can be traced to
human shortcomings in the design, specification, application or
installation, or operation and maintenance of the chosen protective
system. A dozen or so
examples of failures due to these human-based causes were presented.
These included:
o the ludicrous results of entrusting the company accountant with the design of a CP system to case,
o the predictable results of failing to commission an impressed current CP on a ship,
o the consequence of failing to appreciate that even sacrificial anode systems can give rise to stray current interaction problems under some circumstances, and
o several examples where either a weak coating specification, or inadequate control of the coating application, caused failures leading to expensive litigation.
1.2 ‘Electrochemical Behaviour of CuNi 90/10 in Chlorinated Seawater under Stagnating Conditions’, Wilhelm Schleich , KM Europa Metal
The copper-nickel alloy CuNi 90/
10 has been extensively used on different
marine engineering structures. Its main application is piping for seawater
handling and heat exchangers. Regardless
of the macro fouling resistance of the material, seawater is often
chlorinated. As there is only
limited information available about the effect of chlorination on the
corrosion rate of copper-nickel, a systematic approach is required to
understand the effect of free chlorine on its corrosion behaviour.
The aim of the first part
of this project has been the characterization of the electrochemical
behaviour of CuNi 90/10 in chlorinated seawater under stagnating
conditions at room temperature. The results were obtained in synthetic
seawater containing up to 5 ppm free chlorine using fresh and pre-exposed
samples. Testing involved
free corrosion potential and polarization resistance measurements,
cathodic and anodic polarization as well as gravimetric and metallographic
examinations. The data
obtained did not reveal a distinct relationship between corrosion
behaviour and the free chlorine concentration. Instead, the corrosion rate
was found to decrease with increasing formation of oxide layers. This fact underlines a clear advantage of CuNi 90/10 versus
high alloy materials in applications where the presence of chlorine is
often combined with high seawater temperatures.
The data obtained is being used as the basis for a second part of the project, which compares chlorination under erosive conditions with previously reported data. This will be used to validate current practical service recommendations.
3.1
'The
Effect of Surface Finish on Corrosion Behaviour of Super-duplex Stainless
Steel', Bill Deans, Department of
Engineering, Kings College, Aberdeen
The surface
condition of real engineering components is often implicated in service
failures but is not often considered in current materials selection
procedures. The effect of surface finish on the pitting corrosion
behaviour of a 25Cr super duplex stainless steel has been investigated
using a variety of surface finishes from a downhole tubular component and
a polished sample of the same material.
Testing was carried out to ASTM G150 to determine the critical
pitting temperature (CPT) in a 3.5% NaCl solution.
The results showed that the surface roughness parameter, Ra, was not sufficient to determine the effect of surface finish as it only describes the amplitude of surface grooves and not the spacing. A surface with steep, narrow grooves has a poorer resistance to pitting than a more ‘open’ surface, albeit with a lower value of Ra. Both depth and spacing have an effect on diffusion rates from surface grooves and, hence, have a role in nucleation leading to the creation of metastable and stable pitting attack.
‘Exemplary Long-Term Performance of Certain Materials in Marine Environments’, Bopinder Phull, Corrosion Consultant, Wilmington, NC, USA (ex LaQue)The
long-term performance of a number of materials in a harsh, natural, marine
atmospheric environment – where extra specimens exposed at test
commencement decades ago served as “museum” pieces – are reviewed.
The exposures at the world-renowned Kure Beach, NC, USA test site
were finally terminated in 2005.
53
years’ exposure, 250-meters from the ocean - 150-µm thick
thermal-sprayed coatings of Zn, Al, and Zn-Al mixed and alloy powders
(ranging from 90 to 10% Zn – balance Al), and dual-layer coatings (75-µm
Zn over 75-µm Al; and vice-versa); all applied by arc or flame-spraying,
with no top coat sealant, protected the carbon steel substrate.
Based on these results, thermal-sprayed Zn and Al coatings are
widely used in marine atmospheres, often with sealants to extend life.
21 years’ exposure, 25-meters from the ocean - Galvanic corrosion panels simulating corrosion of iron support-bars inside the Statue-of-Liberty due to contact with the copper skin and riveted copper saddle brackets. Very severe corrosion of carbon steel in contact with copper produced voluminous corrosion products that duplicated joint failure of the copper saddle brackets. Candidate replacement materials tested: Type 316L SS, 2205 duplex SS, 70/30 CuNi, and aluminium-bronze, were all compatible with the copper saddles brackets. Type 316L stainless steel which was selected as replacement material for the actual Statue-of-Liberty application is performing extremely well after 20+ years.
60 years’ exposure, 250-meters from the ocean - Mill-finished Types 301 and 304 stainless steel exposed 250-meters from the ocean for exhibited some visible rust spots and staining; this decreased markedly in the order for Types 308, 309, 310 and 316 SS.
38
years’ exposure, 250-meters from the ocean - Cold-rolled Type 430 SS
exhibited moderate rust spots and staining for the following surface
finishes: Nos. 2, 4, 2B, 8, and electropolished.
Type 316 SS exhibited minimal corrosion compared to Type 302 SS
which performed better than Type 430 SS.
In all cases, the electropolished surface exhibited the least
corrosion.
55
years’ exposure 250-meters from the ocean - Cr-plated Type 302 SS
still appeared shiny.
64
years’ exposure 250-meters from the ocean - Ni-Cr-Mo-Fe “alloy
C” exhibited no corrosion. It
still had a mirror finish - the surface never having been subjected to any
cleaning.