August_AMP_Digital

energy, steel, and concrete production needs to be reduced

dramatically over the coming decades. CO

capture and

storage is an important technology in reducing greenhouse

gas emissions. In the case of large-scale underground stor-

age of CO

in depleted gas reservoirs or aquifers, the in-

tegrity of carbon steel down-hole tubing is strongly

endangered by CO

corrosion due to much more severe en-

vironmental conditions than normally encountered in tradi-

tional oil and gas production. Severe corrosion problems

may occur due to unknown concentrations of impurities

such as oxygen, carbon monoxide, sulfur dioxide, or hydro-

gen sulfide that are inevitably present in exhaust gases. One

alternative could be the use of CRAs, but most likely at a

huge cost. CRAs may not be completely immune to corro-

sion attack due to the high salinity, impurities, temperature,

and design-caused crevices found in these service condi-

tions

[3,4]

Newand sustainable corrosion control concepts are needed

to ensure equipment integrity and achieve long, failure-free life-

times. Reliable material use requires suitable prediction mod-

els concerning corrosion behavior under such conditions.

Models are needed not only for lifetime prediction, but also for

a better understanding of the corrosionprocesses under condi-

tions of CO

sequestration. The respectivemodelsmust be de-

veloped and verified using experimental data. Testingmethods

with high repeatability and reproducibility need to be devel-

oped and standardized as well

[4]

Offshore wind energy:

Land-based wind farms have

reached a significant level of reliability. However, an addi-

tional challenge from a corrosion standpoint comes into play

when wind energy plants move offshore. All major compo-

nents are susceptible to corrosion and many

types are possible due to a much more severe

corrosive environment stemming from sea

spray and seawater conditions in conjunction

with higher wind forces. One of the main chal-

lenges is to prevent corrosion fatigue under

these severe environments

[5]

. For example,

coatings in general may have strain limits that

can be exceeded by the wind forces or the pe-

riodic action of rotating blades. Evaluation of

the effect of superimposed mechanical stresses

on corrosion protection systems is needed to avoid failures.

Fuel cells:

Independent of the type of fuel cell (proton

exchange membrane, solid oxide, or molten carbonate ver-

sions), the electrochemical reactions and reactant/product

gases within these cells promote harsh environments.

Therefore, corrosion of metal plates is something that is vir-

tually unavoidable over the required lifetime of a fuel cell. A

significant issue is corrosion of the bipolar plates, where a

compromise must be reached between high conductivity

and high corrosion resistance in the fuel cell environ-

ment

[6, 7]

. The solid oxide fuel cell (SOFC), in particular, op-

erates at intermediate (500°-700°C) or high (700°-900°C)

temperatures, which offer the advantages of fuel flexibility

and lower cost catalysts. Operation at elevated tempera-

tures, however, also results in high-temperature materials

degradation issues. A major issue is the resistance of these

materials to oxidation at elevated temperatures, where oxide

scales are formed that are usually not of high conductivity.

The oxidation of Fe-Cr interconnect materials also forms

with simultaneous volatilization of the oxide, which

results in Cr-poisoning of the cathode. In order to avoid

SOFC failures, development of very thin and protective—

yet highly conductive—oxide scales on the surface of inter-

connect alloys with long stability under operating conditions

is necessary.

Lightweight structures:

Lightweight materials are of high

importance to the transportation industry to reduce both

fuel consumption and greenhouse gas emissions. Alloys

based on both magnesium and titanium aluminides are of

special interest for certain applications due to a combina-

tion of their low specific density and mechanical properties.

Although magnesium is a light material, it is among those

with the least corrosion resistance, especially when in con-

tact with other metallic materials. Generally, the method of

modifying the alloy through alloying compounds for in-

creased corrosion resistance is limited and coating systems

must be developed.

Intermetallic titanium aluminides are another useful

alloy for lightweight construction, as they feature half the

weight of steel or nickel-base materials currently used in car

engines while offering similar mechanical performance. The

biggest drawback of these materials is, however, their lim-

ited oxidation resistance at temperatures above 700°-800⁰C.

Again, innovative surface protection systems are needed.

Corrosion failure prevention strategies

Given the severe environments materials will face as

critical enablers of future global energy industries, prevent-

ing corrosion failures requires developments in four key di-

rections according to the tetrahedron in Fig. 2, including

materials design, corrosion mitigation, lifetime prediction,

and inspection.

Materials design:

For the most challenging environ-

ments in which low cost, inexpensive metals may not be em-

ployed, corrosion-resistant alloys need to be developed.

CRAs can be designed based on principles that reduce the

number of corrosion failures by optimizing microstructure

and composition, and possibly employing nanotechnology.

Techniques such as grain-boundary engineering can be used

to design materials resistant to failure from intergranular

corrosion and stress corrosion cracking (SCC). Modern

high-performance computing has enabled the integrated

ADVANCED MATERIALS & PROCESSES •

AUGUST 2014

Fuel cell vehicle

Hydrogen

tank

Batteries

Anode

Power electronics

Membrane

Cathode Electro

engine