**2.5. Stainless steel and their coatings**

which are both hygroscopic and proton conductors. Functionalized one-dimensional carbon nanotubes can also be incorporated into the Nafion membrane to improve the membrane

By definition, coating is a covering that can be applied to the surface of substrate for enhance‐ ment, functional, and modification purposes. The major purpose of coating on bipolar plate is to serve as corrosion resistance interface between the substrate and the environment, thereby reducing or eliminating the interfacial contact resistance that affects the overall power output of the fuel cell. They impact special surface properties of hardness, wear control, corrosion, and oxidation resistance without changing the substrate bulk properties. Therefore, improves

Coating is mostly needed for application of metallic bipolar plates because of the possible interaction with the stringent acidic fuel cell environments that affects its overall performance. Oxide formation and ion dissolution as a result of metal bipolar plates can be prevented by applications of various coatings. Metallic bipolar plates[24, 25] are often coated with protective coating layers which serve as a barrier between its substrate and the corrosive media thereby preventing corrosion. The coating must be able to satisfy the following important criteria:

**•** It must have good adhesion to the substrate material without exposing its corrosive media. Proper adhesion of coating to the substrate is achieved by selecting coating materials with thermal expansion coefficients similar to those of the chosen bipolar plate's material to

**•** It must be conductive so as to enhance electron conduction through it to the external circuit. In the absence of corrosion, there will be no formation of metallic ions poisoning the membrane assembly electrode and reducing its potency for proton transport. Formation of oxide films in stainless steel as a self-protection against progression of corrosion that eventually result into

Two types of coatings[26–29] that have been investigated over the years as suitable candidates

**•** Carbon-based coatings: includes conductive polymer, graphite, diamond-like carbon, and

**•** Metal-based coatings: comprises metal nitrides, metal carbides, metal oxides, and noble

The investigation of metallic bipolar plates is divided into two major parts:

performance operated under low relative humidity and dry conditions [22, 23].

**2.4. Surface modification through coating**

244 Electrodeposition of Composite Materials

minimize micro- and macro-crack formation.

**•** It must be impermeable to the fuel cell reactant gases.

high surface contact resistance will also be eliminated.

for bipolar plates are as follows:

**•** Stainless steel and their coatings

organic self-assembled monopolymers.

metals like gold, platinum, and ruthenium.

**•** It must be chemically stable or inert and give low contact resistance.

surface properties.

Bare or uncoated stainless steel cannot satisfy DOE criteria for bipolar plate. There are three types of stainless steel with varying chromium contents: austenitic (AISI SS300)[30] has 18– 20% Cr, ferritic (AISI SS400)[31, 32] has 17% Cr, and martensitic Cr quantities ≥ 11.5%. Chromium acts to produce a thin layer oxide of Cr2O3 which gives it self-surface protection and stop progression of corrosion. The passive layer of the thick oxide film however increases interfacial contact resistance between the bipolar plates and the gas diffusion layers which amount to the overall voltage drop. As a result of this, majority of studies[33, 35] on bipolar plates use measurements of interfacial contact resistance as the main criteria for material suitability.

In the stainless steel group, austenitic stainless steel is the most corrosion resistant due to its high Ni composition coupled with substantial level of chromium that gives it a higher formability at all temperatures from the cryogenic region to the melting point of the alloy.

They are the largest produced stainless steel accounting for about 70%. As a result, the grades 316, 310, and 304 SS have been investigated by many researchers as suitable candidates to replace nonporous graphite bipolar plates.

The primary selection criterion for austenitic stainless steel bipolar plates is the Cr, N, and Mo content that comes in different compositions and therefore makes them to behave differently in various environments. Addition of Mo and N is intended to enhance crevice and pitting corrosion resistance. Nickel and chromium addition is to improve strength and high-temper‐ ature oxidation resistance.

Over the years, ferritic stainless steel has been considered as bipolar plate material due to its low nickel content that reduces the overall cost of the material and eliminates the potential problem of Ni ion contamination of the membrane.
