15.4 Chemical Processes: Corrosion, Oxidation & Embrittlement

15.4 Chemical Processes: Corrosion, Oxidation & Embrittlement

Chemical and electrochemical attack is one of the most common ways machinery deteriorates in service. The blueprint groups these processes under chemical processes: aqueous corrosion, high-temperature oxidation and the embrittlement mechanisms that turn ductile metals brittle. A designer must recognize each mechanism, estimate its rate, and choose materials, coatings or details that prevent it.

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Electrochemical Corrosion Mechanics & Prevention

Metallic corrosion is an electrochemical process that oxidizes refined metals back to low-energy mineral compounds.

The Four Essentials of an Electrochemical Corrosion Cell

An electrochemical corrosion cell requires four concurrent components:

1. Anode: The metallic site where oxidation occurs, shedding electrons into the metal and dissolving metal ions into the solution ($M \rightarrow M^{z+} + z e^-$).
2. Cathode: The metallic site where reduction occurs, consuming electrons supplied by the anode (e.g., oxygen reduction $O_2 + 2H_2O + 4e^- \rightarrow 4OH^-$ in neutral water, or hydrogen evolution $2H^+ + 2e^- \rightarrow H_2$ in acids).
3. Electrolyte: An electrically conductive liquid (e.g., moisture, saltwater, soil) that conducts current via ionic transport.
4. Metallic Path: Direct electrical connection transferring electrons from the anode to the cathode.

Galvanic Corrosion & The Critical Area Ratio Effect

When two electrochemically dissimilar metals are in electrical contact within an electrolyte, a galvanic cell develops. The more active (anodic) metal corrodes preferentially, while the more noble (cathodic) metal is protected.

$$\text{Galvanic Series in Seawater (Active } \rightarrow \text{ Noble):}$$ $$\text{Mg} \rightarrow \text{Zn} \rightarrow \text{Al Alloys} \rightarrow \text{Carbon Steel} \rightarrow \text{Cast Iron} \rightarrow \text{304/316 SS (Active)} \rightarrow \text{Brass/Bronze} \rightarrow \text{Copper} \rightarrow \text{Nickel} \rightarrow \text{Titanium} \rightarrow \text{Gold}$$

• The Area Ratio Rule: The rate of galvanic penetration at the anode depends directly on the cathode-to-anode surface area ratio:

$$i_{\text{anode}} = i_{\text{cathode}} \left( \frac{A_{\text{cathode}}}{A_{\text{anode}}} \right)$$

• Dangerous Configuration: A small anode coupled to a large cathode (e.g., carbon steel rivets fastening large stainless steel plates). The large cathode draws massive total reduction current, forcing extreme localized current density onto the small anode, causing rapid perforation.
• Acceptable Configuration: A large anode coupled to a small cathode (e.g., stainless steel fasteners joining large carbon steel structural beams).

Specific Corrosion Morphologies

• Pitting Corrosion: Autocatalytic localized attack where passive oxide films (e.g., on 304/316 stainless steel) break down in chloride ($Cl^-$) environments. The pit interior becomes an anode; restricted fluid exchange hydrolyzes metal ions, creating concentrated hydrochloric acid inside the pit.
• Crevice Corrosion: Localized attack in stagnant micro-gaps beneath gaskets, washers, or overlapping joints. Oxygen is depleted within the crevice, establishing a differential aeration cell where the oxygen-starved crevice becomes anodic relative to the oxygenated exterior surface.
• Stress Corrosion Cracking (SCC):
  • Requires the simultaneous presence of three conditions: (1) a susceptible alloy, (2) a sustained tensile stress (applied or residual), and (3) a specific aggressive chemical environment.
  • Classical pairs: Austenitic stainless steels in hot aqueous chlorides ($> 60^\circ\text{C}$); Brass in ammonia ("season cracking"); High-strength carbon steels in hydrogen sulfide ($H_2S$, sulfide stress cracking).

Corrosion Mitigation Strategies

1. Cathodic Protection (CP):
   • Sacrificial Anode (Galvanic CP): Electrically bonding a sacrificial active metal (zinc, aluminum, magnesium) to the protected structure (underground pipeline, ship hull, water heater tank). The sacrificial metal corrodes, continuously supplying protective electrons.
   • Impressed Current Cathodic Protection (ICCP): Uses an external DC power supply connected between an inert anode (mixed metal oxide) and the structure, driving electrons into the structure.
2. Protective Coatings & Anodizing:
   • Barrier Coatings: Epoxy, polyurethane, and powder paints isolate metal from electrolytes.
   • Galvanizing: Hot-dip zinc coating on steel provides both a physical barrier and sacrificial cathodic protection at exposed scratches.
   • Anodizing: An electrochemical process that thickens the natural aluminum oxide ($\text{Al}_2\text{O}_3$) passive layer, which is then sealed in hot deionized water or nickel acetate.

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Intergranular Corrosion, Dezincification & Erosion-Corrosion

• Sensitization of austenitic stainless steels: holding 300-series steels between about $425^\circ\text{C}$ and $815^\circ\text{C}$ ($800^\circ\text{F}$ to $1{,}500^\circ\text{F}$), as in a weld heat-affected zone, precipitates chromium carbides on grain boundaries. The chromium-depleted zones next to them corrode preferentially ("weld decay"). Remedies: low-carbon L grades ($\le 0.03\%$ C, such as 304L), titanium- or niobium-stabilized grades (321, 347), or solution annealing after welding.
• Dezincification selectively removes zinc from brasses with high zinc content in some waters, leaving weak, porous copper; inhibited or low-zinc alloys avoid it.
• Erosion-corrosion occurs where high-velocity or turbulent flow strips protective films (pump impellers, elbows, heat-exchanger tube inlets). Fretting corrosion is wear plus oxidation at clamped joints that slip microscopically (Section 6.1).

High-Temperature Oxidation

Above a few hundred degrees Celsius, metals react directly with oxygen to form scale. The Pilling-Bedworth ratio (oxide volume per metal volume consumed) indicates whether the scale protects: ratios between about 1 and 2 usually give an adherent, protective film (aluminum is about 1.3), ratios below 1 (magnesium, about 0.8) give porous scale, and much larger ratios build compressive stress that cracks and spalls the scale. When a protective scale controls the rate by diffusion, growth follows the parabolic law $x^2 = k_p t$, so doubling the exposure time increases scale thickness by only about $41\%$. Chromium (at least about $10.5\%$ for stainless steels), aluminum and silicon are added to form protective $\text{Cr}_2\text{O}_3$, $\text{Al}_2\text{O}_3$ or $\text{SiO}_2$ scales.

Embrittlement Mechanisms

• Hydrogen embrittlement: atomic hydrogen from acid pickling, electroplating, welding with damp consumables, cathodic overprotection or sour ($\text{H}_2\text{S}$) service diffuses into steel and causes delayed brittle fracture under sustained tensile stress, often hours or days after assembly. High-strength steels (roughly above 39 HRC) are most susceptible. Plated high-strength parts are baked promptly after plating (typically around $190^\circ\text{C}$ to $220^\circ\text{C}$ for several hours), and hot-dip galvanizing of the highest-strength structural bolts (such as ASTM A490) is prohibited.
• Caustic embrittlement: concentrated sodium hydroxide at boiler seams and crevices causes intergranular cracking of stressed steel.
• Liquid metal embrittlement: certain liquid metals wet and penetrate grain boundaries, for example zinc on high-strength steel during welding of galvanized parts, or mercury on aluminum and brass.
• Temper embrittlement: alloy steels held or slowly cooled between about $375^\circ\text{C}$ and $575^\circ\text{C}$ allow impurities (P, Sb, Sn, As) to segregate to grain boundaries, raising the ductile-to-brittle transition temperature.

Corrosion Rate and Corrosion Allowance

A weight-loss coupon test converts directly to a penetration rate:

$$\text{mpy} = \frac{534\,W}{\rho\,A\,t} \qquad \left(W \text{ in mg},\ \rho \text{ in g/cm}^3,\ A \text{ in in}^2,\ t \text{ in h}\right)$$ $$\text{mm/y} = \frac{87.6\,W}{\rho\,A\,t} \qquad \left(W \text{ in mg},\ \rho \text{ in g/cm}^3,\ A \text{ in cm}^2,\ t \text{ in h}\right)$$

where mpy is mils (thousandths of an inch) per year. The design corrosion allowance is the expected rate times the design life, added to the pressure or structural thickness (as in the ASME vessel and piping examples of Section 11.1).

Worked example: A carbon steel coupon ($\rho = 7.87\,\text{g/cm}^3$) with $10.0\,\text{in}^2$ exposed area loses $485\,\text{mg}$ in $1{,}000\,\text{h}$ of service-water exposure.

$$\text{mpy} = \frac{534(485)}{7.87(10.0)(1{,}000)} = \frac{258{,}990}{78{,}700} = 3.29\,\text{mpy} = 0.084\,\text{mm/y}$$

For a 20-year design life, the corrosion allowance is $20 \times 0.084 = 1.67\,\text{mm}$ (about $0.066\,\text{in}$), so a $1/16\text{ in}$ ($1.6\,\text{mm}$) allowance would be marginal and $2\,\text{mm}$ is a sensible choice.