Localized corrosion in which chloride ions or similar species locally break down the passive film and small pits are dug deeper. It is a problem for stainless steels and aluminum.
Japanese name: 孔食
Pitting corrosion appears in 2 practice questions across 2 topics, including Corrosion and surface treatment, Biomaterials.
Pitting begins when chloride ions locally break down weak points in the passive film. In stainless steels, sites where inclusions such as manganese sulfide (MnS) have dissolved are known to be favored starting points. Most small pits repassivate partway and stop (metastable pitting), but inside a pit that keeps growing, dissolved metal ions hydrolyze and make the solution acidic, and chloride ions gather to maintain electrical neutrality, so the film can no longer repair itself. The inside of the pit becomes a small anode and the wide passive surface around it the cathode, so only the bottom of the pit advances rapidly.
The pitting resistance equivalent number PREN = Cr + 3.3Mo + 16N (each element in mass%) is widely used as a guide to the pitting resistance of stainless steels. SUS304 is about 18–20, SUS316 about 24, and the duplex stainless steel SUS329J3L (equivalent to 2205) about 35; for long exposure to seawater, grades of 40 or more are sometimes chosen. The critical pitting temperature (CPT), found by raising the temperature in a solution such as ferric chloride until pitting begins, is also used for comparison. PREN is a guide calculated from composition, so actual resistance varies with welds and surface condition.
In pitting, even if only a tiny amount of metal has dissolved overall, a single pit that penetrates the wall makes a pipe or tank leak and puts it out of service. Service life therefore cannot be assessed from weight loss or average corrosion rate as with uniform corrosion. Pits are small, and the surface is often covered with rust or deposits, so they are easily missed in visual inspection. In practice, the maximum pit depth measured on limited areas is used with extreme value statistics (the Gumbel distribution) to estimate the maximum depth over the whole installation.
Aluminum is also a passive metal and pits in neutral water containing chloride. In particular, when the water contains traces of copper ions, copper deposits on the aluminum surface and acts as a cathode, promoting pitting. For this reason, layouts that place aluminum parts downstream of copper piping, where dissolved copper can reach them, are avoided. Countermeasures include forming a thick oxide film by anodizing, covering heat exchanger tubes and similar parts with a zinc-containing layer that sacrifices itself (clad material), and simply avoiding aluminum in environments with high chloride concentrations.
Answer: B. Pitting corrosion; switch to a Mo-bearing grade such as SUS316
Chloride ions locally break down the passive film at weak points and cause pitting corrosion, in which small holes are dug deeper and deeper. Inside the pit, hydrolysis of the dissolved metal ions acidifies the solution, and chloride ions collect to maintain electrical neutrality, so the corrosion becomes self-accelerating. The large surrounding passive surface acts as the cathode and the bottom of the pit as a small anode, so the pit can advance rapidly to perforation. The countermeasure is to switch to a grade richer in Mo, Cr and N, which strengthen the film (such as SUS316, equivalent to AISI 316, with about 2–3% Mo); the pitting resistance equivalent number calculated from these contents is a guide for selection. Because this is not corrosion that thins the whole surface uniformly, increasing the wall thickness is not an effective countermeasure. More carbon makes Cr carbides form more readily at grain boundaries (sensitization) and actually lowers corrosion resistance.
Answer: A. In a moist environment, tetragonal grains transform to monoclinic from the surface with a volume expansion, roughening the surface and lowering the strength
Y-TZP gets its high toughness from transformation toughening, in which metastable tetragonal grains transform to monoclinic and expand under the stress at a crack tip. In a moist environment, however, the tetragonal → monoclinic transformation proceeds slowly from the surface even at temperatures as low as body temperature. The volume expansion of about 4% creates microcracks and grain pull-out at the surface, roughening it and lowering the strength and wear resistance. This is low-temperature degradation (hydrothermal degradation); manufacturing conditions accelerated the transformation and fractures of femoral heads became a problem. For this reason, alumina–zirconia composites and similar materials have been developed. Transformation to the cubic phase does not occur at low temperature and does not strengthen transformation toughening. Pitting is corrosion in which the passive film of a metal breaks down locally; it is not a degradation mechanism of oxide ceramics.
Both are forms of corrosion in which the passive film breaks down locally, but pitting can start anywhere on a flat surface, whereas crevice corrosion occurs inside crevices, such as under bolt heads or gaskets. Crevice corrosion generally occurs at lower temperatures and chloride concentrations than pitting.
In mass%, it is calculated as Cr + 3.3 × Mo + 16 × N. For example, Cr 17%, Mo 2.2% and N 0.05% give about 25. Higher values tend to mean greater pitting resistance, but it should be used only as a guide.
Because where and when it starts is decided probabilistically by small film defects, inclusions and local differences in the environment. Once a pit starts growing it advances quickly, so both material selection and inspection are needed to guard against it.
Chloride ions locally break down the passive film at weak points and cause pitting corrosion, in which small holes are dug deeper and deeper.
When SUS304 (JIS; AISI 304) is used in chloride-rich seawat…Pitting is corrosion in which the passive film of a metal breaks down locally; it is not a degradation mechanism of oxide ceramics.
Which statement correctly describes the low-temperature deg…Passivity Sensitization Transformation toughening Faraday's law of electrolysis Galvanic corrosion Local cell Sacrificial anode Standard electrode potential Stress corrosion cracking