9.3.Practical Consequences 165 9.3.Practical Consequences Finally,we shall briefly summarize the corrosion properties of some selected materials and designs.This list contains observa- tions which have been accumulated over a time span of many decades and belong to the "must-know"repertoire of the corro- sion-conscious engineer. Low-alloy steel (up to 2%of total alloy content)has a much better resistance to atmospheric corrosion than plain carbon steel. Stainless steels (iron containing at least 12%chromium)de- rive their corrosion resistance from the high reactiveness of Cr and its alloys (Table 9.1),which leads to a protective (i.e.,in- sulating)coating or Cr-containing corrosion products and thus to an interruption of the corrosion current.This mechanism is called passivation.(Note in this context the ranking of "active" stainless steel compared to "passive"stainless steel in Table 9.2) Iron can be passivated by immersing it momentarily into highly concentrated nitric acid,which causes a thin film of iron hy- droxide to form.Iron,protected this way,is then no longer at- tacked by nitric acid of lower concentrations. Martensitic stainless steels usually display a better corrosion resistance when in the hardened state compared to its annealed condition. Cold worked metals are often more severely attacked by cor- rosion than annealed metals. A two-phase alloy is generally more severely attacked by cor- rosion than a single-phase alloy or a pure metal.This is due to the possible presence of a composition cell,where one phase may be cathodic to the other.As an example,cementite is ca- thodic with respect to ferrite.This leads to microcorrosion cells within the sample consisting of a and Fe3C plates as shown in Figure 9.7(a).Under these circumstances,the a tends to be sac- rificed by protecting the Fe3C. Precipitation of a second phase or segregation at grain bound- aries may cause a galvanic cell and leads to intergranular corrosion.As an example,chromium carbide particles may pre- cipitate at high temperatures (due to welding or heat treat- ments)along the grain boundaries of stainless steel.This causes a chromium depletion in the vicinity of these grain boundaries.Again,a concentration cell is formed which may lead to corrosion at the grain boundaries [Figure 9.7(b)]. Stress corrosion cracking occurs in specific metals containing regions that have different stress levels and are exposed to spe-Finally, we shall briefly summarize the corrosion properties of some selected materials and designs. This list contains observa￾tions which have been accumulated over a time span of many decades and belong to the “must-know” repertoire of the corro￾sion-conscious engineer. • Low-alloy steel (up to 2% of total alloy content) has a much better resistance to atmospheric corrosion than plain carbon steel. • Stainless steels (iron containing at least 12% chromium) de￾rive their corrosion resistance from the high reactiveness of Cr and its alloys (Table 9.1), which leads to a protective (i.e., in￾sulating) coating or Cr-containing corrosion products and thus to an interruption of the corrosion current. This mechanism is called passivation. (Note in this context the ranking of “active” stainless steel compared to “passive” stainless steel in Table 9.2.) • Iron can be passivated by immersing it momentarily into highly concentrated nitric acid, which causes a thin film of iron hy￾droxide to form. Iron, protected this way, is then no longer at￾tacked by nitric acid of lower concentrations. • Martensitic stainless steels usually display a better corrosion resistance when in the hardened state compared to its annealed condition. • Cold worked metals are often more severely attacked by cor￾rosion than annealed metals. • A two-phase alloy is generally more severely attacked by cor￾rosion than a single-phase alloy or a pure metal. This is due to the possible presence of a composition cell, where one phase may be cathodic to the other. As an example, cementite is ca￾thodic with respect to ferrite. This leads to microcorrosion cells within the sample consisting of  and Fe3C plates as shown in Figure 9.7(a). Under these circumstances, the  tends to be sac￾rificed by protecting the Fe3C. • Precipitation of a second phase or segregation at grain bound￾aries may cause a galvanic cell and leads to intergranular corrosion. As an example, chromium carbide particles may pre￾cipitate at high temperatures (due to welding or heat treat￾ments) along the grain boundaries of stainless steel. This causes a chromium depletion in the vicinity of these grain boundaries. Again, a concentration cell is formed which may lead to corrosion at the grain boundaries [Figure 9.7(b)]. • Stress corrosion cracking occurs in specific metals containing regions that have different stress levels and are exposed to spe- 9.3 • Practical Consequences 165 9.3 • Practical Consequences