The corrosion resistance of stainless steels largely depends on the stability and uniformity of the passive layer formed on their surface when exposed to corrosive environments.

The effectiveness of this passive layer against pitting corrosion is primarily influenced by the percentages of chromium and molybdenum present in the stainless steel. Additionally, the element nitrogen plays a significant role in enhancing resistance to pitting corrosion, offering a cost-effective alternative to molybdenum.

Factors Affecting the Passive Layer

The breakdown of the passive layer can occur due to various factors, including:

• Defects in the passive layer
• Mechanical damage
• Surface irregularities such as impurities, scales, and deposits
• Presence of chloride ions in the environment

High concentrations of chloride ions can lead to the breakdown of the passive layer. The intensity of the corrosion attack is influenced by factors such as the percentage of chloride, acidity, pH, and the presence of oxygen or other oxidizers.

Welding-Related Factors

Welding introduces various factors that can initiate pitting corrosion, including:

• Impurities and secondary phases
• Composition variations within a phase
• Sensitization
• Arc instability and droplet formation
• Localized compositional non-uniformities

In the microstructure, MnS inclusions are significant sites for initiating pitting corrosion. Delta ferrite and sigma phase also exacerbate pitting susceptibility.

Pitting Resistance Number (PREN)

The pitting resistance of stainless steels improves with higher chromium content, but the addition of molybdenum enhances this effect, particularly in grades like 316 (with 18% Cr, 2.5% Mo, and 12% Ni). Adding nitrogen further increases pitting resistance. Consequently, the combined effects of chromium, nitrogen, and molybdenum are used as a measure for assessing the pitting resistance of stainless steel. This measure is referred to as the Pitting Resistance Equivalent Number (PREN), defined as:

PREN = Cr% + 3.3Mo% + 16N%

The table below classifies different metals based on their corrosion resistance:

Critical Pitting Temperature (CPT)

The critical pitting temperature (CPT) is the specific temperature at which pitting corrosion begins for a particular grade of stainless steel. This temperature is influenced by surface conditions, the presence of deposits, and chloride ions, as well as environmental temperature. Therefore, selecting a grade that will not experience pitting at the operating temperature is crucial.

CPT values can be determined using ASTM A48 standards in ferric chloride solutions (10% FeCl3) and in mixed acidic chloride and sulfate solutions (4M HCl + 1% FeSO4 + 3% NaCl). Laboratory tests assessing pitting behavior are typically conducted through electrochemical tests.

Preferred Grades for Pitting Resistance

Austenitic stainless steels with higher percentages of chromium and molybdenum, such as types 304, 316, and 317, exhibit superior pitting resistance. Alternatively, materials like nickel-based alloys (such as Inconel 625, Hastelloy, and Alloy X-6) or special steels like 317L, Jessop 700, LM, titanium, copper-nickel, and nickel-copper alloys can also be employed.

Crevice Corrosion of Stainless Steel

Crevices, formed in conditions involving metal-to-metal contact, gaskets, and corrosion deposits, limit oxygen access and result in crevice corrosion. Various factors contribute to the initiation and growth of crevice corrosion in austenitic stainless steels:

Factors Contributing to Crevice Corrosion

1. Geometric Factors: Type of crevice (metal-to-metal, metal-to-non-metal), width and depth of the crevice, and surface area ratio.
2. Environmental Factors: Oxygen concentration, pH level, chloride concentration, temperature, turbulence, diffusion, and convection.
3. Electrochemical Factors: Metal solution, oxygen reduction, hydrogen generation.
4. Metallurgical Factors: Impurities in the alloy composition and characteristics of the passive layer.

Mitigating Crevice Corrosion

To minimize the effects of crevice corrosion, it is essential to prevent the formation of crevices. This can be improved by maintaining a uniform flow rate in heat exchangers and using grades of steel with higher chromium and molybdenum content, which are more resistant to crevice corrosion. Austenitic stainless steels with higher molybdenum content, such as L904, L316, and 254 SMO, as well as ferritic grades like Mo-2Cr18 and duplex steels like 2205, show substantial resistance to crevice corrosion.

Critical Crevice Corrosion Temperature (CCCT)

For specific grades of stainless steel, crevice corrosion is also influenced by ambient temperature. Above the critical crevice corrosion temperature (CCCT), crevice corrosion initiates, while below this temperature, it does not occur. Thus, selecting a steel grade that will not be subjected to crevice corrosion is feasible, provided the ambient temperature does not exceed critical values.

CCCT values are obtained through ASTM B48G standards in ferric chloride (6% FeCl3 solution for 72-hour crevice tests).

Comparing Pitting and Crevice Corrosion

Although the mechanisms of pitting and crevice corrosion initiation differ, their propagation mechanisms are similar. Crevice corrosion does not require severely corrosive conditions to initiate. A steel resistant to pitting in a specific solution may still experience crevice corrosion in the same environment. By controlling metallurgical factors that enhance pitting resistance, it is also possible to improve resistance to crevice corrosion. Preventing the presence of manganese sulfides will enhance both pitting and crevice corrosion resistance.

Strategies for Reducing Manganese Sulfide Impurities

There are three potential approaches for reducing manganese sulfide impurities:

1. Reduce the manganese content to below the solubility limit of MnS.
2. Lower the sulfur content below the solubility limit of MnS.
3. Add alloying elements like titanium and zirconium to form stronger and more beneficial sulfides.