Introduction to Tribocorrosion and Its Effects on Stainless Steel
Austenitic stainless steel, particularly the 316L grade, is favored for its excellent corrosion resistance, mechanical strength, and durability in harsh environments. It is commonly used in applications exposed to corrosive agents like seawater, such as ship equipment, marine resource extraction systems, and deep-sea diving instruments. One of the most common issues in such environments is pitting corrosion, a form of localized corrosion that occurs when chloride ions, typically present in seawater, attack the protective passive oxide layer on stainless steel.
In addition to the general corrosion caused by exposure to chloride ions, stainless steel components, especially those in mechanical applications such as pumps, bearings, and hydraulic systems, are often subjected to tribocorrosion. Tribocorrosion is the combined effect of mechanical wear and electrochemical corrosion, where surface wear due to frictional forces accelerates the material’s degradation by promoting electrochemical reactions. In marine environments, this synergistic process speeds up the breakdown of stainless steel, leading to increased susceptibility to pitting corrosion.
Tribocorrosion: Accelerating the Degradation of Stainless Steel
Under normal conditions, stainless steel exhibits a high resistance to pitting corrosion due to the protective chromium oxide passive film that forms on its surface. However, during tribocorrosion, mechanical wear exposes fresh metal surfaces, creating defects that allow corrosion to initiate more easily. These defects, coupled with the aggressive chloride ions, make the steel more vulnerable to pitting corrosion. The pitting potential, or the voltage at which pitting corrosion typically begins, is significantly reduced in tribocorrosion conditions.
Previous studies, including those on 304 and 316L stainless steels, have shown that corrosion pits form at much lower potentials than usual when the steel is subjected to tribocorrosion. This results in a much higher density of pits, causing accelerated degradation of the steel. In particular, the transition from metastable pits (initial, small pits) to stable, larger pits is altered, significantly impacting the material’s long-term integrity.
Role of Chromium in Pitting Corrosion Resistance
Chromium is an essential alloying element in stainless steel that enhances its corrosion resistance. It contributes to the formation of a chromium-rich oxide film on the surface of the steel, which protects the material from aggressive ions like chlorides. However, when chromium is depleted from certain areas of the stainless steel particularly at grain boundaries due to carbide precipitation, the protective oxide layer becomes compromised. This makes the steel more prone to localized corrosion, such as pitting.
In this study, the researchers found that Cr-depleted layers were formed on the surface of 316L stainless steel during tribocorrosion. These layers, which were up to ten nanometers thick, played a significant role in decreasing the material’s resistance to pitting corrosion. The Cr-depleted areas were associated with an increased defect density and a reduction in the chromium content in the passive film. This weakened the protective oxide layer, making it easier for chloride ions to penetrate and initiate pitting.
Formation and Effects of the Cr-Depleted Layer
The Cr-depleted layer forms as a result of mechanical wear during tribocorrosion, where carbide precipitation at the grain boundaries depletes the chromium content in these regions. This layer weakens the protective oxide layer on the stainless steel, creating areas more susceptible to pitting. The study further demonstrated that the dissolution of the Cr-depleted layer provided a high concentration of cations that sustained the growth of corrosion pits. Once these pits began to form, the aggressive electrolyte around them further facilitated their development, leading to high-density metastable pits, which could grow into stable pits at lower potentials.
Surface Roughness and Nanocrystallization in Tribocorrosion
Apart from the Cr-depleted layer, other factors contribute to the increased pitting corrosion susceptibility during tribocorrosion, including surface roughness and nanocrystallization. The mechanical wear involved in tribocorrosion roughens the stainless steel surface, creating more occluded sites where corrosion can initiate. These sites require less current to sustain diffusion-controlled processes that promote pit growth. This phenomenon accelerates the formation of metastable pits, which can later develop into stable pits.
Additionally, tribocorrosion induces nanocrystallization in the surface layer of stainless steel. The repeated mechanical deformation of the material during tribocorrosion leads to the formation of nanocrystals, which introduce a high density of grain boundaries and dislocations. These defects increase the reactivity of the surface, providing additional nucleation sites for metastable pits. This combination of surface roughening and nanocrystallization, together with the Cr-depleted layer, dramatically reduces the steel’s resistance to pitting corrosion.
Investigating the Combined Effects of Tribocorrosion Factors
To better understand the combined effects of surface roughness, nanocrystallization, and Cr-depletion, the researchers prepared four different types of 316L stainless steel samples with varying surface and subsurface characteristics. These samples were then subjected to electrochemical tests under non-frictional conditions to investigate their metastable pitting corrosion behavior.
Electrochemical methods allowed the researchers to evaluate several parameters related to the metastable pits, such as:
• Metastable pit peak current: This indicates the intensity of corrosion current during the formation of metastable pits.
• Metastable pit lifetime: The duration for which metastable pits exist before they either stabilize or disappear.
• Metastable pit radius: The size of the pits that form, which is an indicator of their growth potential.
• Metastable pit stability product: A measure of the stability of the metastable pits.
By statistically evaluating these parameters, the study aimed to identify which factors, surface roughness, nanocrystallization, or Cr-depletion, had the most significant impact on the formation and growth of metastable pits during tribocorrosion.
Impact of Tribocorrosion on Stainless Steel in Marine Environments
The findings of this study are particularly important for understanding the degradation of stainless steel in marine environments, where tribocorrosion is a common issue. Components made from 316L stainless steel, such as ship parts, offshore drilling equipment, and underwater instruments, are often exposed to both mechanical stress and corrosive seawater. The study highlights that tribocorrosion can lead to the formation of high-density pits at much lower potentials than expected, thus increasing the rate of material degradation and potentially leading to catastrophic failures.
By understanding how factors like Cr-depletion, surface roughness, and nanocrystallization contribute to the pitting corrosion process, this research can help in the development of more resistant materials or better design guidelines for stainless steel components in marine applications.
Key Takeaways:
• Tribocorrosion Reduces Pitting Resistance: The pitting corrosion resistance of 316L stainless steel significantly decreases during tribocorrosion due to the formation of a Cr-depleted layer.
• Chromium Depletion Is Crucial: The formation of Cr-depleted layers, which weaken the passive oxide film, increases susceptibility to pitting corrosion in tribocorrosion conditions.
• Surface Roughness and Nanocrystallization: Mechanical wear roughens the surface and induces nanocrystallization, providing more sites for pit nucleation and reducing the steel’s resistance to corrosion.
• Electrochemical Methods for Analysis: The study used electrochemical techniques to assess key parameters related to metastable pit formation, such as peak current, pit lifetime, and pit radius.
• High-Density Pit Formation: Tribocorrosion leads to the formation of high-density metastable pits at lower potentials, accelerating degradation and weakening the material.
• Implications for Marine Environments: This study provides critical insights for improving the durability and design of stainless steel components in marine environments, where tribocorrosion is prevalent.
