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Journal of Failure Analysis and Prevention

, Volume 10, Issue 6, pp 458–462 | Cite as

Premature Corrosion of Steak Knives Due to Extensive Precipitation of Chromium Carbides

  • Marina Banuta
  • Isabelle Tarquini
Case History---Peer-Reviewed

Abstract

Steak knives from three different suppliers were brought to our facility to conduct a comparative analysis of their metallurgical properties. Although they have allegedly been used and cleaned in similar conditions in several restaurants across North America, only knives coming from one supplier showed signs of premature corrosion. Our investigation found that even though they have similar chemical and mechanical properties, and had been submitted to the same heat treatment, major microstructure discrepancies made some of these knives prone to rapid corrosion. As per our examination, the prematurely corroded knives experienced extensive chromium carbides precipitation during heat treatment, which determined a massive chromium depletion of the matrix and lowered their corrosion resistance. On the contrary, the knives that never corroded kept all their chromium content inside the matrix, thus contributing to better corrosion resistance.

Keywords

Martensitic stainless steel Corrosion Heat treated Chromium carbides precipitation 

Background

Eight steak knives, coming from three different manufacturers (identified A, B, and C) and having being used in several restaurants across North America, were brought to our facility to conduct a comparative analysis of their metallurgical properties and to investigate the most probable cause of the premature corrosion of some of them. Regardless of their origin, all knives were allegedly used in the same conditions and the same cleaning methods were applied. However, only knives coming from one manufacturer rapidly lost their shining appearance, became stained and developed rust spots after some time of use. The other ones showed no sign of degradation. Our client wanted to know why only some of the knives corroded and what difference between their properties could explain their different behavior. General images of the steak knives under investigation are shown in Fig. 1. According to their origin, the knives were labeled from A to C, the group A being the corroded ones. Groups B and C came from two other manufacturers and bore no sign of degradation. All knives were made of stainless steel martensitic grade AISI 420.
Fig. 1

General images of the submitted knives: (a) Group A, (b) Group B, and (c) Group C

The Investigation

Visual Examination and Corrosion Mode Identification

General visual and low-magnification examination of corroded knives from group A found indeed the presence of staining, rust deposits, and pitting corrosion, localized on both sides of the blades and at the handles junctions. All these indications are typical evidence of corrosion degradation on stainless steel. Figure 2 shows images of the most representative areas. Low-magnification examination was also conducted on knives of the other two groups. They showed no sign of degradation, rust deposits, or staining. Overall, our visual observation corroborated the presence of an in-progress corrosion process in knives coming from group A. Staining and pitting corrosion are typical degradation phenomena for stainless steels used in corrosive environments rich in chlorides and chlorine solutions.
Fig. 2

Corrosion degradation on knives

EDS Analysis

EDS analyses were next performed to evaluate the presence of chemical elements in the corroded areas and in deposits. Typical micrograph and spectrum of the examined degraded regions are shown in Figs. 3 and 4. It can be noted that the corroded surfaces contain several chemical elements, some of which are normally present in the base metal (like, carbon, iron, chromium, and nickel), iron and oxygen form iron oxide (rust), and calcium, phosphorus, sodium and chlorine come from food as well as from cleaning solutions. Considering the base material is stainless steel, phosphorus, and chlorine were particularly aggressive for the knives under investigation.
Fig. 3

SEM micrograph of a corroded area

Fig. 4

EDS spectrum

Comparative Metallurgical Characterization

One of the specific requests of our client was an explanation concerning the different behavior of the knives in similar condition of use. In order to answer to this question, a comparative metallurgical characterization was conducted on one knife selected from each group (A, B, and C). Chemical analysis, hardness measurement, and microstructure examination were thus performed on samples coming from these knives.

Chemical analyses were carried out using optical emission spectroscopy on roughly polished specimens of knives A, B, and C. The obtained results, which are shown in Table 1, indicate that, regardless of some differences, all knives are made of grade AISI 420 stainless steel.
Table 1

Chemical analyses

Sample

C

Si

Mn

P

S

Cr

AISI 420

0.15 min.

1.0 max.

1.0 max.

0.040 max.

0.030 max.

12.0–14.0

Knife A

0.33

0.58

0.46

0.024

0.006

13.9

Knife B

0.25

0.53

0.30

0.027

0.013

12.22

Knife C

0.24

0.89

0.26

0.023

0.018

12.71

Hardness measurements found that all three samples have average hardness values of 56 HRC, which is consistent with quenching and tempering of this grade [1].

Microstructure analysis was conducted on the three samples, on specimens cut in longitudinal direction. Typical micrographs are presented in Figs. 5, 6, and 7. As it can be easily noted in these micrographs, the discrepancy between the microstructure in corroded and non-corroded knives is outstanding. While all knives were undoubtedly quenched and tempered, possessing thus an essentially martensitic matrix, the amount of secondary chromium carbides is very different. As such, the corroded knife exhibits a significant amount of small chromium carbides evenly distributed in the martensite matrix. On the contrary, in the case of the intact ones, only few scattered chromium carbides are noticeable. This discrepancy played an important role in the corrosion resistance of the submitted knives, as it will be further explained.
Fig. 5

Typical microstructure of knife A, showing extensive secondary carbides precipitation

Fig. 6

Typical microstructure of knife B, showing virtually no secondary carbides precipitation

Fig. 7

Typical microstructure of knife C, showing virtually no secondary carbides precipitation

Discussion

It is well known that the great corrosion resistance exhibited by stainless steels lies in their chromium and/or nickel content. Nickel deals with harmful acids while chromium is responsible for the formation, at the stainless steel surface, of an invisible protection film, known as a “passivation film” [2]. This process may be spontaneous or chemically induced. Regardless of its nature, the passivation consists in creating a chrome oxide thin film with a high corrosion resistance. As such, the stainless steel becomes practically inert to most aggressive environments. Chromium is thus the key element that provides these steels with their outstanding corrosion resistance.

However, not all stainless steels families possess the same mechanical and corrosion resistance. Two stainless steel families are usually used in cutlery manufacturing, that is, austenitic stainless steels for forks, spoons, and any other non-cutting items, and martensitic stainless steel for knives. While the first possess better corrosion resistance, the latter exhibit better mechanical properties. Indeed, in order to achieve a lasting edge for the knives a steel that is much harder but of lower corrosion resistance than the spoons and forks must be used (martensitic). This is one of the reasons ordinary knives are more prone to corrode given the right circumstances.

The right circumstances in cutlery are usually related to cleaning conditions. On one hand, common salt in food and liquid bleaches or disinfectants are very corrosive and will greatly accelerate the corrosive action of water. On the other hand, some water softeners are regenerated with salt. If they are not functioning correctly, the softened water can have a high salt content. Detergents are normally harmless when fully dissolved. If, however, certain powder detergents are allowed to come into contact with blades in hot water before the detergent is fully dissolved, pits and/or stains can form within a few minutes [3]. Un-dissolved table salt can have a similar effect.

In the present case, knives coming from three different suppliers and being exposed to similar using and cleaning conditions exhibited different corrosion behaviors. As our investigation found, knives from group “A” have undergone pitting corrosion and staining on the blades and at the blade/handle junctions, which is a typical corrosion phenomenon in stainless steels working in chloride environment [4]. It was found that the cause of their different behavior lies in their microstructure characteristics. As such, even though all have tempered martensite matrixes, only knife “A” exhibit significant precipitation of secondary chromium carbide particles. On the contrary, this phenomenon is almost inexistent in knives “B” and “C.” Considering that the normal use of cutlery does not imply temperatures in the range susceptible to produce secondary chromium carbide precipitation, it is reasonable to believe that the origin of this phenomenon was related to heat treatment parameters.

Martensitic stainless steels represent an interesting choice in cutlery because of their capacity of being hardened as to acquire optimal combination of high strength, toughness, and corrosion resistance. In order to achieve the best corrosion resistance, these steels must contain at least 11% of Cr for the formation of protective oxide film, and the chromium must be dissolved in the matrix [5]. Several studies showed that the corrosion resistance of this class of stainless steels is sensitive to the carbide volume fraction dissolved in the matrix after austenitizing for quenching and is close related to the carbide precipitation during tempering [5]. It is thus obvious that heat treatment is an important processing step in controlling the corrosion resistance of these steels.

Recent studies also suggest that, to avoid massive chromium carbides precipitation, austenitization should take place at higher temperatures than the ones commonly used in these steels, usually at the upper end of the austenitization range [6]. In this respect, The ASM’s Heat Treater’s Guide recommends austenitizing at 1950 °F (1065 °C) for maximum corrosion resistance in 420 stainless steel grade. This practice would enable the presence of lesser primary chromium carbides in solution during austenitizing and lesser secondary chromium carbides in the matrix after quenching.

Although the carbides precipitation from the solution may have a great positive impact on the hardness, it will also cause an important decrease of the corrosion resistance. As already stated above, the chromium is the key element in the corrosion resistance of stainless steels. However, to achieve its role, it must remain in atomic form in the metallic matrix of the alloy. When it combines with carbon to form chromium carbides, the chromium is taken from the matrix and no longer participates in the chromium oxide passivation film. Consequently, the corrosion resistance of the material will drastically decrease with the amount of chromium carbide precipitates.

Conclusion

All findings of this metallurgical investigation led to the conclusion that the knives coming from supplier “A” corroded by a pitting corrosion mechanism due to exposure to chlorides during using and cleaning. As noted during microscopic analyses, these knives had poor corrosion resistance because of the chromium depletion of the metallic matrix by chromium carbides precipitation during heat treatment (secondary carbide precipitation). On the contrary, even though they are made of the same material and underwent the same heat treatment sequence, and they were used and cleaned under the same conditions, the knives coming from two others suppliers never corroded because they possess better corrosion resistance, due to the absence of a phenomenon of chromium depletion in the matrix. This discrepancy between their microstructures was thus the decisive factor in their different behavior. It was suggested that, in order to improve the corrosion resistance of its products, supplier A should consider utilization of higher austenitizing temperatures.

References

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Copyright information

© ASM International 2010

Authors and Affiliations

  1. 1.SGS Canada Inc.—Materials EngineeringMontrealCanada

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