Doctoral Dissertations

Date of Award

6-1987

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Metallurgical Engineering

Major Professor

E. E. Stansbury

Abstract

The cyclic polarization pitting test (IN sulfuric acid with 0.5M NaCl) has been used to rank the relative pitting resistance of CF and CFM alloys as a function of alloy composition and heat treatment. The highest and most reproducible pitting potentials were determined for alloys that were subjected to a surface prepassivation treatment (30 minutes in 50% nitric acid at 50-60°C) prior to covering the epoxy/metal interface of the sample mount with enamel to avoid crevice corrosion at this interface. In the absence of the prepassivation treatment, sporadic crevice corrosion at the enamel/metal interface led to low values of the pitting potential and tended to obscure improvements in pitting resistance derived by alloying with nitrogen. For solution treated CF alloys so prepassivated, a composition normalizing factor of the form

Creff = %Cr + %Mo + 20(%C + %N) in austenite

was found to correlate adequately the pitting resistance of a wide range of solution treated compositions.

A group of CF alloys that effectively isolated nitrogen as a compositional variable was studied extensively. It was shown that nitrogen (0.025%N to 0.170%N) does improve the corrosion resistance of solution treated CF alloys as measured by the cyclic polarization pitting test. For a wide variety of conditions, such as 27-42° C solution temperature, 0.0-0.5M NaCl concentration, 0.05-3.0v/hr scan rate, and a 9 micron to 1 grit surface finish, the high nitrogen alloys exhibited superior pitting resistance. A critical review of the literature indicated that no mechanism has been proposed that adequately explains the observed improvement in pitting resistance due to nitrogen additions.

However, critical manipulation, observation, and analysis of cyclic polarization curves of CF alloys, as a function of nitrogen content, has led to a description of a mechanism of improvement in pitting resistance due to nitrogen. In addition to increasing pitting potentials, nitrogen was found to: 1) shift the open circuit potential and the potential at which the critical passivation current density occurs to more negative potentials; 2) increase the magnitude of the critical passivation current density; 3) influence the shape of the anodic peak current such that, at high nitrogen contents, the active loop actually "splits" into two distinct active loops; 4) aid the repassivation process at the onset of pitting (even for very high potentials); and 5) influence the pit initiation sites such that the ferrite-austenite boundaries (as opposed to the austenite phase) are the most susceptible to pitting in the high nitrogen alloys.

In addition to the above, atomic absorption analysis of test solutions following potentiostatic dissolution at various potentials revealed that increasing amounts of nitrogen in CF alloys increased the amount of iron and decreased the amount of chromium found in the solution. This observation indicates that the amount of chromium remaining in the passive film is increasing with increasing nitrogen content, thus leading to improved corrosion resistance. The trend of chromium accumulation in the passive film was observed at all dissolution potentials examined but this effect was greatest at potentials corresponding to the lower of the "split" anodic loops described previously. These observations lead to the proposal that, on an atomic scale, a chromium/nitrogen interaction at the passive film/alloy substrate interface discourages chromium dissolution. Although this mechanism was not proven analytically, this concept is shown to be consistent with other observations noted in the literature; particularly that nitrogen retards chromium diffusion in austenite and accumulates during corrosion at the metal/oxide interface.

For sensitized CP alloys, carbon was found to be the controlling compositional variable. Independent of nitrogen content, the low carbon alloys were much more resistant to sensitization for a fixed chromium content. The 0.05%C CP alloys (each about 20.8%Cr) were found to sensitize similarly in as little as 20 minutes at 650°C. Nitrogen did not improve the pitting resistance of alloys so heat treated due to the very rapid precipitation of carbides at ferrite-austenite boundaries. Microstructural examination following intergranular corrosion tests (EPR, JEPR), however, indicates that nitrogen influences carbide growth by causing preferential carbide growth into the ferrite (chromium in the ferrite phase rather than in austenite is incorporated into carbides) at ferrite-austenite boundaries in duplex alloys. Since ferrite contains more chromium than austenite, the chromium depletion is reduced which in turn implies that nitrogen additions lead to improved overall corrosion resistance. It was also shown (EPR test) that a significantly lower downscan potential was required to "reactivate" the chromium depleted regions occurring in ferrite compared to those in austenite, implying a more stable passive film for slightly sensitized nitrogen bearing alloys compared to nitrogen free alloys.

In contrast to the CF alloys, nitrogen was found to have little or no effect on the pitting resistance of CFM alloys as measured by the cyclic polarization pitting test. It is concluded that the total alloy content (principally high Cr content in addition to 2.2%Mo) was sufficiently high in the variable nitrogen alloys that improvements due to nitrogen additions could not be detected. In particular, the improvement in pitting resistance due to the presence of 2.2%Mo in the CFM alloys overshadows any benefits due to nitrogen additions. This factor is apparent from the fact that the test solution temperature was raised significantly (compared to the CF alloy tests) before pitting was observed in any of the CFM alloys. It is also likely that nitrogen influences the initial stages of anodic dissolution in CFM alloys in a manner similar to that described for CF alloys because similar changes in shape of the anodic curve ("loop splitting") were noted as a function of nitrogen content. However, at the increased level of pitting resistance provided by the Mo content of the CFM alloys, this has a minor influence on the pitting resistance.

Download
Files over 3MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS