Application of a nickel silicide based alloy as a plasma sprayed coating for corrosion resistance to aerated sulfuric acid solutions

Author

Kathleen A. McNelis

Keywords and Phrases

Binary nickel silicide

Abstract

"Past research projects at the University of Missouri, Rolla (UMR) have resulted in the development of a series of nickel silicide alloys for resistance to corrosion by aqueous sulfuric acid solutions. One of the alloys developed, nominally Ni_75.5Si₂₂Nb₂B_0.5, showed excellent corrosion resistance, but poor ductility. This alloy was selected for use as a coating material in this thesis.

In this work, the Ni₂₂Si₂₃Nb₂B_0.5 alloy was applied to several substrates by atmospheric plasma spray to create a coating. Spray parameters were selected to minimize coating porosity, with best results producing coatings whose porosity is estimated to be between 2 and 5% by volume. Adhesion of coatings onto 410 stainless steel substrates ranged from 5 to 6 MPa, and onto 316 stainless steel substrates ranged from 7 to 11 MPa.

The microstructure of the coating is different than the cast material. While in the cast material large regions of different phases are visible, there is no visible distinction of phases within the coatings. Standardless EDS analysis indicates, however, that the composition is not entirely homogeneous, and X RD results indicate the presence of γ - (Ni₃₁Si₁₂) and α_ss-(Ni,Si,Nb).

Corrosion of both the coating and underlying substrates in aerated boiling (70% by weight) sulfuric acid were significant. An estimated 30% of the coating mass was lost during 24 hours of testing. Corrosion appears to have occurred by both dissolution of the coating followed by attack of underlying substrate as well as permeation of the coating through interconnected cracks and pores to create pitting at the substrate surface. Corrosion was most pronounced at the outer perimeter of the specimens, where residual stresses are expected to be greatest, however significant corrosion also occurred along flat surfaces. Corrosion was less than the comparison C-276 coating, which was almost entirely removed during the first day of testing.

Hardness measurements indicate that the average hardness of the coating (295 ±79 HK, 500g) is roughly half that of the cast alloy (572 ±150 HK, 500g), and abrasive wear results indicate a similar trend. Abrasive wear rates of the coating were approximately 0.5 mm3/m using a 600 grit SiC drum"--Abstract, page iii.

Advisor(s)

Newkirk, Joseph William

Committee Member(s)

Van Aken, David C.
Smith, Jeffrey D.

Department(s)

Materials Science and Engineering

Degree Name

M.S. in Metallurgical Engineering

Publisher

University of Missouri--Rolla

Publication Date

Summer 2004

Pagination

xiii, 90 pages

Note about bibliography

Includes bibliographical references (pages 87-89).

Rights

© 2004 Kathleen A. McNelis, All rights reserved.

Document Type

Thesis - Restricted Access

File Type

text

Language

English

Subject Headings

Nickel alloysCoatings

Thesis Number

T 8603

Print OCLC #

62266369

Recommended Citation

McNelis, Kathleen A., "Application of a nickel silicide based alloy as a plasma sprayed coating for corrosion resistance to aerated sulfuric acid solutions" (2004). Masters Theses. 3635.
https://scholarsmine.mst.edu/masters_theses/3635