Time resolved laser induced plasma spectroscopy of stainless steel type 304

Abstract

Laser induced plasma spectroscopy (LIPS) is a technique for determining the elemental composition of a material. In this method, a pulsed laser beam is focused to sufficiently high intensity to excite and vaporize a sample. A high temperature transient plasma is formed either through multiphoton or avalanche ionization. It expands in the forward direction and normal to the sample. Aside from free electrons, ions, excited species of atoms and molecules, the ejected materials also include particles, clusters and molten globules. Spectroscopic analysis of the plasma optical emission is used to determine elemental composition and concentration of the sample. The emission is initially characterized by an intense featureless continuum due to the recombination of electrons with ions (free-bound), elastic collisions of electrons with ions and atoms (free-free). As the plasma expands and cools with time, the radiative relaxation of the excited species produces the discrete emission lines. The sensitivity and precision of LIPS can be increased through time resolved measurement. An optimum period of time for measurement is selected based on the decay of the background continuum and the appearance of the emission lines. The exact timing depends on the sample material, excitation wavelength, pulse duration and other factors.
Compared to methods based on atomic emission spectroscopy, LIPS has several advantages. The most important features are: (1) applicable to solid, liquid, and gaseous samples; (2) conductors, dielectrics and semiconductors can be analyzed; (3) elaborate sample preparation is not required; (4) only a very small amount (0.1 μg to 1 mg) of sample is needed; (5) sampling regions of 10 microns in diameter are possible; (6) study of sample inhomogeneities can be made; (7) real-time analysis can be performed; (8) contact with the sample is not necessary; and (9) analysis can be done in situ in harsh or dangerous environments.Jowever, LIPS also has its limitations. Due to the complex nature of laser-sample interactions, there is an uncertainty if the ablated material represents the bulk sample. To obtain quantitative results, normalization techniques are needed to account for variations in the ablated material. Despite these issues, LIPS has been successfully applied in detecting the presence of trace elements, determination of the carbon content in steel, quantitative analysis of elemental composition in iron ore, analysis of aluminum alloy, particulates in combustion and detection of toxic gases in air.
The aim of this study is to investigate the effects of a corrosive environment on stainless steel through the use of LIPS. Stainless steels are materials of great technological importance due to its mechanical strength and corrosion resistance. However, long term exposure to harsh environment leads to its decomposition. In this paper, we present the preliminary results of our investigation.