A long standing research field in material science has been the trade-off between strength and ductility of steels. Advanced High Strength Steels (AHSS) aim to combine these two properties and provide steels that can increase safety and lower CO2 consumption in automotive applications, by decreasing weight. To further decrease the environmental impact, novel quenched and partioned (Q&P) martensitic stainless steel is considered to prevent corrosion. Quenching and partitioning is a heat treatment that aims for a martensite/retained austenite microstructure with specific phase fractions to optimize strength and ductility. While mechanical properties are well researched for Q&P treated martensitic stainless steel, the corrosion properties are not that widely documented. This thesis aims to discover the influence of the microstructural development in terms of phase fractions due to the Q&P treatment, and the manganese content on the corrosion properties of martensitic stainless steel. Two alloys with chemical composition 0.2C-12.5Cr-0.35Si-XMn, where X stands for 0.7 for the low manganese alloy, and 3 for high manganese alloy, are considered for this study, and are compared to a commercial AISI 420 martensitic stainless steel. The microstructure was investigated by the use of optical microscopy, energy dispersive X-ray spectroscopy (EDS), electron probe microanalysis (EPMA), scanning Kelvin probe force microscopy (SKPFM). This was combined with electrochemical experiments including Open-circuit potential measurements, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). When compared to commercially available AISI 420 martensitic stainless steel, the novel Q&P MSS showed improved corrosion properties. This can mainly be attributed to the lack of chromium rich carbides (Cr23C6 or Cr7C3) in the Q&P treated MSS. However, this improvement in corrosion properties was only observed for the samples with low manganese content. The increase of fraction retained austenite did show an increase in corrosion potential, making the steel more cathodic, but the improvements in terms of pitting potential and passive film properties are limited. Samples with high fresh martensite fractions also showed an increase in corrosion potential, but only marginally increased in terms of length of the passive region. EIS data shows that the addition of fresh martensite reduces the passivity properties of the material. Volta-potential measurements done by SKPFM showed a clear difference between primary martensite and fresh martensite of up to 20mV and were overlapping with the topography maps. This can be an indication of increased tendency to form micro-galvanic cells between martensite phases. Manganese was found to be detrimental for the corrosion properties of the Q&P treated MSS. A clear drop in the length of the passive region shows that the high manganese samples are more susceptible to pitting. EDS measurements and EPMA elemental distribution maps showed local zones of decreased chromium and increased manganese and silicon. This indicates that there are secondary phase particles present in the material in the form of manganese silicide or manganese sulfide. Volta-potential measurements done by SKPFM showed these particles behave more anodic in comparison to the matrix of the material. The number of particles per unit area was increased for the samples with high manganese content. These inclusions greatly reduce corrosion performance, showing that the addition of manganese is detrimental for corrosion performance. Several other microstructural features are discussed in terms of influence on the corrosion performance of the material. Which include prior austenite grain boundaries, elemental banding of chromium and manganese and Volta-potential differences between martensite laths.