Changing material changes roblox physics?

script.Parent.CornerWedgeAlpha.Touched:Connect(function()
        if debounce2 == false then 
            debounce2 = true
        for i,v in pairs (script.Parent:GetDescendants()) do
            if v:IsA("Part") or v:IsA("CornerWedgePart") then
        local fire = Instance.new("Fire") 
        fire.Parent = v
            end
        end
end
    local explosion = Instance.new("Explosion")
        explosion.Parent = script.Parent.Part
        explosion.BlastPressure = 40000
end)

^ this explodes like normal

script.Parent.CornerWedgeAlpha.Touched:Connect(function()
        if debounce2 == false then 
            debounce2 = true
        for i,v in pairs (script.Parent:GetDescendants()) do
            if v:IsA("Part") or v:IsA("CornerWedgePart") then
v.Material = ("Corroded Metal")         
local fire = Instance.new("Fire") 
        fire.Parent = v
            end
        end
end
    local explosion = Instance.new("Explosion")
        explosion.Parent = script.Parent.Part
        explosion.BlastPressure = 40000
end)

^ this explodes with much less force

do materials have effects on mass or is my script wrong in some way?