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?